T h e   H i s t o r y   o f S  u  p  e  r  c  o  n  d  u  c  t  o  r

s

     Superconductors, materials that have no resistance to the flow of electricity, are one of the last great frontiers of scientific discovery. Not only have the limits of superconductivity not yet been reached, but the theories that explain superconductor behavior seem to be constantly under review. In 1911 superconductivity was first observed in mercury by Dutch physicist Heike Kamerlingh Onnes of Leiden University (shown above). When he cooled it to the temperature of liquid helium, 4 degrees Kelvin (-452F, -269C), its resistance suddenly disappeared. The Kelvin scale represents an "absolute" scale of temperature. Thus, it was necessary for Onnes to come within 4 degrees of the coldest temperature that is theoretically attainable to witness the phenomenon of superconductivity. Later, in 1913, he won a Nobel Prize in physics for his research in this area.

Walter Meissner

     The next great milestone in understanding how matter behaves at extreme cold temperatures occurred in 1933. German researchers Walter Meissner (above) and Robert Ochsenfeld discovered that a superconducting material will repel a magnetic field (below graphic). A magnet moving by a conductor induces currents in the conductor. This is the principle on which the electric generator operates. But, in a superconductor the induced

currents exactly mirror the field that would have otherwise penetrated the superconducting material - causing the magnet to be repulsed. This phenomenon is known as strong diamagnetism and is today often referred to as the "Meissner effect" (an eponym). The Meissner effect is so strong that a magnet can actually be levitated over a superconductive material.

     In subsequent decades other superconducting metals, alloys and compounds were discovered. In 1941 niobium-nitride was found to superconduct at 16 K. In 1953 vanadium-silicon displayed superconductive properties at 17.5 K. And, in 1962 scientists at Westinghouse developed the first commercial superconducting wire, an alloy of niobium and titanium (NbTi). High-energy, particle-accelerator electromagnets made of copper-clad niobium-titanium were then developed in the 1960s at the Rutherford-Appleton Laboratory in the UK, and were first employed in a superconducting accelerator at the Fermilab Tevatron in the US in 1987.

     The first widely-accepted theoretical understanding of superconductivity was advanced in 1957 by American physicists John Bardeen, Leon Cooper, and John Schrieffer (above). Their Theories of Superconductivity became know as the BCS theory - derived from the first letter of each man's last name - and won them a Nobel prize in 1972. The mathematically-complex BCS theory explained superconductivity at temperatures close to absolute zero for elements and simple alloys. However, at higher temperatures and with different superconductor systems, the BCS theory has subsequently become inadequate to fully explain how superconductivity is occurring.

Brian Josephson

     Another significant theoretical advancement came in 1962 when Brian D. Josephson (above), a graduate student at Cambridge University, predicted that electrical current would flow between 2 superconducting materials - even when they are separated by a non-superconductor or insulator. His prediction was later confirmed and won him a share of the 1973 Nobel Prize in Physics. This tunneling phenomenon is today known as the "Josephson effect" and has been applied to electronic devices such as the SQUID, an instrument capabable of detecting even the weakest magnetic fields. (Below SQUID graphic courtesy Quantum Design.)

     The 1980's were a decade of unrivaled discovery in the field of superconductivity. In 1964 Bill Little of Stanford University had suggested the possibility of organic (carbon-based) superconductors. The first of these theoretical superconductors was successfully synthesized in 1980 by Danish researcher Klaus Bechgaard of the University of Copenhagen and 3 French team members. (TMTSF)2PF6 had to be cooled to an incredibly cold 1.2K transition temperature (known as Tc) and subjected to high pressure to superconduct. But, its mere existence proved the possibility of "designer" molecules - molecules fashioned to perform in a predictable way.

     Then, in 1986, a truly breakthrough discovery was made in the field of superconductivity. Alex Müller and Georg Bednorz (above), researchers at the IBM Research Laboratory in Rüschlikon, Switzerland, created a brittle ceramic compound that superconducted at the highest temperature then known: 30 K. What made this discovery so remarkable was that ceramics are normally insulators. They don't conduct electricity well at all. So, researchers had not considered them as possible high-temperature superconductor candidates. The Lanthanum, Barium, Copper and Oxygen compound that Müller and Bednorz synthesized, behaved in a not-as-yet-understood way. (Original article printed in Zeitschrift für Physik Condensed Matter, April 1986.) The discovery of this first of the superconducting copper-oxides (cuprates) won the 2 men a Nobel Prize the following year. It was later found that tiny amounts of this material were actually superconducting at 58 K, due to a small amount of lead having been added as a calibration standard - making the discovery even more noteworthy.

     Müller and Bednorz' discovery triggered a flurry of activity in the field of superconductivity. Researchers around the world began "cooking" up ceramics of every imaginable combination in a quest for higher and higher Tc's. In January of 1987 a research team at the University of Alabama-Huntsville substituted Yttrium for Lanthanum in the Müller and Bednorz molecule and achieved an incredible 92 K Tc. For the first time a material (today referred to as YBCO) had been found that would superconduct at temperatures warmer than liquid nitrogen - a commonly available coolant. Additional milestones have since been achieved using exotic - and often toxic - elements in the base perovskite ceramic. The current class (or "system") of ceramic superconductors with the highest transition temperatures are the mercuric-cuprates. The first synthesis of one of these compounds was achieved in 1993 at the University of Colorado and by the team of A. Schilling, M. Cantoni, J. D. Guo, and H. R. Ott of Zurich, Switzerland. The world record Tc of 138 K is now held by a thallium-doped, mercuric-cuprate comprised of the elements Mercury, Thallium, Barium, Calcium, Copper and Oxygen. The Tc of this ceramic superconductor was confirmed by Dr. Ron Goldfarb at the National Institute of Standards and Technology-Colorado in February of 1994. Under extreme pressure its Tc can be coaxed up even higher - approximately 25 to 30 degrees more at 300,000 atmospheres.

     The first company to capitalize on high-temperature superconductors was Illinois Superconductor (today known as ISCO International), formed in 1989. This amalgam of government, private-industry and academic interests introduced a depth sensor for medical equipment that was able to operate at liquid nitrogen temperatures (~ 77K).

     In recent years, many discoveries regarding the novel nature of superconductivity have been made. In 1997 researchers found that at a temperature very near absolute zero an alloy of gold and indium was both a superconductor and a natural magnet. Conventional wisdom held that a material with such properties could not exist!  Since then, over a half-dozen such compounds have been found. Recent years have also seen the discovery of the first high-temperature superconductor that does NOT contain any copper (2000), and the first all-metal perovskite superconductor (2001).

     Also in 2001 a material that had been sitting on laboratory shelves for decades was found to be an extraordinary new superconductor. Japanese researchers measured the transition temperature of magnesium diboride at 39 Kelvin - far above the highest Tc of any of the elemental or binary alloy superconductors. While 39 K is still well below the Tc's of the "warm" ceramic superconductors, subsequent refinements in the way MgB2 is fabricated have paved the way for its use in industrial applications. Laboratory testing has found MgB2 will outperform NbTi and Nb3Sn wires in high magnetic field applications like MRI.

     Though a theory to explain high-temperature superconductivity still eludes modern science, clues occasionally appear that contribute to our understanding of the exotic nature of this phenomenon. In 2005, for example, Superconductors.ORG discovered that increasing the weight ratios of alternating planes within the layered perovskites can often increase Tc significantly. This has led to the discovery of more than 35 new high-temperature superconductors, including a candidate for a new world record.

     The most recent "family" of superconductors to be discovered is the "pnictides". These iron-based superconductors were first observed by a group of Japanese researchers in 2006. Like the high-Tc copper-oxides, the exact mechanism that facilitates superconductivity in them is a mystery. However, with Tc's over 50K, a great deal of excitement has resulted from their discovery.

     Researchers do agree on one thing: discovery in the field of superconductivity is as much serendipity as it is science. Stay tuned!

     [For additional, more-obscure history, visit the "Atypical" and "Type 2"pages.]

     [Last page rev: March 2009]

     

Superconducting Materials

Highly oriented oxide superconductor films are fabricated by a dipping-pyrolysis process using metal-organic compounds as starting materials. Epitaxial YBa2Cu3O7-y(YBCO) films prepared by two-step annealing give high critical current densities at 77K: 1.2x 106A/cm2 in zero-applied fields, and 1.0x105A/cm2 in a magnetic field of 8T. These values are comparable to those of the YBCO films prepared by in situ deposition processes such as sputtering and chemical vapor deposition.

High-temperature oxide superconductors and related materials are synthesized and their crystal structures and physical properties are evaluated. The figure shows the crystal structure of the Cu-based superconductor Cu1-xBa2Ca3Cu4O12-

y(transition temperature of 117K): determined by single crystal x-ray structure analysis method for the first time.

Crystal structure of Cul-xBa2Ca3Cu4O12-y

The Yamanashi MLX01 MagLev train.

Uses  for  Superconductors

     Magnetic-levitation is an application where superconductors perform extremely well. Transport vehicles such as trains can be made to "float" on strong superconducting magnets, virtually eliminating friction between the train and its tracks. Not only would conventional electromagnets waste much of the electrical energy as heat, they would have to be physically much larger than superconducting magnets. A landmark for the commercial use of MAGLEV technology occurred in 1990 when it gained the status of a nationally-funded project in Japan. The Minister of Transport authorized construction of the Yamanashi Maglev Test Line which opened on April 3, 1997. In December 2003, the MLX01 test vehicle (shown above) attained an incredible speed of 361 mph (581 kph).

     Although the technology has now been proven, the wider use of MAGLEV vehicles has been constrained by political and environmental concerns (strong magnetic fields can create a bio-hazard). The world's first MAGLEV train to be adopted into commercial service, a shuttle in Birmingham, England, shut down in 1997 after operating for 11 years. A Sino-German maglev is currently operating over a 30-km course at Pudong International Airport in Shanghai, China. The U.S. plans to put its first (non-superconducting) Maglev train into operation on a Virginia college campus. Click this link for a website that lists other uses for MAGLEV.

MRI of a human skull.

     An area where superconductors can perform a life-saving function is in the field of biomagnetism. Doctors need a non-invasive means of determining what's going on inside the human body. By impinging a strong superconductor-derived magnetic field into the body, hydrogen atoms that exist in the body's water and fat molecules are forced to accept energy from the magnetic field. They then release this energy at a frequency that can be detected and displayed graphically by a computer. Magnetic Resonance Imaging (MRI) was actually discovered in the mid 1940's. But, the first MRI exam on a human being was not performed until July 3, 1977. And, it took almost five hours to produce one image! Today's faster computers process the data in much less time. A tutorial is available on MRI at this link. Or read the latest MRI news at this link.

     The Korean Superconductivity Group within KRISS has carried biomagnetic technology a step further with the development of a double-relaxation oscillation SQUID (Superconducting QUantum Interference Device) for use in Magnetoencephalography. SQUID's are capable of sensing a change in a magnetic field over a billion times weaker than the force that moves the needle on a compass (compass: 5e-5T, SQUID: e-14T.). With this technology, the body can be probed to certain depths without the need for the strong magnetic fields associated with MRI's.

      Probably the one event, more than any other, that has been responsible for putting "superconductors" into the American lexicon was the Superconducting Super-Collider project planned for construction in Ellis county, Texas. Though Congress cancelled the multi-billion dollar effort in 1993, the concept of such a large, high-energy collider would never have been viable without superconductors. High-energy particle research hinges on being able to accelerate sub-atomic particles to nearly the speed of light. Superconductor magnets make this possible. CERN, a consortium of several European nations, is doing something similar with its Large Hadron Collider (LHC) now under construction along the Franco-Swiss border.

     Other related web sites worth visiting include the proton-antiproton collider page at Fermilab. This was the first facility to use superconducting magnets. Get information on the electron-proton collider HERA at the German lab pages of DESY (with English text). Lastly, Brookhaven National Laboratory features a page dedicated to its RHIC heavy-ion collider.

     Electric generators made with superconducting wire are far more efficient than conventional generators wound with copper wire. In fact, their efficiency is above 99% and their size about half that of conventional generators. These facts make them very lucrative ventures for power utilities. General Electric has estimated the potential worldwide market for superconducting generators in the next decade at around $20-30 billion dollars. Late in 2002 GE Power Systems received $12.3 million in funding from the U.S. Department of Energy to move high-temperature superconducting generator technology toward full commercialization. To read the latest news on superconducting generators click Here.

     Other commercial power projects in the works that employ superconductor technology include energy storage to enhance power stability. American Superconductor Corp. received an order from Alliant Energy in late March 2000 to install a Distributed Superconducting Magnetic Energy Storage System (D-SMES) in Wisconsin. Just one of these 6 D-SMES units has a power reserve of over 3 million watts, which can be retrieved whenever there is a need to stabilize line voltage during a disturbance in the power grid. AMSC has also installed more than 22 of its D-VAR systems to provide instantaneous reactive power support.

The General Atomics/Intermagnetics General superconductingFault Current Controller, employing HTS superconductors.

     Recently, power utilities have also begun to use superconductor-based transformers and "fault limiters". The Swiss-Swedish company ABB was the first to connect a superconducting transformer to a utility power network in March of 1997. ABB also recently announced the development of a 6.4MVA (mega-volt-ampere) fault current limiter - the most powerful in the world. This new generation of HTS superconducting fault limiters is being called upon due to their ability to respond in just thousandths of a second to limit tens of thousands of amperes of current. Advanced Ceramics Limited is another of several companies that makes BSCCO type fault limiters. Intermagnetics General recently completed tests on its largest (15kv class) power-utility-size fault limiter at a Southern California Edison (SCE) substation near Norwalk, California. And, both the US and Japan have plans to replace underground copper power cables with superconducting BSCCO cable-in-conduit cooled with liquid nitrogen. (See photo below.) By doing this, more current can be routed through existing cable tunnels. In one instance 250 pounds of superconducting wire replaced 18,000 pounds of vintage copper wire, making it over 7000% more space-efficient.

     An idealized application for superconductors is to employ them in the transmission of commercial power to cities. However, due to the high cost and impracticality of cooling miles of superconducting wire to cryogenic temperatures, this has only happened with short "test runs". In May of 2001 some 150,000 residents of Copenhagen, Denmark, began receiving their electricity through HTS (high-temperature superconducting) material. That cable was only 30 meters long, but proved adequate for testing purposes. In the summer of 2001 Pirelli completed installation of three 400-foot HTS cables for Detroit Edison at the Frisbie Substation capable of delivering 100 million watts of power. This marked the first time commercial power has been delivered to customers of a US power utility through superconducting wire. Intermagnetics General has announced that its IGC-SuperPower subsidiary has joined with BOC and Sumitomo Electric in a $26 million project to install an underground, HTS power cable in Albany, New York, in Niagara Mohawk Power Corporation's power grid. Sumitomo Electric's DI-BSCCO cable was employed in the first in-grid power cable demonstration project sponsored by the U.S. Department of Energy and New York Energy Research & Development Authority. After connecting to the grid successfully on July 2006, the DI-BSCCO cable has been supplying the power to approximately 70,000 households without any problems. The long-term test will be completed in the 2007-2008 timeframe.

Hypres Superconducting Microchip,

Incorporating 6000 Josephson Junctions.

      The National Science Foundation, along with NASA and DARPA and various universities, are currently researching "petaflop" computers. A petaflop is a thousand-trillion floating point operations per second. Today's fastest computers have reached "petaflop" speeds - quadrillions of operations per second. Currently the fastest is a U.S. Military Supercomputer call the "Road Runner" operating at 1.026 petaflops per second (with multiple CPU's). The fastest single processor is a Lenslet optical DSP running at 8 teraflops. It has been conjectured that devices on the order of 50 nanometers in size along with unconventional switching mechanisms, such as the Josephson junctions associated with superconductors, will be necessary to achieve the next level of processing speeds. TRW researchers (now Northrop Grumman) have quantified this further by predicting that 100 billion Josephson junctions on 4000 microprocessors will be necessary to reach 32 petabits per second. These Josephson junctions are incorporated into field-effect transistors which then become part of the logic circuits within the processors. Recently it was demonstrated at the Weizmann Institute in Israel that the tiny magnetic fields that penetrate Type 2 superconductors can be used for storing and retrieving digital information. It is, however, not a foregone conclusion that computers of the future will be built around superconducting devices. Competing technologies, such as quantum (DELTT) transistors, high-density molecule-scale processors , and DNA-based processing also have the potential to achieve petaflop benchmarks.

     In the electronics industry, ultra-high-performance filters are now being built. Since superconducting wire has near zero resistance, even at high frequencies, many more filter stages can be employed to achive a desired frequency response. This translates into an ability to pass desired frequencies and block undesirable frequencies in high-congestion rf (radio frequency) applications such as cellular telephone systems. ISCO International and Superconductor Technologies are companies currently offering such filters.

      Superconductors have also found widespread applications in the military. HTSC SQUIDS are being used by the U.S. NAVY to detect mines and submarines. And, significantly smaller motors are being built for NAVY ships using superconducting wire and "tape". In mid-July, 2001, American Superconductor unveiled a 5000-horsepower motor made with superconducting wire (below). An even larger 36.5MW HTS ship propulsion motor was delivered to the U.S. Navy in late 2006

      The newest application for HTS wire is in the degaussing of naval vessels. American Superconductor has announced the development of a superconducting degaussing cable. Degaussing of a ship's hull eliminates residual magnetic fields which might otherwise give away a ship's presence. In addition to reduced power requirements, HTS degaussing cable offers reduced size and weight.

      The military is also looking at using superconductive tape as a means of reducing the length of very low frequency antennas employed on submarines. Normally, the lower the frequency, the longer an antenna must be. However, inserting a coil of wire ahead of the antenna will make it function as if it were much longer. Unfortunately, this loading coil also increases system losses by adding the resistance in the coil's wire. Using superconductive materials can significantly reduce losses in this coil. The Electronic Materials and Devices Research Group at University of Birmingham (UK) is credited with creating the first superconducting microwave antenna. Applications engineers suggest that superconducting carbon nanotubes might be an ideal nano-antenna for high-gigahertz and terahertz frequencies, once a method of achieving zero "on tube" contact resistance is perfected.

      The most ignominious military use of superconductors may come with the deployment of "E-bombs". These are devices that make use of strong, superconductor-derived magnetic fields to create a fast, high-intensity electro-magnetic pulse (EMP) to disable an enemy's electronic equipment. Such a device saw its first use in wartime in March 2003 when US Forces attacked an Iraqi broadcast facility.

A photo of Comet 73P/Schwassmann-Wachmann 3, in the act of disintegrating ,

taken with the European Space Agency S-CAM.       Among emerging technologies are a stabilizing momentum wheel (gyroscope) for earth-orbiting satellites that employs the "flux-pinning" properties of imperfect superconductors to reduce friction to near zero. Superconducting x-ray detectors and ultra-fast, superconducting light detectors are being developed due to their inherent ability to detect extremely weak amounts of energy. Already Scientists at the European Space Agency (ESA) have developed what's being called the S-Cam, an optical camera of phenomenal sensitivity (see above photo). And, superconductors may even play a role in Internet communications soon. In late February, 2000, Irvine Sensors Corporation received a $1 million contract to research and develop a superconducting digital router for high-speed data communications up to 160 Ghz. Since Internet traffic is increasing exponentially, superconductor technology may be called upon to meet this super need. Irvine Sensors speculates this router may see use in facilitating Internet2.

      According to June 2002 estimates by the Conectus consortium, the worldwide market for superconductor products is projected to grow to near US $5 billion by the year 2010 and to US $38 billion by 2020. Low-temperature superconductors are expected to continue to play a dominant role in well-established fields such as MRI and scientific research, with high-temperature superconductors enabling the newer industries. The above ISIS graph gives a rough breakdown of the various markets in which superconductors are expected to make a contribution.

      All of this is, of course, contingent upon a linear growth rate. Should new superconductors with higher transition temperatures be discovered, growth and development in this exciting field could explode virtually overnight.

     Another impetus to the wider use of superconductors is political in nature. The reduction of green-house gas (GHG) emissions has becoming a topical issue due to the Kyoto Protocol which requires the European Union (EU) to reduce its emissions by 8% from 1990 levels by 2012. Physicists in Finland have calculated that the EU could reduce carbon dioxide emissions by up to 53 million tons if high-temperature superconductors were used in power plants.

      The future melding of superconductors into our daily lives will also depend to a great degree on advancements in the field of cryogenic cooling. New, high-efficiency magnetocaloric-effect compounds such as gadolinium-silicon-germanium are expected to enter the marketplace soon. Such materials should make possible compact, refrigeration units to facilitate additional HTS applications. Stay tuned !

June

Type 1 Superconductors

And a Periodic Chart Comparison

      The Type 1 category of superconductors is mainly comprised of metals and metalloids that show some conductivity at room temperature. They require incredible cold to slow down molecular vibrations sufficiently to facilitate unimpeded electron flow in accordance with what is known as BCS theory. BCS theory suggests that electrons team up in "Cooper pairs" in order to help each other overcome molecular obstacles - much like race cars on a track drafting each other in order to go faster. Scientists call this process phonon-mediated coupling because of the sound packets generated by the flexing of the crystal lattice.

      Type 1 superconductors - characterized as the "soft" superconductors - were discovered first and require the coldest temperatures to become superconductive. They exhibit a very sharp transition to a superconducting state (see above graph) and "perfect" diamagnetism - the ability to repel a magnetic field completely. Below is a list of known

Type 1 superconductors along with the critical transition temperature (known as Tc) below which each superconducts. The 3rd column gives the lattice structure of the solid that produced the noted Tc. Surprisingly, copper, silver and gold, three of the best metallic conductors, do not rank among the superconductive elements. Why is this ?

Lead (Pb) Lanthanum (La) Tantalum (Ta) Mercury (Hg) Tin (Sn) Indium (In) Palladium (Pd)* Chromium (Cr)* Thallium (Tl) Rhenium (Re) Protactinium (Pa) Thorium (Th) Aluminum (Al) Gallium (Ga) Molybdenum (Mo) Zinc (Zn) Osmium (Os) Zirconium (Zr) Americium (Am) Cadmium (Cd) Ruthenium (Ru) Titanium (Ti) Uranium (U) Hafnium (Hf) Iridium (Ir) Beryllium (Be) Tungsten (W) Platinum (Pt)* Lithium (Li) Rhodium (Rh)

7.196 K 4.88 K 4.47 K 4.15 K 3.72 K 3.41 K 3.3 K 3 K 2.38 K 1.697 K 1.40 K 1.38 K 1.175 K 1.083 K 0.915 K 0.85 K 0.66 K 0.61 K 0.60 K 0.517 K 0.49 K 0.40 K 0.20 K 0.128 K 0.1125 K 0.023 K  (SRM 768) 0.0154 K 0.0019 K 0.0004 K 0.000325 K

FCC HEX BCC RHL TET TET (see note 1) (see note 1) HEX HEX TET FCC FCC ORC BCC HEX HEX HEX HEX HEX HEX HEX ORC HEX FCC HEX BCC (see note 1) BCC FCC

*Note 1: Tc's given are for bulk (alpha form), except for Palladium, which has been irradiated with He+ ions, Chromium as a thin film, and Platinum as a compacted powder.

     Many additional elements can be coaxed into a superconductive state with the application of high pressure. For example, phosphorus appears to be the Type 1 element with the highest Tc. But, it requires compression pressures of 2.5 Mbar to reach a Tc of 14-22 K. The above list is for elements at normal (ambient) atmospheric pressure. See the periodic table below for all known elemental superconductors (including Niobium, Technetium and Vanadium which are technically Type 2).

**Note 2: Normally bulk carbon (amorphous, diamond, graphite, white) will not superconduct at any temperature. However, a Tc of 15K has been reported for elemental carbon when the atoms are configured as highly-aligned, single-walled nanotubes. And non-aligned, multi-walled nanotubes have shown superconductivity near 12K. Since the penetration depth is much larger than the coherence length, nanotubes would be characterized as "Type 2" superconductors.

***Note 3: For a list of elements that are naturally diamagnetic, click HERE.

Author's Comment:  The information posted on this page was obtained from a variety of sources including, but not limited to, the CRC Handbook of Chemistry and Physics, the Technische Universität München, Reade Metals and Minerals Corp., industry news sources, and various private researchers. A special thanks to Professor Bertil Sundqvist, Department of Experimental Physics, Umea University, Sweden, also to Dr. Jeffery Tallon, Industrial Research Ltd., New Zealand, and to Dr. James S. Schilling, Department of Physics, Washington University.

A Type 2 Layered Cuprate

Type 2 Superconductors

     Except for the elements vanadium, technetium and niobium, the Type 2 category of superconductors is comprised of metallic compounds and alloys. The recently-discovered superconducting "perovskites" (metal-oxide ceramics that normally have a ratio of 2 metal atoms to every 3 oxygen atoms) belong to this Type 2 group. They achieve higher Tc's than Type 1 superconductors by a mechanism that is still not completely understood. Conventional wisdom holds that it relates to the planar layering within the crystalline structure (see above graphic). Although, other recent research suggests the holes of hypocharged oxygen in the charge reservoirs are responsible. (Holes are positively-charged vacancies within the lattice.) The superconducting cuprates (copper-oxides) have achieved astonishingly high Tc's when you consider that by 1985 known Tc's had only reached 23 Kelvin. To date, the highest Tc attained at ambient pressure for a material that will form stoichiometrically (by formula) has been 138 K. And the highest Tc overall is 233K for a material which does not form stoichiometrically (see below list). One theory predicts an upper limit of about 200 K for the layered cuprates (Vladimir Kresin, Phys. Reports 288, 347 - 1997). Others assert there is no limit. Either way, it is almost certain that other, more-synergistic compounds still await discovery among the high-temperature superconductors.

     The first superconducting Type 2 compound, an alloy of lead and bismuth, was fabricated in 1930 by W. de Haas and J. Voogd. But, was not recognized as such until later,

after the Meissner effect had been discovered. This new category of superconductors was identified by L.V. Shubnikov at the Kharkov Institute of Science and Technology in the Ukraine in 1936(1) when he found two distinct critical magnetic fields (known as Hc1 and Hc2) in PbTl2. The first of the oxide superconductors was created in 1973 by DuPont researcher Art Sleight when Ba(Pb,Bi)O3 was found to have a Tc of 13K. The superconducting oxocuprates followed in 1986.

     Type 2 superconductors - also known as the "hard" superconductors - differ from Type 1 in that their transition from a normal to a superconducting state is gradual across a region of "mixed state" behavior. Since a Type 2 will allow some penetration by an external magnetic field into its surface, this creates some rather novel mesoscopic phenomena like superconducting "stripes" and "flux-lattice vortices". While there are far too many to list in totality, some of the more interesting Type 2 superconductors are listed below by similarity and with descending Tc's. Where available, the lattice structure of the system is also noted.

Tl5Ba4Ca2Cu9Oy

     (As a 9212/2212C intergrowth.)

(Sn5In)Ba4Ca2Cu11Oy

     (As a B212/2212C intergrowth.)

(Sn5In)Ba4Ca2Cu10Oy

     (As a B212/1212C intergrowth.)

Sn6Ba4Ca2Cu10Oy

     (As a B212/1212C intergrowth.)

(Sn1.0Pb0.5In0.5)Ba4Tm6Cu8O22+

     (As a 1256/1212 intergrowth.)

(Sn1.0Pb0.5In0.5)Ba4Tm5Cu7O20+

     (As a 1245/1212 intergrowth.)

(Sn1.0Pb0.5In0.5)Ba4Tm4Cu6O18+

     (As a 1234/1212 intergrowth)

Sn3Ba4Ca2Cu7Oy

     (As a 5212/1212C intergrowth.)

 ~233 K 

 ~218 K 

 ~212 K 

 ~200 K 

 ~195 K 

 ~185 K  

 ~163 K  

 ~160 K  

(Hg0.8Tl0.2)Ba2Ca2Cu3O8.33 HgBa2Ca2Cu3O8

   138 K*  133-135 K

HgBa2Ca3Cu4O10+  HgBa2(Ca1-xSrx)Cu2O6+ HgBa2CuO4+ 

 125-126 K  123-125 K    94-98 K

 Lattice: TET

* Note: As a result of a topological "defect", Hg will also go into the Cu atomic sites. Thus, the volume fraction of the intended structure type is considerably less than 100%.

Tl2Ba2Ca2Cu3O10 (Tl1.6Hg0.4)Ba2Ca2Cu3O10+ TlBa2Ca2Cu3O9+ (TlSn)Ba4TmCaCu4Ox (Tl0.5Pb0.5)Sr2Ca2Cu3O9 Tl2Ba2CaCu2O6 TlBa2Ca3Cu4O11 TlBa2CaCu2O7+ Tl2Ba2CuO6 TlSnBa4Y2Cu4Ox

  127-128 K     126 K     123 K

  ~121 K   (Superconductors.ORG - 2005)   118-120 K     118 K     112 K     103 K      95 K      86 K    (Superconductors.ORG - 2007)

 

 Lattice: TET

Sn4Ba4(Tm2Ca)Cu7Ox

Sn2Ba2(Tm0.5Ca0.5)Cu3O8+

SnInBa4Tm3Cu5Ox

Sn3Ba4Tm3Cu6Ox

Sn3Ba8Ca4Cu11Ox

SnBa4Y2Cu5Ox

Sn4Ba4Tm2YCu7Ox

Sn4Ba4TmCaCu4Ox

Sn4Ba4Tm3Cu7Ox

Sn2Ba2(Y0.5Tm0.5)Cu3O8+

Sn3Ba4Y2Cu5Ox

SnInBa4Tm4Cu6Ox

Sn2Ba2(Sr0.5Y0.5)Cu3O8

Sn4Ba4Y3Cu7Ox

 ~127 K    (TmTm-Ca structure only) ~115 K   (Superconductors.ORG - 2005) ~113 K   (Superconductors.ORG - 2005)   109 K   (Superconductors.ORG - 2007)   109 K   (One-of-a-Kind Resonant - 2006)   107 K    (Superconductors.ORG - 2007)  ~104 K   (First Hi-Tc Reentrant - 2007)  ~100 K   (Superconductors.ORG - 2007)  ~98 K   (Superconductors.ORG - 2006)  ~96 K   (Superconductors.ORG - 2007)  ~91 K   (Superconductors.ORG - 2006)    87 K    (Superconductors.ORG - 2005)    86 K     (Aleksandrov, et al - 1989)  ~80 K    (Superconductors.ORG - 2005)

Bi1.6Pb0.6Sr2Ca2Sb0.1Cu3Oy Bi2Sr2Ca2Cu3O10

*** Bi2Sr2CaCu2O9

*** Bi2Sr2(Ca0.8Y0.2)Cu2O8 Bi2Sr2CaCu2O8

   115 K  (thick film on MgO substrate)    110 K     110 K    95-96K   91-92K

 Lattice: ORTH

*** Though not always listed as a component, a small amount of Lead (x=.2-.26) is often used with Bismuth compounds to help facilitate a higher-Tc crystalline phase.

(Ca1-xSrx)CuO2

YSrCa2Cu4O8+

(Ba,Sr)CuO2

BaSr2CaCu4O8+

(La,Sr)CuO2

 110 K   (Highest Tc quaternary compound) 101 K    (Superconductors.ORG - 2007)   90 K   90 K     (Superconductors.ORG - 2007)   42 K

*** The above 5 compounds are all "infinite layer".

Pb3Sr4Ca3Cu6Ox

Pb3Sr4Ca2Cu5O15+

(Pb1.5Sn1.5)Sr4Ca2Cu5O15+

Pb2Sr2(Ca, Y)Cu3O8

 106 K    (Superconductors.ORG - 2007) 101 K    (Superconductors.ORG - 2005) ~95 K    (Superconductors.ORG - 2006)   70 K   (Cava, et al - 1989)

AuBa2Ca3Cu4O11 

AuBa2(Y, Ca)Cu2O7 

AuBa2Ca2Cu3O9 

  99 K    (Kopnin, et al - 2001)  82 K  30 K

 Lattice: ORTH

YBa3Cu4Ox   (9223C structure)

(Y0.5Lu0.5)Ba2Cu3O7 (Y0.5Tm0.5)Ba2Cu3O7 Y3Ba5Cu8Ox (Y0.5Gd0.5)Ba2Cu3O7 Y2CaBa4Cu7O16 Y3Ba4Cu7O16 Y2Ba5Cu7Ox NdBa2Cu3O7 Y2Ba4Cu7O15 GdBa2Cu3O7 YBa2Cu3O7 TmBa2Cu3O7 YbBa2Cu3O7 YSr2Cu3O7

Lattice: TET

177 K   (Superconductors.ORG - 2009) 107 K    (Superconductors.ORG - 2005) 105 K    (Superconductors.ORG - 2005) 105 K    (Superconductors.ORG - 2008)   97 K    (Superconductors.ORG - 2005)   97 K  (YY-Ca phase)  (Superconductors.ORG - 2006)   96 K    (Superconductors.ORG - 2005)   96 K    (Superconductors.ORG - 2008)   96 K   95 K   94 K   92 K   (See above graphic)   90 K   89 K   62 K

 Comment: "1-2-3" superconductors actually have the 1212C structure. Thus, the formula for YBCO could be written CuBa2YCu2O7.

GaSr2(Ca0.5Tm0.5)Cu2O7 Ga2Sr4Y2CaCu5Ox Ga2Sr4Tm2CaCu5Ox La2Ba2CaCu5O9+

  (Sr,Ca)5Cu4O10 GaSr2(Ca, Y)Cu2O7 (In0.3Pb0.7)Sr2(Ca0.8Y0.2)Cu2Ox

 99 K    (Superconductors.ORG - 2006)  85 K    (Superconductors.ORG - 2006)  81 K    (Superconductors.ORG - 2006)  79 K   (Saurashtra Univ., Rajkot, India - 2002)  70 K  70 K  60 K

(La,Sr,Ca)3Cu2O6 La2CaCu2O6+ (Eu,Ce)2(Ba,Eu)2Cu3O10+ (La1.85Sr0.15)CuO4 SrNdCuO**** (La,Ba)2CuO4 (Nd,Sr,Ce)2CuO4 Pb2(Sr,La)2Cu2O6 (La1.85Ba.15)CuO4

 58 K  45 K  43 K  40 K  40 K  35-38 K  35 K  32 K  30 K   (First HTS ceramic SC discovered - 1986)

**** First ceramic superconductor discovered without a non-superconducting oxide layer.

Comment: All of the above are copper perovskites, even though their metal-to-oxygen ratios are not exactly 2-to-3. The best performers are those compounds that contain one or more of the electron-emitters BaO, SrO or CaO, along with a Period 6 heavy metal like Mercury, Thallium, Lead, Bismuth, or Gold.

 GdFeAsO1-x  (Ca,Sr,Ba)Fe2As2

 LiFeAs

  53.5 K  (Highest Tc iron-based compound)    38 K       18 K   

Comment: The above are members of the newly discovered iron pnictide family.

 MgB2  Ba0.6K0.4BiO3

  39 K  (Highest Tc Non-Fullerene Alloy)   30 K   (First 4th order phase compound)

 Nb3Ge  Nb3Si  Nb3Sn  Nb3Al  V3Si  Ta3Pb  V3Ga  Nb3Ga  V3In

 23.2 K  19 K  18.1 K  18 K  17.1 K  17 K  16.8 K  14.5 K  13.9 K

  Lattice: A15

  Comment: Among the binary alloys, these are some of the best performers; combining Group 5B metals in a ratio of 3-to-1 with 4A or 3A elements.

PuCoGa5  18.5 K    (First SC transuranic compound)

NbN  16.1 K

Comment: After NbTi (below) NbN is the most widely used low-temperature superconductor.

Nb0.6Ti0.4

MgCNi3  9.8 K      (First superconductive wire) 7-8 K      (First all-metal perovskite superconductor)

C Nb Tc V

 15 K      (as highly-aligned, single-walled nanotubes)  9.25 K  7.80 K  5.40 K

 Lattice: C=Fullerene, Nb=BCC, Tc=HEX, V=BCC

 Comment: These four are the only elemental Type 2 superconductors.

RuSr2(Gd,Eu,Sm)Cu2O8 ErNi2B2C YbPd2Sn UGe2 URhGe2 AuIn3

Tc ~58 K   (Ruthenium-oxocuprate) Tc  10.5 K    (Nickel-Borocarbide)

Tc ~2.5 K  (Heusler compound)

Tc ~1K    (Heavy fermion)

Tc ~1K             ( " )

Tc  50 uK

Comment: The above 6 compounds are all rare ferromagnetic superconductors.

Sr.08WO3 Tl.30WO3 Rb.27-.29WO3

  2-4 K     (Tungsten-bronze) 2.0-2.14 K          (")   1.98 K               (")

 Lattice: TET

(1.)  "History of Physics Research in Ukraine", by Oleksandr Bakai and Yurij Raniuk, Kharkov Institute of Science and Technology, 1993.

Author's Comment:  The Tc's noted on this page were obtained from a variety of sources including, but not limited to, the CRC Handbook of Chemistry and Physics, the N.I.S.T. database, Physica C, industry news sources, and various private researchers. In cases where there was a discrepancy between sources, the higher Tc or a range of Tc's has been listed. If you desire the Tc of a compound or alloy not listed on this page, feel free to write.

Atypical Superconductorsand the Future

      As if ceramic superconductors were not strange enough, even more mysterious superconducting systems have been discovered. One is based on compounds centered around the "Fullerene". The fullerene name comes from the late designer-author Buckminster Fuller. Fuller was the inventor of the geodesic dome, a structure with a soccer ball shape. The fullerene - also called a buckminsterfullerene or "buckyball" - exists on a molecular level when 60 carbon atoms join in a closed sphere. When doped with one or more alkali metals the fullerene becomes a "fulleride" and has produced Tc's ranging from 8 K for Na2Rb0.5Cs0.5C60 up to 40 K for Cs3C60. In 1993 researchers at the State University of New York at Buffalo reported Tc's between 60 K and 70 K for C-60 doped with the interhalogen compound ICl.

     Fullerenes, like ceramic superconductors, are a fairly recent discovery. In 1985, professors Robert F. Curl, Jr. and Richard E. Smalley of Rice University in Houston and Professor Sir Harold W. Kroto of the University of Sussex in Brighton, England, accidentally stumbled upon them. The discovery of superconducting alkali metal fullerides came in 1991 when Robert Haddon and Bell Labs announced that K3C60 had been found to superconduct at 18 K.

      Larger, non-spherical pure carbon fullerenes that will superconduct have only recently been discovered. In April of 2001, Chinese researchers at Hong Kong University found 1-dimensional superconductivity in single-walled carbon nanotubes at around 15 Kelvin. And in February 2006, Physicists in Japan showed non-aligned, multi-walled carbon nanotubes were superconductive at temperatures as high as 12 K. Silicon-based fullerides like Na2Ba6Si46 will also superconduct. However, they are structured as infinite networks, rather than discrete molecules. Fullerenes are technically part of a larger family of organic conductors which are described below.

(TMTSF)2PF6

The first organic superconductor discovered.

Courtesy Laboratoire de Physique des Solides

      "Organic" superconductors are part of the organic conductor family which includes: molecular salts, polymers and pure carbon systems (including carbon nanotubes and C60 compounds). The molecular salts within this family are large organic molecules that exhibit superconductive properties at very low temperatures. For this reason they are often referred to as "molecular" superconductors. Their existence was theorized in 1964 by Bill Little of Stanford University. But the first organic superconductor (TMTSF)2PF6 was not actually synthesized until 1980 by Danish researcher Klaus Bechgaard of the University of Copenhagen and French team members D. Jerome, A. Mazaud, and M. Ribault. About 50 organic superconductors have since been found with Tc's extending from 0.4 K to near 12 K (at ambient pressure). Since these Tc's are in the range of Type 1 superconductors, engineers have yet to find a practical application for them. However, their rather unusual properties have made them the focus of intense research. These properties include giant magnetoresistance, rapid oscillations, quantum hall effect, and more (similar to the behavior of InAs and InSb). In early 1997, it was, in fact (TMTSF)2PF6 that a research team at SUNY discovered could resist "quenching" up to a magnetic field strength of 6 tesla. Ordinarily, magnetic fields a fraction as strong will completely kill superconductivity in a material.

     Organic superconductors are composed of an electron donor (the planar organic molecule) and an electron acceptor (a non-organic anion). Below are a few more examples of organic superconductors.

(TMTSF)2ClO4

[tetramethyltetraselenafulvalene + acceptor]

(BETS)2GaCl4

[bis(ethylenedithio)tetraselenafulvalene + acceptor]

(BEDO-TTF)2ReO4H2O[bis(ethylenedioxy)tetrathiafulvalene + acceptor]

Courtesy: Jeremy Quallsquallsj@sonoma.edu, Science Program - CM/Tand Klause Bechgaard, Risoeklause.bechgaard@risoe.dk

NOTE: For more on organics, click here.

      Discovered in 1993 by Bob Cava (currently at Princeton University) and Bell Labs, "Borocarbides" are one of the least-understood superconductor systems of all. It has always been assumed that superconductors cannot be formed from ferromagnetic transition metals - like iron, cobalt or nickel. It's the equivalent of trying to mix oil and water. However, in some borocarbides there is a "soap" that acts to bring these adversaries together. The crystallographic sites for the magnetic ions are thought to be isolated from the conduction path. This allows the cooper pairs to detour around the magnetic ions. Further, when combined with an element that has unusual magnetic properties - like holmium - "reentrant" behavior can also be in evidence in some borocarbides. Below Tc, where it should remain superconductive, there is a discordant temperature at which the material retreats to a "normal", non-superconductive state (see above graphic).

      To date only one superconductor has been found that has zero resistance at a single temperature - the opposite of reentrant superconductivity. Click HERE to read more about this "resonant" superconductor.

      Not only the borocarbides recede from a superconductive state at extreme low temperatures. In the compounds HoMo6S8 (Chevrel) and ErRh4B4 superconductivity suddenly disappears at around 1K. Click here to read more about this phenomenon. As can be seen from some of the below examples, the first metal site in the molecule is always occupied by a rare earth atom.

YPd2B2C                    23 KLuNi2B2C                 16.6 KYNi2B2C                   15.5 KTmNi2B2C                 11 K   (resistance increases below Tc)ErNi2B2C                  10.5 K        (ferromagnetic)HoNi2B2C                  7.5 K      (see above graphic) Source: Dr. Harald SchmidtUniversity of Bayreuth, Germanyand Sandie DannLoughborough University, Leics, England NOTE: Other elements that exhibit ferromagnetism have been - by one means or another - integrated into superconducting compounds. See the below section on Heavy Fermions.

Flux lattices in UPt3

Courtesy Bell Labs

      The "Heavy Fermions" sound like a family of overweight circus performers. But, they are yet another example of atypical superconductors. Heavy fermions are compounds containing rare-earth elements such as Ce or Yb, or actinide elements such as U. Their (inner shell) conduction electrons often have effective masses (known as quasiparticle masses) several hundred times as great as that of  "normal" electrons, resulting in what's known as low "Fermi energy" (Ef). This makes them reluctant superconductors. Yet, at cryogenic temperatures, many of these materials are magnetically ordered, others show strong paramagnetic behavior, and some display superconductivity through a mechanism that quickly runs afoul of BCS theory. Research suggests cooper-pairing in the heavy fermion systems arises from the magnetic interactions of the electron spins (D-wave, P-wave, S-wave), rather than by lattice vibrations.

      The first observation of superconductivity in a heavy fermion system was made by E. Bucher, et al, in 1973 in the compound UBe13; but, at the time was attributed to precipitated uranium filaments. Superconductivity was not actually recognized, per se, in a heavy fermion compound until 1979 when Dr. Frank Steglich of the Max Planck Institute for Chemical Physics in Solids (Dresden, Germany) realized it was a bulk property in CeCu2Si2. Heavy fermion superconductivity has since been observed in over 20 Ce compounds.

      In April 2003 a heavy-fermion compound unambiguously exhibited the so-called "FFLO" state, where magnetism and superconductivity have a beneficial coexistence. The compound CeCoIn5 confirmed a theoretical model first put forth in 1964 by Fulde, Ferrell, Larkin, and Ovchinnikov (FFLO). Below are some heavy fermion compounds that will superconduct, along with their Tc's. As can be seen, their transition temperatures are in the range of Type 1 superconductors, which severely limits their use in practical applications.

CeCoIn5                        2.3 K   (first confirmed FFLO compound)UPd2Al3                         2 KPd2SnYb                       1.79 KURu2Si2                        1.2 KUNi2Al3                         1 KAl3Yb                           0.94 KUBe13                          0.87 KCeCo2                          0.84 KCePt3Si                        0.75 K   (first heavy fermion superconductor without a center of symmetry)UPt3                             0.48 K   (see above graphic)CeCu2Si2                     .1-.7 K

Note: UGe2 and URhGe2 exhibit simultaneous ferromagnetism and superconductivity. Read more about this phenomenon in the below section on Ruthenates.

Courtesy: Zbigniew Koziol, Amsterdam

Dr. A. de Visser, University of Amsterdam, The Netherlandsand Dr. Henri A. Radovan, National High Magnetic Field Laboratory, USA

Magnetic topography of Sr2RuO4

      In the mid 1990's, it was discovered that copper-oxygen planes are not the only superconducting facilitators within the layered perovskites. In 1994 physicists at IBM Zurich and at Hiroshima University collaborated to study the atomic planes of ruthenium-oxygen due to their similarity to copper-oxygen planes. Yoshiteru Maeno and colleagues found that the compound Sr2RuO4 exhibited superconductivity at 1.5 K. While this is an extremely cold Tc for a superconducting perovskite, it revealed a new area of potential among what are known as "Ruthenates". Shortly after that SrRuO and SrYRuO6 were also found to superconduct at similarly low temperatures.

      When the crystalline structure of some of these materials is broken apart, its surface becomes increasingly ferromagnetic at low temperatures. This phenomenon flies in the face of condensed matter theory. So much so, that researchers have characterized them as an analog to superfluid Helium-3. And, when copper is added to the mix, even stranger things happen. In June 1999 New Zealand researcher Dr. Jefferey Tallon and his German colleague Dr. Christian Bernard discovered a ruthenium-cuprate** whose bulk is both a superconductor and a magnet. Although it was not the first compound discovered that exhibits coexisting ferromagnetism and superconductivity, it's remarkably high Tc of 58 K makes it truly distinct in the world of superconductors. Unlike "normal" superconductors, this compound becomes diamagnetic at about one-half Tc.

     [**RuSr2(Gd,Eu,Sm)Cu2O8 or any parenthetical element partially substituted by Y.]

NOTE: To learn more about this material, click here.

Ba0.6K0.4BiO3

Courtesy University of Texas

     There are many phase transitions that matter goes through on its way to another state (e.g. ice changing to water requires a sudden increase in heat energy). Among the superconductors this is also the case at Tc, Hc and Jc. However, a superconductor has been discovered that exhibits no measurable change in its specific heat (the amount of energy required to increase its temperature by one degree) while going through up to 3 different "critical" magnetic fields. Ba0.6K0.4BiO3 seems to be the first material discovered that enters a "fourth order" phase transition (according to Ehrenfest's phase transition classification scheme) - something that has never before been observed in nature. Roy Goodrich of Louisiana State University and Donovan Hall of the National High Magnetic Field Laboratory announced this discovery in May, 1999. Not only is this an amazing anomaly in the field of superconductivity. It suggests that even higher order phases may exist in nature.

     While no one can predict what future discoveries will be made in the field of superconductivity, some recent developments in the tungsten-bronze system suggest a new vista may be emerging. In July of 1999 researchers Y. Tsabba and S. Reich of the Weizmann Institute in Israel reported possible superconductivity near 91 K in the sodium-doped tungsten-bronze Na0.05WO3. This would be the first known HTS that is not a cuprate. Most tungsten-bronze compounds that are known to superconduct have Tc's below 4K - making this a truly tantalizing find. To learn more, click here.

     Other categories of materials that theory suggests may produce superconductors are the higher silver fluorides and complex fluorides -- known as fluoroargentates. Fluoroargentates bear a strong similarity to oxocuprates, compounds that currently have the highest transition temperatures of all known superconductors. In October 2003 researchers Wojciech Grochala, Adrian Porch and Peter P. Edwards reported sudden drops in magnetic susceptibility within a large number of samples of Be-Ag-F. They attribute this to possible spherical regions of superconductivity - with a Tc up to 64 K - couched inside a ferromagnetic host. For more on this, click HERE.

     With few exceptions (e.g. polysulphur-nitrides), most polymers resist being coaxed into a superconductive state. However, some organic polymers exhibit electrical resistance many orders of magnitude lower than the best metallic conductors. And, they do this at room temperature! These ultraconductors™, materials such as oxidized atactic polypropylene (OAPP), do not have zero resistance. But, their enhanced conductivity at ambient temperatures and pressures may actually allow them to compete with superconductors in certain fields. Polypropylene, for example, is normally an insulator. In 1985, however, researchers at the Russian Academy of Sciences discovered that as an oxidized thin-film, polypropylene can have a conductivity 105 to 106 higher than the best refined metals. The Meissner effect - the classic criterion for superconductivity - cannot be observed, as the critical transition temperature appears to be above the point at which the polymer breaks down (>700K). However, strong (giant) diamagnetism has been confirmed. A primer on ultraconductors™ is available by clicking here.

Superconductor Terminology

and the Naming Scheme

Anneal: To heat and then slowly cool a material to reduce brittleness. Annealing of ceramic superconductors usually follows sintering and is done in an oxygen-rich atmosphere to restore oxygen lost during calcination. The oxygen content of a ceramic superconductor is critical. For example, YBCO with 6.4 atoms of oxygen will not superconduct. But YBCO with 6.5 atoms will. Click here to see a graphical representation of this.

Anti-ferromagnetism: A state of matter where adjacent ions in a material are aligned in opposite or "anti-parallel" arrays. Such materials display almost no response to an external magnetic field at low temperatures and only a weak attaction at higher temperatures. There is evidence that anti-ferromagnetism in the copper oxides plays a role in the formation of Cooper pairs and, thus, in facilitating a superconductive state in some compounds.

BCS Theory: The first widely-accepted theory to explain superconductivity put forth in 1957 by John Bardeen, Leon Cooper, and John Schreiffer. The theory asserts that, as electrons pass through a crystal lattice, the lattice deforms inward towards the electrons generating sound packets known as "phonons". These phonons produce a trough of positive charge in the area of deformation that assists subsequent electrons in passing through the same region in a process known as phonon-mediated coupling. This is analogous to rolling a bowling ball up the middle of a bed. 2 people, one lying on each side of the bed, will tend to roll toward the center of the bed, once the ball has created a depression in the mattress. And, a 2nd bowling ball, placed at the foot of the bed, will now, quite easily, roll toward the middle. For a more technical explanation click here.

Borocarbides: Superconducting borocarbides are compounds containing both boron and carbon in combination with rare-earth and transition elements; some of which exhibit the

unusual ability to return to a normal, non-superconductive state at temperatures below Tc. For more on this, click here.

BSCCO: An acronym for a ceramic superconductor system containing the elements Bismuth, Strontium, Calcium, Copper and Oxygen. Typically, a small amount of lead is also included in these compounds to promote the highest possible Tc. BSCCO has probably found the widest acceptance among high-Tc superconductor applications due to its unique properties. BSCCO compounds exhibit both an intrinsic Josephson effect and anisotropic (directional) behavior. You can view a list of BSCCO compounds on the "Type 2" page.

Ceramics: Ceramic superconductors are inorganic compounds formed by reacting a metal with oxygen, nitrogen, carbon or silicon. The best-known of these are the copper-perovskites. Ceramics are typically hard, brittle, heat-resistant materials formed by a process known as solid-state reaction.

Charge Reservoirs: In superconductors, charge reservoirs are the layers that may control the oxidation state of adjacent superconducting planes (even though they themselves are not superconducting). In the layered cuprates, these consist of copper-oxide chains.

Chevrel (phases): A class of molybdenum chalcogenides (compounds containing Group VI elements S, Se or Te along with molybdenum and a positively charged metal ion) - named for Roger Chevrel of the University of Rennes, whose research brought them to the attention of the scientific community in the early 1970's. Recently, the Superconductivity Group at the University of Durham (UK) reported a novel fabrication technique that increases Bc2 (upper critical field) in the chevrel PbMo6S8 from 50 T in bulk materials up to > 100 T. Click HERE to read a [technical] writeup on this (as a PDF file). Or click HERE to see a short list of some of these compounds alongside their Tc's.

Coherence Length: The size of a cooper pair - representing the shortest distance over which superconductivity can be established in a material. This is typically on the order of 1000Å; although it can be as small as 30Å in the copper oxides.

Cooper Pair: Two electrons that appear to "team up" in accordance with theory - BCS or other - despite the fact that they both have a negative charge and normally repel each other. (Named for Leon Cooper.) Below the superconducting transition temperature, paired electrons form a condensate - a macroscopically occupied single quantum state - which flows without resistance. However, since only a small fraction of the electrons are paired, the bulk does not qualify as being a "bose-einstein condensate". Click here to see an animation of a cooper pair.

DAC: An acronym for "diamond anvil cell". Often the Tc of a superconductor can be coaxed upward with the application of high pressure. The DAC is used to accomplish this in the laboratory. A DAC is composed of 2 specially-cut diamonds and a stainless steel gasket. The gasket goes between the diamonds and seals a small chamber in which a fluid is placed. Since neither the diamonds nor the liquid will compress, hydrostatic forces in

excess of a million atmospheres can be brought to bear on a sample suspended within the fluid. Click here to see a graphic of a DAC.

Diamagnetism: The ability of a material to repel a magnetic field. Many naturally-occurring substances (like water, wood and paraffin, and many of the elements) exhibit weak diamagnetism. Superconductors exhibit strong diamagnetism below Tc. In a few rare compounds, a material may become superconductive at a higher temperature than the point at which diamagnetism appears. But, as a rule, the onset of strong diamagnetism is one of the most reliable ways to ascertain when a material has become superconductive. To see a short movie of a magnet being levitated by a superconductor, click here.

D-Wave: A form of electron pairing in which the electrons travel together in orbits resembling a four-leaf clover. Wave functions help theoreticians describe (and predict) electron behavior. The d-wave models have gained substantial support recently over s-wave pairing as the mechanism by which high-temperature superconductivity might be explained. Click here to see a graphic.

Energy Gap: This is the energy required to break up a pair of electrons. According to BCS theory, the formula for determining the energy gap (in meV) is Eg=7/2 KTc. Where K = Boltzmann's constant (8.62e-5 eV/K). And where Tc is the critical transition temperature in Kelvin. Since electron-pairing is universally agreed to be the method by which superconductivity occurs, this is the amount of energy required to disrupt the superconducting state.

ESR: An acronym for "Electron Spin Resonance" (also EPR: Electron Paramagnetic Resonance). This is another mechanism by which superconductivity might be explained in some materials. Simply put, ESR is the response of electrons to electromagnetic radiation or magnetic fields at discrete frequencies. Electrons, as they move, create tiny magnetic moments. Nearby electrons are influenced either beneficially or adversely. When the moments are complementary, the electrons become paired and can help each other move through a crystal lattice.

Ferrite: Ferrites are ceramics with magnetic properties. They are included on this page because many of the same elements used in ferrites (e.g. Ba, Sr, Tm, O) are also key constituents in ceramic superconductors. This may be an important clue in understanding high-temperature superconductivity.

Ferromagnetism: A state wherein a material exhibits magnetization through the alignment of internal ions (neighboring magnetic moments). This contrasts with paramagnetism, which is temporary, much weaker and results from unpaired electrons.

Flux-Lattice: A configuration created when flux lines from a strong magnetic field try to penetrate the surface of a Type 2 superconductor. The tiny magnetic moments within each resulting vortex repel each other and a periodic lattice results as they array themselves in an orderly fashion.

Fluxon: The smallest magnetic flux (flux quantum) that exists in nature. Just as electrons are quantized charge, fluxons are a quantized flux. The term is used in association with vortices, which result from magnetic fields penetrating Type 2 superconductors in single fluxon quanta. Click here to view a hollographic depiction of waves of fluxons on the surface of superconducting Niobium.

Flux-Pinning: The phenomenon where a magnet's lines of force (called flux) become trapped or "pinned" inside a superconducting material. This pinning binds the superconductor to the magnet at a fixed distance. Flux-pinning is only possible when there are defects in the crystalline structure of the superconductor (usually resulting from grain boundaries or impurities). Flux-pinning is desirable in high-temperature ceramic superconductors in order to prevent "flux-creep", which can create a pseudo-resistance and depress Jc and Hc. Click here to see a superconductor suspended in air by flux-pinning.

Four-point Probe: The most common method of determining the Tc of a superconductor. Wires are attached to a material at four points with a conductive adhesive. Through two of these points a voltage is applied and, if the material is conductive, a current will flow. Then, if any resistance exists in the material, a voltage will appear across the other two points in accordance with Ohm's law (voltage equals current times resistance). When the material enters a superconductive state, its resistance drops to zero and no voltage appears across the second set of points. By using the four-point method, instead of just two points, resistance in the adhesive and wires can be ignored; as the second set of points do not themselves conduct any current and can, therefore, only reflect what voltage exists across the body of the material.

Hall Effect: When a magnetic field is applied perpendicularly to a thin metal film or semiconductor film that is conducting an electric current, a small voltage will appear perpendicular to the axis of both the film and the magnetic field. This voltage is proportional to the strength of the applied field. However, the output is typically not linear. The Hall resistance (the ratio of the Hall voltage to the current) changes in steps, pursuant to the laws of quantum mechanics. This is known as the Integral Quantum Hall Effect or just Quantum Hall Effect. Discovered in 1879, the Hall effect was named for its discoverer Edwin H. Hall, a graduate student at Johns Hopkins University.

Hc: The scientific notation representing the "critical field" or maximum magnetic field that a superconductor can endure before it is "quenched" and returns to a non-superconducting state. Usually a higher Tc also brings a higher Hc.

Heavy Fermions: Compounds containing the elements cerium, ytterbium or uranium; whose (inner shell) conduction electrons often have effective masses (called quasiparticle masses) several hundred times as great as that of a "free" (normal) electron mass. This gives them what's known as a low "Fermi energy" and makes them unlikely - and unusual - superconductors. Research suggests cooper-pairing in heavy fermion systems arises from the magnetic interactions of the electron spins.

Hole: A positively-charged vacancy within a crystal lattice resulting from the shortage of an electron in that region. Holes are typically induced by doping a material with an impurity. However, they can also be synthesized electronically with devices like the field-effect transistor (FET). Modern electronic devices rely heavily on holes (as p-type semiconductors) to function. There is evidence that the holes of hypocharged oxygen in charge-reservoirs are, in fact, what makes possible high-temperature superconductivity in the layered cuprates.

HTS: An acronym for "High-Temperature Superconductor" (or Superconductivity). There is no widely-accepted temperature that separates HTS from LTS (Low-Temperature Superconductors). However, all the superconductors known before the 1986 discovery of the superconducting oxocuprates would be classified LTS. The barium-lanthanum-cuprate fabricated by Müller and Bednorz, with a Tc of 30K, is generally considered to be the first HTS material. Certainly any compound that will superconduct above the boiling point of liquid nitrogen (77K) would be HTS.

Hysteresis (loop): Hysteresis, as it applies to a superconductor, relates to the dynamic response of a superconductor to a strong magnetic field impinged upon it. As the strength of a nearby magnetic field (H) increases, the critical transition temperature (Tc) of a superconductor will decrease. And, at some point superconductivity will completely disappear, as it becomes "quenched". However, as the magnetic field is gradually withdrawn, the superconductor may NOT immediately return to a superconductive state. Herein lies the hysteresis. The graph of H-vs-Tc is different retreating than it is advancing (creating a "loop" shape). This fact must be weighed carefully in high-current applications where the superconductor Hc may, even briefly, be exceeded; as significant power losses can result.

Infinite layer: Infinite layer compounds have no clear separation between molecules. Rather than electrostatic bonding between discrete molecules to form a bulk crystalline aggregate, all the atoms are bound together by covalent or co-ionic bonding to form the equivalent of one huge molecule.  (Ba,Sr)CuO2 and Na2Ba6Si46 are examples of "infinite layer" or "infinite network" superconductor compounds.

Isotope Effect: The influence atomic mass contributes to the critical transition temperature of a superconductor. For example, 203.4Hg has a Tc of 4.126K. While 198Hg has a Tc of 4.177K. Since both forms of mercury have the same lattice structure, this difference in Tc can be attributed solely to the difference in mass. To learn more, click here.

Jc: The scientific notation representing the "critical current density" or maximum current that a superconductor can carry. Also note that, as the current flowing through a superconductor increases, the Tc will usually decrease.

Josephson Effect (also DC Josephson Effect): A phenomenon named for Cambridge graduate student Brian Josephson, who predicted that electrons would "tunnel" through a narrow (<10 angstroms) non-superconducting region, even in the absence of an external voltage. In a normal conductor, electrical current only flows when there's a voltage

differential and contiguous electrical connection. It has been theorized that the Josephson Effect arises from the incoherent phase relationships between superconducting electrons in the two (separated) superconductors. The AC Josephson Effect is where the current flow oscillates as an external magnetic field impinged upon it increases beyond a critical value. [at a frequency of 2eV/h, where e is the electron charge, V is the voltage that appears, and h is Planck's constant] Sidebar: This oscillation frequency has, in fact, resulted in an upward revision of Planck's constant from 6.62559e-34 to 6.626196e-34.

Josephson Junction: A thin layer of insulating material sandwiched between 2 superconducting layers. Electrons "tunnel" through this non-superconducting region in what is known as the "Josephson effect" (see above). Sidebar: The standard volt is now defined as the voltage required to produce a frequency of 483,597.9 GHz in a Josephson Junction.

Kelvin: A scale of temperature measurement that starts at "absolute zero", the coldest theoretical temperature attainable. (Named for Lord William Thomson Kelvin.)

Meissner Effect: Exhibiting diamagnetic properties to the total exclusion of all magnetic fields. (Named for Walter Meissner.) This is a classic hallmark of superconductivity and can actually be used to levitate a strong rare-earth magnet. To see a movie of a magnet being levitated by a superconductor, click here.

Mott Transition: The Mott transition is the shift from an insulating to a metallic state in a material. The high- temperature copper oxides are composed of CuO2 planes that are separated from each other by ionic "blocking layers". Although it has one conduction electron (or hole) per Cu site, each CuO2 plane is originally insulating because of the large electron correlation. That behavior is typical of the Mott insulator state, in which all the conduction electrons are tied to the atomic sites. The superconducting state emerges when holes from the blocking layers dope the CuO2 layers in a way that alters the number of conduction electrons and triggers the Mott transition. Researchers believe that the strong antiferromagnetic correlation, which originates in the Mott-insulating CuO2 sheets and persists into the metallic state, could be a possible mechanism of high-temperature superconductivity. (courtesy Science Week)

Organics: Organic superconductors are a sub-class of organic conductors that include molecular salts, polymers and pure carbon systems (including carbon nanotubes and C60 compounds). They may also be referred to as "molecular" superconductors. They are typically large, carbon-based molecules of 20 or more atoms, consisting of a planar organic molecule and a non-organic anion. For a non-technical write-up on organics, click here. Or, for a more technical paper on this subject, click here.

Penetration Depth (also London Penetration Depth): This term relates to how deeply a magnetic field will penetrate the surface of a superconductor. An external magnetic field impinged upon a Type 2 superconductor will decay exponentially into the surface based on the paired electron density within the superconductor (only a small fraction of the electrons are in a superconductive state). The "London" name comes from brothers F. London and

H. London, who in 1935 created a theoretical model of superconductivity. For a more technical explanation and the actual formula to calculate penetration depth, click here.

Perovskites: A large family of crystalline ceramics that derive their name from a mineral known as a perovskite. They are the most abundant minerals on earth and have a metal-to-oxygen ratio of approximately 2-to-3. Copper-oxide superconductors are layered perovskites. The perovskite name comes from Russian mineralogist Count Lev Aleksevich von Perovski.

Phase-Slip (also Quantum Phase-Slip): A point where a material in a superconductive state spontaneously changes from one state to another, generating a topological "defect". This defect causes paired electrons to become "out of step" with each other, producing a voltage and, ergo, non-zero electrical resistance. This phenomenon has been observed in ultra-thin wires less than a few tens-of-nanometers in diameter. Though bulk superconductivity may persist (T<Tc), one consequence of phase slip is a lower current-carrying state. A similar phenomenon occurs in Josephson Junctions.

Planar Weight Disparity (PWD): A term referring to the method by which Tc can often be increased by adjusting the relative weights of alternating layers in copper-oxide superconductors. The greatest improvements usually occur when making the insulating layers heavy/light OR the Cu-O2 planes heavy/light - but not both. Click HERE to read more about this discovery.

Proximity Effect: The phenomenon where a thin film of non-superconductive material in close proximity with a superconductor takes on superconductive properties. The Josephson junction is a device that takes advantage of this phenomenon. The Inverse Proximity Effect is where just the opposite occurs. A non-superconductive metal can enhance the Tc of an adjacent superconductor. This inverse effect has been observed with silver and lead.

P-Wave: A rare form of electron pairing in which two electrons travel together in spherical orbits; with both having the same direction of rotation. (See "D-Wave" explanation above.)

Quasiparticle: A bare particle that is "dressed" or "clothed" by a cloud of other surrounding particles. Quasiparticles behave similarly to bare (normal) particles, but usually have a larger effective mass due to this cloud moderating interactions with other particles.

Quench: The phenomenon where superconductivity in a material is suppressed; usually by exceeding the maximum current the material can conduct (Jc) or the maximum magnetic field it can withstand (Hc).

Re-entrant (behavior): A condition where a material retreats from its superconductive state and then re-enters it. This can be caused by a strong external magnetic field that dynamically exceeds the Hc of the material and/or is mis-aligned (in the case of some organic superconductors), a discordant temperature below Tc (in the case of some

borocarbides), or by Jc hysteresis (momentarily exceeding the critical current density, causing the Tc to shift downward).

Resistance: The opposition of a material to the flow of electrical current through it. Energy lost due to resistance is a result of vibrations at the molecular level and manifests itself as heat in proportion to the square of the current flow. In a superconductor all resistance disappears below a certain temperature. However, this applies only to direct current (DC) electricity. Other types of losses result when transporting alternating current (AC). Examples of this include hysteresis, reactive-coupling and radiational losses. In the new high-temperature ceramic superconductors, the power loss in applications like transmission lines is inversely proportional to the critical current density for low magnetic field applications. This limitation can be compensated for to some degree by increasing the ratio of voltage to current. In Type 2 superconductors carrying high-frequency alternating current, "skin effect" losses also result as the energy tends to migrate to the surface where the conductive medium is incontiguous, producing a pseudo-resistance. In some materials the amount of resistance may also depend on the direction of current flow (anisotropic resistivity) and/or presence of an external magnetic field (hall effect).

Room-temperature Superconductor: There are NO confirmed room-temperature superconductors (as was once reported for lithium-beryllium-hydride and for lead-silver-carbonate). However, it has been theorized that a metallic form of hydrogen might be a room-temperature superconductor. In 1996 physicists at Lawrence Livermore Laboratory were able to briefly create metallic hydrogen. But, its existence was fleeting and no measurement of the Meissner effect was possible. Zero resistance has been observed at room temperatures in ballistic quantum wire. However, having one-dimensional geometry, this wire does not exhibit the Meissner effect, except when configured as a closed loop.

Sinter: The process of heating a material to just below its melting point. An extended period of sintering is the method by which the constituent components of a ceramic superconductor are combined in a solid-state reaction. Since ceramic superconductors are inherently brittle, sintering helps promote intergranular bonding and hardness.

SQUID: A superconducting loop interrupted in 2 places by Josephson junctions. When sufficient electrical current is conducted across the squid body, a voltage is generated proportional to the strength of any nearby magnetic field. The SQUID, an acronym for Superconducting QUantum Interference Device, is the most sensitive detector known to science. Click here to see a graphic.

Stripes: Stripes are microscopic rivers of charge that flow across the surface of a Type 2 superconductor. It is theorized that stripes encourage "holes" to pair up and, as such, may play a role in facilitating charge transfer. Recently, at the Stripes 2000 conference in Rome, Italy, it was shown that there exists a critical value of micro-strain that must be exerted upon the CuO2 planes for stripes to form. Click here to learn more.

Superconductor: An element, inter-metallic alloy, or compound that will conduct electricity without resistance below a certain temperature. However, this applies only to direct

current (DC) electricity and to finite amounts of current. All known superconductors are solids. None are gases or liquids. And all require extreme cold to enter a superconductive state. Once set in motion, current will flow forever in a closed loop of superconducting material - making it the closest thing to perpetual motion in nature. Scientists refer to superconductivity as a "macroscopic quantum phenomenon". In addition to being classified Type 1 and Type 2, superconductors can be categorized further by their dimensionality. Most are 3-D. But some compounds, like surface-doped NaWO3 and some organic superconductors are 2-D. Li2CuO2 and single-walled carbon nano-tubes have shown rare 1-D superconductivity. In addition to repelling magnetic fields, enhanced thermal conductivity, higher optical reflectivity and reduced surface friction are also properties of superconductors. The term "superconductor" is also used in some instances to refer to materials that have near infinite thermal conductivity - such as carbon nanotubes. However, on this website it is used in the context of electrical conductivity only.

Susceptibility: A measure of the relative amount of induced magnetism in a material. Magnetic susceptibility is often used in lieu of resistance measurements to determine the transition temperature of a superconductor. Although, on occasion, the two techniques produce very different Tc's. In a typical superconductor, the (arbitrary) value of susceptibility will change from zero to a negative number as the temperature drops through Tc. However, in some materials it changes from positive to negative, as paramagnetism yields to diamagnetism.

S-Wave: A form of electron pairing in which the electrons travel together in spherical orbits, but in opposite directions. (See "D-Wave" explanation above.)

Tc: The scientific notation representing the critical transition temperature below which a material begins to superconduct. The sudden loss of resistance in a superconductive medium may occur across a range as small as 20 millionths of a degree or, in the case of some stoichiometrically imperfect compounds, tens of degrees. Click here to see a graphic example. ("Tc" is not to be confused with the atomic symbol for Technetium.)

Thin Film (Deposition): A method of fabricating ceramic superconductors to more precisely control the growth of the crystalline structure to eliminate grain boundaries and achieve a desired Tc. This can involve Pulsed-Laser Deposition (PLD) or Pulsed-Electron Deposition (PED) of the material. A variation of this technique can be used to increase the Tc of a superconductor by growing it on a supporting material with a smaller interatomic spacing. The supporting material acts as a molecular "girdle" to compress the atomic lattice of the superconductor, thereby raising its transition temperature. Superconductive tape is made using thin film deposition technology.

Translational Symmetry: As it applies to superconductivity, translational symmetry is where the process of charge transfer is repeated exactly as the charge carriers (paired electrons) traverse the solid. In a normal conductor, latent heat continuously vibrates the atomic lattice, deflecting mobile free electrons and preventing "perfect" translational symmetry. In a superconductor this scattering tendency is overcome.

Tungsten-bronze: A nebulous term used to describe alkali metal tungstenates, vanadates, molybdates, titanates and niobates. The term was originally coined to describe NaxWO3 compounds; the crystals of which look much like the copper-tin alloy known as bronze. There have been reports of superconductivity as high as 91K for a surface-doped sodium tungsten-bronze. This material was the first high-temperature superconductor discovered that does not contain any copper.

Ultraconductor: Materials known as ultraconductors™ display room-temperature resistance many orders of magnitude lower than the best metallic conductors. Examples of these materials include oxidized atactic polypropylene (OAPP) and other polymers. Since ultraconductor™ is a colloquial term, these materials might better be described as "hyperconductors". The Meissner effect cannot be confirmed in them, but strong (giant) diamagnetism is in evidence. Some of them may actually find acceptance in high-current applications ahead of superconductors as a result of their low losses at ambient temperatures and pressures. (A primer on ultraconductors is available by clicking HERE.)

Undressing: The process by which a quasiparticle becomes more like a bare (normal) particle. It is theorized this may be a driving force behind superconductivity, as undressed electrons are significantly lighter and can, thus, conduct current more readily. To learn more, click here.

Unit Cell: A unit cell is the smallest assemblage of atoms, ions, or molecules in a solid, beyond which the structure repeats to form the 3-dimensional crystal lattice.

Vortices (plural of vortex): Swirling tubes of electrical current induced by an external magnetic field into the surface of a superconducting material that represent a topological singularity in the wavefunction. These are particularly evident in Type 2 superconductors during "mixed-state" behavior when the surface is just partially superconducting. Superconductivity is completely suppressed within these volcano-shaped structures. Recent research suggests that flux vortices may NOT possess quantum values (equal to multiples of Planck's constant divided by 2 times electron charge). But may instead have but a tiny fraction of the basic unit of magnetism. The movement of vortices can produce a pseudo-resistance and, as such, is undesirable. While superconductivity is a "macroscopic" phenomenon, vortices are a "mesoscopic" phenomenon. (See the graphic at the top of this page.)

YBCO: An acronym for a well-known ceramic superconductor composed of Yttrium, Barium, Copper and Oxygen. This was the first truly "high temperature" ceramic superconductor discovered; having a transition temperature well above the boiling point of liquid nitrogen - a commonly available coolant. Its actual molecular formula is YBa2Cu3O7, making it a "1-2-3" superconductor. YBCO compounds exhibit d-wave electron pairing. The patent for YBCO is held by Lucent Technologies. (You can view a list of the best-performing 1-2-3 compounds on the "Type 2" page.)

       With the discovery of the ceramic superconductors, came a need for a classification system to describe structure types. The cuprate superconductors all have blocks of conducting CuO2 planes, alternating with insulating, spacing and separating layers. This makes possible a systematic Naming Scheme that allows for identification and comparison. The scheme chosen uses four numbers. The first denotes the number of insulating layers between adjacent conducting blocks. The second represents the number of spacing layers between identical CuO2 blocks. The third gives the number of layers that separate adjacent CuO2

planes within the conducting block. And, the fourth is the number of CuO2planes within a conducting block.

      Using the TlBa2Ca2Cu3O9 molecule depicted at left as an example, there is 1 insulating TlO layer, 2 spacing BaO layers, 2 separating Ca layers, and 3 conducting CuO2 planes - making it a "1223" type.

      

       There are also occasionally letters of the alphabet in the 5th and 6th positions. Suffixes like "T*", "C", and "F" describe to researchers structure detail in the oxygen positions.

       To further complicate matters, several of the more popular structures have names that do NOT follow the 4-number scheme. For example, the compound Y1Ba2Cu3O7 is often referred to by the 3-number name "123". This delineates the number of metal atoms (the stoichiometry) - without regard to atom location - within the structure. Using the 4-number scheme, it would be classified a "1212".

     

               

15 October 2008Superconductors.ORG

Superconductors.ORG herein reports synthesis of the first 200K superconductor in conjunction with the discovery of a new superconductor system.

The 200K material is believed to have a B212/1212C intergrowth structure, where B=11 and C=copper chain. This structure is shown below left and has the chemical formula Sn6Ba4Ca2Cu10Oy. The general formula for this new family of superconductors is SnxBa4Ca2Cu(x+4)Oy. Within this new family, unit cells with 3 to 6 atoms of tin (x) have been found to superconduct, with 6 atoms of tin producing a new record high Tc near 201K.

This intergrowth is highly lopsided, as was the Sn-In-Pb-Tm cuprate that produced a 195K superconductor in July 2008. However, it achieves its very high transition temperature through application of planar weight disparity (PWD) in the insulating layers, instead of the CuO2 planes.

Above is a composite magnetization test, showing the Meissner transition occuring just above 202K. A flashing line has been added to illustrate the diamagnetic shift in slope below Tc. The quiescent noise is a result of zooming in on a small volume fraction.

Four resistance tests were averaged and four magnetization tests were averaged, producing a mean resistive Tc of 200.8 Kelvin and a magnetic Tc of 202.4 Kelvin.

       The first hint that a new superconductor system might exist came in 2006 when three unrelated proto-compounds showed a small resistive "blip" around 160 K. The only elements that were common to these three materials were Sn, Ba, Ca, Cu and O. Through

trial-and-error, that 160 K material was finally identified on October 4, 2008, as being Sn3Ba4Ca2Cu7Oy (see below left) with structure 5212/1212C (see below right). This was the first member of the new family of superconductors.

       Analysis of the 160 K compound, dubbed Sn-3427, indicated that it could be an analog to the Sn-In-Pb-Tm copper-oxide family that produced Tc's up to 195 K. This became the impetus to add more Sn/Cu layers to increase the unit cell size and attempt to reach 200 K. However, instead of stacking 5 thulium atoms (heavy) opposite a single thulium atom (light), 6 tin atoms (heavy) would be stacked opposite a single copper chain (light). This attempt proved successful as a 200 K minority phase resulted.

       It may be possible to increase Tc further by doping the molecule with indium (In) or by adding another Sn/Cu layer. However, the increase would only be slight. This is because the inclination of the curve representing Tc-v-PWD becomes nearly flat with 22 layers in the unit cell. This was seen in the Sn-In-Pb-Tm copper-oxide family and can be seen in the below results as well.

       Since the 200K material contains only inexpensive and non-toxic elements, a method of refinement to increase its volume fraction is now all that is required for it to become the first commercial superconductor capable of operating at dry ice temperatures.

       Synthesis of these materials was by the solid state reaction method. Stoichiometric amounts of the below precursors were mixed, pelletized and sintered for 11 hours at 890C. The pellet was then annealed for 10 hours at 500C in flowing O2.

SnO 99.9% (Alfa Aesar) 5.57 molesBaCuOx 99.9% (Alfa Aesar) 5.98 molesCaCO3 99.95% (Alfa Aesar) 1.38 molesCuO 99.995% (Alfa Aesar) 3.29 moles

     The magnetometer employed twin Honeywell SS94A1F Hall-effect sensors with a tandem sensitivity of 50 mv/gauss. The 4-point probe was bonded to the pellet with CW2400 silver epoxy and used 7 volts on the primary.

RESEARCH NOTE: The copper-oxides are strongly hygroscopic. All tests should be performed immediately after annealing.

E. Joe Eck© 2008 Superconductors.ORGPatent Pending #61/196,032All rights reserved.

High-temperature interface superconductivity between metallic and

insulating copper oxides.  

Journal: Nature

Authors: Gozar A, Logvenov G, Kourkoutis LF, Bollinger AT, Giannuzzi LA, Muller DA, Bozovic I

Published: 2008 Oct 9;455(7214):782-5

Pubmed ID: 18843365

The realization of high-transition-temperature (high-T(c)) superconductivity confined to nanometre-sized interfaces

has been a long-standing goal because of potential applications and the opportunity to study quantum phenomena in

reduced dimensions. This has been, however, a challenging target: in conventional metals, the high electron density

restricts interface effects (such as carrier depletion or accumulation) to a region much narrower than the coherence

length, which is the scale necessary for superconductivity to occur. By contrast, in copper oxides the carrier density is

low whereas T(c) is high and the coherence length very short, which provides an opportunity-but at a price: the

interface must be atomically perfect. Here we report superconductivity in bilayers consisting of an insulator

(La(2)CuO(4)) and a metal (La(1.55)Sr(0.45)CuO(4)), neither of which is superconducting in isolation. In these

bilayers, T(c) is either approximately 15 K or approximately 30 K, depending on the layering sequence. This highly

robust phenomenon is confined within 2-3 nm of the interface. If such a bilayer is exposed to ozone, T(c) exceeds 50

K, and this enhanced superconductivity is also shown to originate from an interface layer about 1-2 unit cells thick.

Enhancement of T(c) in bilayer systems was observed previously but the essential role of the interface was not

recognized at the time.

Superconductor World Record Jumps to 233K

A 15 Degree Increase through Thallium Substitution

24 March 2009Superconductors.ORG

       40 degrees below zero is cold by any measure. But, in the world of superconductors it's a record hot day. Superconductors.ORG herein reports an increase in high-Tc to 233K (-40C, -40F) through the substitution of thallium into the tin/indium atomic sites of the X212/2212C structure that produced a 218 Kelvin superconductor in January of 2009.

       The host material producing the 233K signal has the chemical formula Tl5Ba4Ca2Cu9Oy. One of several resistance-v-temperature plots used to confirm this new record is shown above. And a composite magnetization test, showing the Meissner transition, is shown below right. A flashing line has been added to illustrate the

diamagnetic shift in slope below Tc. The quiescent noise is a result of zooming in on a small volume fraction.

This very high Tc material, like previous record discoveries, was designed to exploit planar weight disparity (PWD) along "c" axis of the 9212/2212C structure (shown at left). In this case a very heavy insulating layer in the upper part of the structure (Tl5Cu4) opposes a single copper chain in the lower part. Even though the copper chain branches, the planar weight ratio remains high - around 20:1. The structure that produced the previous 218K record material had a PWR just over 16:1.

       Weaker resistive transitions also appeared near 227K, 223K and 218K. The 227K step (shown above) most likely results from a 9212/1212C structure forming as a minority

phase; the 223K results from a 7212/2212C structure; and the 218K step from a 7212/1212C structure. Below 218K the other minority phases disappeared into the noise. As a result, a 1212C pellet with less thallium and copper was synthesized to discover the Tc of the 5212/1212C and 3212/1212C phases. In that pellet transitions appeared near 204K and 184K. All of these transitions were then plotted, producing the below Tc-v-PWR graph. The rate of slope declination suggests that 240K may be attainable.

        Since we are now just a "stone's throw" from room temperature, this discovery is being released into the public domain without patent protection. Other researchers are encouraged to examine this material and its structure.

       Synthesis of these materials was by the solid state reaction method. Stoichiometric amounts of the below precursors were mixed, pelletized and sintered for 34 hours at 865C. The pellet was then annealed for 10 hours at 500C in flowing O2.

Tl2O3 99.99% (Alfa Aesar) 7.136 moles (gr.)BaCuOx 99.9% (Alfa Aesar) 5.42 molesCaCO3 99.95% (Alfa Aesar) 1.25 molesCuO 99.995% (Alfa Aesar) 2.98 moles

     The magnetometer employed twin Honeywell SS94A1F Hall-effect sensors with a tandem sensitivity of 50 mv/gauss. The 4-point probe was bonded to the pellet with CW2400 silver epoxy and used 7 volts on the primary.

RESEARCH NOTE: The copper-oxides are strongly hygroscopic. All tests should be performed immediately after annealing.

E. Joe Eck© 2009 Superconductors.ORGAll rights reserved.

Coaxing YBCO Above 175K"Hyper YBCO" HY-134

3 March 2009Superconductors.ORG

Since its discovery in 1987 YBCO has established itself as the preeminent industrial superconductor. However, it has consistently resisted efforts to increase its critical transition temperature (Tc) much beyond about 105K.

Superconductors.ORG announces the discovery of a structure type that facilitates an improvement in the Tc of YBCO beyond 175K (see above R-T plot). And, while the volume fraction derived from the method of synthesis is low, the profound Tc improvement again validates planar weight disparity (PWD) as a robust Tc-enhancement mechanism.

92K YBCO (Y-123) has only 6 metal layers in the unit cell and very little PWD. In this new discovery - based on a 9223C theoretical structure type shown at left - there are 16 metal layers and a large amount of PWD. The closest analog to this structure type is the 9212/1212C intergrowth of the Sn-Ba-Ca-Cu-O family, with Tc ~195K.

The chemical formula of this new discovery - dubbed "Hyper YBCO" - is YBa3Cu4Ox. However, HY-134 does not form stoichiometrically. In order to synthesize a sufficent volume fraction to detect, the "layer cake" method must be used.

The layer cake used to produce the prototype pellet had 17 layers, 9 of (BaCuO) and 8 of (Y2O3 + CuO). This resulted in 16 interference regions in which the desired structure was encouraged to form. The layer cake method is depicted in the simplified graphic below.

       Below is a composite plot of three magnetization tests that were digitally summed to improve the signal-to-noise ratio. Weak diamagnetic transitions also appeared near 175K and 185K. These are believed to have resulted from minority phases: one less Ba-Cu layer for the 175K signal (7223C structure) and one more Ba-Cu layer for the 185K signal (B223C structure). These are the highest transition temperatures ever observed in a quaternary (four element) compound.

       Synthesis of HY-134 was by the solid state reaction method. Stoichiometric amounts of the below precursors were mixed, layered, pelletized at 70,000 psi and then sintered for 11 hours at 890C. The pellet was then annealed for 10 hours at 500C in flowing O2.

Y2O3 99.99% (Alfa Aesar) 2.15 moles (gr.)BaCuOx 99.9% (Alfa Aesar) 12.39 molesCuO 99.995% (Alfa Aesar) 1.515 moles

     The magnetometer employed twin Honeywell SS94A1F Hall-effect sensors with a tandem sensitivity of 50 mv/gauss. The 4-point probe was bonded to the pellet with CW2400 silver epoxy and used 7 volts on the primary.

RESEARCH NOTE: The copper-oxides are strongly hygroscopic. All tests should be performed immediately after annealing.

E. Joe Eck© 2009 Superconductors.ORGAll rights reserved.

          

218K Superconductor Signature ResolvedWorld Record Rises 80 Degrees Since May 2006

29 January 2009Superconductors.ORG

Signs of superconductivity near 218 Kelvin, reported by Superconductors.ORG as a minority phase on November 30, 2008, have now been confirmed. Resistive and diamagnetic transitions have been resolved using an established digital summing technique. Those results - typically 10db over noise - were then used to contrast samples of differing stoichiometries to identify the probable structure type.

The material producing the 218K signal appears to have the chemical formula (Sn5In)Ba4Ca2Cu11Oy. This conclusion was reached after comparing samples with 10 atoms of copper per unit cell to those with 11 atoms of copper per unit cell. In brief, those with 10 copper atoms produced a strong 212K signal, and a weak 218K. Those samples with 11 copper atoms produced a strong 218K signal, and a weak 212K, as seen in the below right plot.

        

An additional copper atom positioned into the 1212C chain of the B212/1212C structure (where B=11 and C=copper chain) causes a branching of the copper chain, creating a B212/2212C structure (shown at left). This extra copper atom increases the hole content of that insulating layer and further stiffens the lattice, raising Tc.

Even though the copper chain branches, the planar weight ratio remains high along the C axis. As with the 212K, 200K, 195K, and prior discoveries, planar weight disparity is the driving force behind these extraordinarily high Tc materials.

An analog to this increase in Tc exists in the difference between Y-123 (Tc~92K, shown below left) and Y-247 (Tc~95K, below right). Y-123 has a colinear C axis, while Y-247 branches to accommodate an extra copper atom.

The graph below shows the relationship between Tc and planar weight ratio. In the highest range of Tc's the curve rolls over and begins to flatten.

       Synthesis of these materials was by the solid state reaction method. Stoichiometric amounts of the below precursors were mixed, pelletized and sintered for 36-60 hours at 830C. The pellet was then annealed for 10 hours at 500C in flowing O2.

SnO 99.9% (Alfa Aesar) 4.64 molesIn2O3 99.9% (Alfa Aesar) 0.96 molesCaCO3 99.95% (Alfa Aesar) 1.38 molesBaCuOx 99.9% (Alfa Aesar) 5.98 molesCuO 99.995% (Alfa Aesar) 3.84 moles

     The magnetometer employed twin Honeywell SS94A1F Hall-effect sensors with a tandem sensitivity of 50 mv/gauss. The 4-point probe was bonded to the pellet with CW2400 silver epoxy and used 7 volts on the primary.

RESEARCH NOTE: The copper-oxides are strongly hygroscopic. All tests should be performed immediately after annealing.

E. Joe Eck© 2009 Superconductors.ORGAll rights reserved.

BCS theoryFrom Wikipedia, the free encyclopediaJump to: navigation, search

BCS theory is a microscopic theory of superconductivity, proposed by Bardeen, Cooper, and Schrieffer. It describes superconductivity as a microscopic effect caused by a condensation of pairs of electrons into a boson-like state.

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1     History     2     Overview    

o 2.1      More details     3     Successes of the BCS theory     4     See also     5     References     6     External links     7     Further reading    

[edit] History

The mid 1950s saw rapid progress in the understanding of superconductivity. It began in the 1948 paper On the Problem of the Molecular Theory of Superconductivity where Fritz London proposed that the phenomonological London equations may be consequences of the coherence of a quantum state. In 1953 Brian Pippard, motivated by penetration experiments, proposed that this would modify the London equations via a new scale parameter called the coherence length. John Bardeen then argued in the 1955 paper Theory of the Meissner Effect in Superconductors that such a modification naturally occurs in a theory with an energy gap. The key ingredient, which describes the field whose mass is equal to Bardeen's energy gap, was Leon Neil Cooper's calculation of the bound states of electrons subject to an attractive force in his 1956 paper Bound Electron Pairs in a Degenerate Fermi Gas.

In 1957 Bardeen and Cooper assembled these ingredients and constructed such a theory, the BCS theory, with Robert Schrieffer. The theory was first announced in February 1957 in the letter Microscopic theory of superconductivity. The demonstration that the phase transition is second order, that it reproduces the Meissner effect and the calculations of specific heats and penetration depths appeared in the July 1957 article Theory of superconductivity. They received the Nobel Prize in Physics in 1972 for this theory. The 1950 Landau-Ginzburg theory of superconductivity is not cited in either of the BCS papers.

In 1986, "high-temperature superconductivity" was discovered (i.e. superconductivity at temperatures considerably above the previous limit of about 30 K; up to about 130 K). It is believed that at these temperatures other effects are at play; these effects are not yet fully understood. (It is possible that these unknown effects also control superconductivity even at low temperatures for some materials).

[edit] Overview

In the BCS framework, superconductivity is a macroscopic effect which results from Bose condensation of electron pairs, called Cooper pairs. These behave as bosons which, at sufficiently low temperature, form a large Bose-Einstein condensate. At sufficiently low

temperatures, electrons near the Fermi surface become unstable against the formation of cooper pairs. Cooper showed such binding will occur in the presence of an attractive potential, no matter how weak. In conventional superconductors, such binding is generally attributed to an electron-lattice interaction. The BCS theory, however, requires only that the potential be attractive, regardless of its origin. Superconductivity was simultaneously explained by Nikolay Bogoliubov, by means of the so-called Bogoliubov transformations.

In many superconductors, the attractive interaction between electrons (necessary for pairing) is brought about indirectly by the interaction between the electrons and the vibrating crystal lattice (the phonons). Roughly speaking the picture is the following:

An electron moving through a conductor will attract nearby positive charges in the lattice. This deformation of the lattice causes another electron, with opposite "spin", to move into the region of higher positive charge density. The two electrons are then held together with a certain binding energy. If this binding energy is higher than the energy provided by kicks from oscillating atoms in the conductor (which is true at low temperatures), then the electron pair will stick together and resist all kicks, thus not experiencing resistance.

[edit] More details

BCS theory starts from the assumption that there is some attraction between electrons, which can overcome the Coulomb repulsion. In most materials (in low temperature superconductors), this attraction is brought about indirectly by the coupling of electrons to the crystal lattice (as explained above). However, the results of BCS theory do not depend on the origin of the attractive interaction. The original results of BCS (discussed below) described an "s-wave" superconducting state, which is the rule among low-temperature superconductors but is not realized in many "unconventional superconductors", such as the "d-wave" high-temperature superconductors. Extensions of BCS theory exist to describe these other cases, although they are insufficient to completely describe the observed features of high-temperature superconductivity.

BCS is able to give an approximation for the quantum-mechanical state of the system of (attractively interacting) electrons inside the metal. This state is now known as the "BCS state". In the normal state of a metal, electrons move independently, whereas in the BCS state, they are bound into "Cooper pairs" by the attractive interaction.

[edit] Successes of the BCS theory

BCS derived several important theoretical predictions that are independent of the details of the interaction, since the quantitative predictions mentioned below hold for any sufficiently weak attraction between the electrons and this last condition is fulfilled for many low temperature superconductors - the so-called "weak-coupling case". These have been confirmed in numerous experiments:

Since the electrons are bound into Cooper pairs, a finite amount of energy is needed to break these apart into two independent electrons. This means there is an "energy gap" for "single-particle excitation", unlike in the normal metal (where the state of an electron can be changed 

by adding an arbitrarily small amount of energy). This energy gap is highest at low temperatures but vanishes at the transition temperature when superconductivity ceases to exist. The BCS theory gives an expression that shows how the gap grows with the strength of the attractive interaction and the (normal phase) single particle density of states at the Fermi energy. Furthermore, it describes how the density of states is changed on entering the superconducting state, where there are no electronic states any more at the Fermi energy. The energy gap is most directly observed in tunneling experiments and in reflection of microwaves from the superconductor. 

BCS theory predicts the dependence of the value of the energy gap E on the critical temperature Tc for a superconductor at an arbitrary temperature T. At zero temperature the ratio between the value of the energy gap at zero temperature and the value of the superconducting transition temperature (expressed in energy units) takes the universal value of 3.5, independent of material. Near the critical temperature the relation asymptotes to 

which is of the form suggested the previous year by M. J. Buckingham in Very High Frequency Absorption in Superconductors based on the fact that the superconducting phase transition is second order, that the superconducting phase has a mass gap and on Blevins, Gordy and Fairbank's experimental results the previous year on the absorption of millimeter waves by superconducting tin.

Due to the energy gap, the specific heat of the superconductor is suppressed strongly (exponentially) at low temperatures, there being no thermal excitations left. However, before reaching the transition temperature, the specific heat of the superconductor becomes even higher than that of the normal conductor (measured immediately above the transition) and the ratio of these two values is found to be universally given by 2.5. 

BCS theory correctly predicts the Meissner effect, i.e. the expulsion of a magnetic field from the superconductor and the variation of the penetration depth (the extent of the screening currents flowing below the metal's surface) with temperature. This had been demonstrated experimentally by Walther Meissner and Robert Ochsenfeld in their 1933 article Ein neuer Effekt bei Eintritt der Supraleitfähigkeit. 

It also describes the variation of the critical magnetic field (above which the superconductor can no longer expel the field but becomes normal conducting) with temperature. BCS theory relates the value of the critical field at zero temperature to the value of the transition temperature and the density of states at the Fermi energy. 

In its simplest form, BCS gives the superconducting transition temperature in terms of the electron-phonon coupling potential and the Debye cutoff energy: 

The BCS theory reproduces the isotope effect, which is the experimental observation that for a given superconducting material, the critical temperature is inversely proportional to the mass of the isotope used in the material. The isotope effect was reported by two groups on the 24th of March 1950, who discovered it independently working with different mercury isotopes, although a few days before publication they learned of each other's results at the ONR conference in Atlanta, Georgia. The two groups are Emanuel Maxwell, who published his results in Isotope Effect in the Superconductivity of Mercury and C. A. Reynolds, B. Serin, W. H. Wright, and L. B. Nesbitt who published their results 10 pages later in Superconductivity of Isotopes of Mercury. The choice of isotope ordinarily has little effect on the electrical properties of a material, but does affect the frequency of lattice vibrations, this effect suggested that superconductivity be related to vibrations of the lattice. This is incorporated into the BCS theory, where lattice vibrations yield the binding energy of electrons in a Cooper pair. 

Type I superconductorFrom Wikipedia, the free encyclopedia  (Redirected from Type-I superconductor)

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Type I superconductors are superconductors that cannot be penetrated by magnetic flux lines (Meißner-Ochsenfeld effect). As such, they have only a single critical temperature at which the material ceases to superconduct, becoming resistive. The origin of their superconductivity is fully explained by BCS theory. Elementary superconductors, such as aluminium and lead are typical Type I superconductors.

[edit] Type I Superconductivity

As explained by BCS theory, type I superconductivity is exhibited by materials with a regularly structured lattice. This allows electrons to be coupled over a relatively large distance (compared to the size of an atom). These pairings are called Cooper pairs.

Though normally electrons exhibit Coulomb repulsion when displayed to each other, when they interact within a lattice, via a phonon interaction they display an attractive force. As this is not a normal state for an electron pair to be in, it is only achieved at very low temperatures. This is because, modeling it as a bond, it has a low bond energy, thus requiring very little force to break it. As such, if the temperature of the material is too high, the energy in the vibrations of the lattice are sufficient to break the bond.

The reason for this force of attraction is an effect known as 'the mattress effect'.[1] This comes about due to spins of electrons. As an electron passes through a lattice, the attractive forces between it and the protons in the nuclei of the atoms cause a ripple in the lattice structure. This means that there are ripples through much of the lattice. These ripples are vibrations called 'phonons'. The ripples induced throughout the lattice will affect other electrons passing through it. This creates the weak link between two electrons being affected by one phonon. This coupling

means that even if one electron is presented with resistance the effect of the resistance is minimized as it is 'pulled along' by the other electron.

This type of superconductivity is normally exhibited by pure metals, e.g. aluminium, lead or mercury. (Alloys are typically of Type II.)

Type-II superconductorFrom Wikipedia, the free encyclopediaJump to: navigation, search

This article is in need of attention from an expert on the subject. WikiProject Physics or the Physics Portal may be able to help recruit one. (November 2008)

A Type-II superconductor is a superconductor characterised by its gradual transition from the superconducting to the normal state within an increasing magnetic field. Typically they superconduct at higher temperatures and magnetic fields than Type-I superconductors. This allows them to conduct higher currents.

Contents[hide]

1     Materials     2     Critical temperatures and critical fields     3     Mixed state     4     See also     5     References    

[edit] Materials

Type-II superconductors are usually made of metal alloys or complex oxide ceramics, whereas most pure metals are Type-I superconductors. All high temperature superconductors are Type-II superconductors, and (as of early 2008) are mostly complex copper oxide ceramics. While most pure metal or pure element superconductors are Type-I, Niobium, Vanadium, Technetium, Diamond and Silicon are pure element Type-II superconductors. Some metal alloy superconductors also exhibit Type-II behavior (eg. niobium-titanium, niobium-tin).

Other Type-II examples are the cuprate-perovskite ceramic materials which have achieved the highest temperatures to reach the superconducting state. These include La1.85Ba0.15CuO4, BSCCO, and YBCO (Yttrium-Barium-Copper-Oxide), which is famous as the first material to achieve superconductivity above the boiling point of liquid nitrogen, as well as the highest temperature

superconductor to date: mercury thallium barium calcium copper oxide (Hg12Tl3Ba30Ca30Cu45O125).

[edit] Critical temperatures and critical fields

In comparison to the (theoretically) sharp transition of a Type-I superconductor above the lower temperature Tc1, magnetic flux from external fields is no longer completely expelled, and the superconductor exists in a mixed state. Above the higher temperature Tc2, the superconductivity is completely destroyed, and the material exists in a normal state. Both of these temperatures are dependent on the strength of the applied field. It is more usual to consider a fixed temperature, in which case transition (flux penetration) occurs between critical field strengths Hc1 and Hc2 (the upper critical field).[1]

[edit] Mixed state

Ginzburg–Landau theory defines 2 parameters: The coherence length of a superconductor related to the mean free path of its charge carriers, and a penetration depth.

The earlier London penetration depth is the penetration distance of a weak magnetic field.

In a Type-II superconductor, the coherence length is smaller than the London penetration depth, meaning that magnetic flux lines can pierce the material at high enough external fields. This is known as the vortex state, as the flux lines run through narrow regions of non superconducting material, surrounded by vortices of supercurrents protecting the rest of the superconductor. The vortices can arrange themselves in a regular structure known as the vortex lattice, also named the Abrikosov vortex, after Alexei Alexeyevich Abrikosov, who was awarded the 2003 Nobel Prize in Physics for his pioneering contributions.[2]

List of superconductorsFrom Wikipedia, the free encyclopediaJump to: navigation, search

A table showing major parameters of major superconductors of simple structure (numerous metallic alloys are not shown). X:Y means material X doped with element Y, TC is the highest reported transition temperature in kelvins and HC is a critical magnetic field in teslas. "Non-metals" here refers to materials which are normally not considered as metals, but become superconducting upon heavy doping. "BCS" means whether or not the superconductivity is explained within the BCS theory.

Formula TC (K) HC (T) Type BCS References  

Metals

Al 1.20 0.01 I yes [1][2][3]

Cd 0.52 0.003 I yes [2][3]

Hf 0.165 I yes [2]

a-Hg 4.15 0.04 I yes [2][3]

b-Hg 3.95 0.04 I yes [2][3]

Ga 1.1 0.005 I yes [2][3]

In 3.4 0.03 I yes [2][3]

Ir 0.14 I yes [2]

a-La 4.9 I yes [2]

b-La 6.3 I yes [2]

Mo 0.92 I yes [2]

Nb 9.26 0.82 II yes [2][3]

Os 0.65 0.007 I yes [2]

Pb 7.19 0.08 I yes [2][3]

Re 2.4 0.03 I yes [2][4][3]

Ru 0.49 0.005 I yes [2][3]

Sn 3.72 0.03 I yes [2][3]

Ta 4.48 0.3 I yes [2][3]

Tc 7.46-11.2 0.04 II yes [2][3]

a-Th 1.37 0.013 I yes [2][3]

Ti 0.39 0.01 I yes [2][3]

Tl 2.39 0.02 I yes [2][3]

a-U 0.68 I yes [2]

V 5.03 1 II yes [2][3]

Zn 0.855 0.005 I yes [2][3]

Zr 0.55 0.014 I yes [2][3]

Non-metals

Ba8Si46 8.07 0.008 II yes [5]

C6Ca 11.5 0.95 II [6]

C6Li3Ca2 11.15 II [6]

C8K 0.14 II [6]

C8KHg 1.4 II [6]

C8Rb 0.025 II [6]

C6Sr 1.65 II [6]

C6Yb 6.5 II [6]

C60Cs2Rb 33 II yes [7]

C60K3 19.8 0.013 II yes [8][9]

C60RbX 28 II yes [10]

Diamond:B 11.4 4 II yes [11][12][13][14]

InN 3 II yes [15]

In2O3 3.3 ~3 II yes [16]

Si:B 0.4 0.4 II yes [17]

SiC:B 1.4 0.008 I yes [18]

SiC:Al 1.5 0.04 II yes [18]

Binary alloys

MgB2 39 74 II yes [19]

Nb3Al 18 II yes [2]

Nb3Ge 23.2 37 II yes [20]

NbO 1.38 II yes [21]

NbN 16 II yes [2]

Nb3Sn 18.3 30 II yes [22]

NbTi 10 15 II yes [2]

Superconducting materials

The sudden drop in electrical resistivity to zero of certain materials when cooled to low temperature is called superconductivity. Superconducting materials exhibit a phenomenon known as Meissner effect in which the magnetic flux is excluded from the material. Depending on the response of the material to the magnetic field the materials can be classified as Type I or Type II superconductors.

Superconducting Materials Processing and Characterization

(1) YBCO Coated Conductors

In large-scale applications of superconductors, the conductors are required not only to carry high critical current density at respectable magnetic fields, but also to possess high ductility, mechanical strength, and stability. To improve the transport properties of high-temperature superconductors, extensive experimental investigations have been carried out in developing highly textured thin films on a variety of substrates. The fabrication of YBa2Cu3Ox (YBCO) films by epitaxial deposition on rolling assisted biaxially textured substrates (RABiTS) has proved to be successful in the conductor development for large-scale applications. The RABiTS technique uses well-established, industrially scaleable, thermomechanical processes to impart a high degree of grain

texture to a base metal. Buffer layers are then deposited to yield chemically and structurally compatible surfaces. Epitaxial YBCO films are grown on such a surface,

resulting in a critical current density at 77 K on the order of 106 A/cm2. This approach has shown a promise for developing long conductors for industrial applications.

We have developed the so-called non-fluorine sol gel synthesis of YBCO on single crystal substrates including YSZ and LAO. The YBCO sol-gel solutions were deposited on both YSZ and LAO substrates via spin coating. The x-ray diffraction (XRD) analysis of the film on YSZ and LAO showed a well-textured, c-axis oriented grain structure. A similar XRD pattern indicated similar structures for the YBCO film on LAO. The oxygenated films on all substrates were characterized by four-probe measurement. Both films on YSZ and LAO exhibited sharp Tc values near 90K. Preliminary measurements on the transport critical current density were performed using the four-probe method using a 10-6 V/cm voltage criterion. The sample dimensions were 15 x 3 x 0.0004 mm3. The transport Jc has been determined as a function of temperature in zero-field for YBCO on LAO. At 77 K, the transport Jc of YBCO on LAO has reached a value above 106A/cm2 at 77K and zero magnetic fields.  However, this value is expected to improve as the processing parameters are optimized. For details, please see our papers from “The Latest.”

 (2) Development of Single Domain YBCO RF Cavities for Wireless Telecommunications

Microwave Cavity Filters are widely used in RF systems for high Q signal filtering. However, their characteristically bulky size at lower operating frequencies make them quite inconvenient to be used especially in the frequency range 1-4 GHz, which is the range of most of the present day commercial wireless applications. This study aims to enhance the usability of cavity filters in these lower frequency ranges by exploring a unique re-entrant structure for the cavity filter design. The study further explores the use of high temperature superconductors in this particular design for improving the performance of the filters by reducing the associated losses. After verifying the basic workability of the concept, a coupled re-entrant cavity filter design is designed with an equivalent high Q.

 (3) Suppression of ab-Plane Cracks by a c-axis uniaxial Pressure

A uniaxial c-axis pressure has been applied to YBa2Cu3Ox (YBCO) single domain during a prolonged oxygenation experiment up to 72 h. It has been found that, due to this c-axis pressure, the local tensile stress responsible for ab-plane crack formation can be compensated effectively. The optical microscopy experimental results indicate that there is a sharp contrast in microstructure between the samples with and without c-axis pressure. The ab-plane cracks in the sample with c-axis pressure appear to be suppressed. X-ray diffraction results show that, for a 72h-oxygen anneal, both samples with and without pressure are well oxygenated with equal orthorhombicity on the polished surfaces where the microstructures are studied. Also discussed is the mechanism of ab-plane crack suppression by the c-axis pressure.

　

Friendly link: http://www.superconductors.org

SUPERCONDUCTING MATERIALS

Superconductivity is the phenomenon wherein the electrical resistance of a metal disappears when the metal is cooled. Superconductivity occurs in a variety of metals, but only when they are cooled to extremely low temperatures, near absolute zero.

Ever since the discovery of superconductivity in 1911, researchers have sought to raise the temperature at which superconductivity occurs, because that would make possible exciting applications of resistance-free electricity long thought impractical in light of the cooling requirement - for example, supercomputers with speed and capacity several orders of magnitude beyond today's capabilities; frictionless superspeed trains levitated by magnetic force; transmission lines that suffer no power loss through resistance; and supermagnets that could significantly enhance the medical diagnostic technology of magnetic resonance imaging (MRI).

In 1986, researchers achieved a breakthrough by identifying a new class of high temperature superconducting (HTS) materials. The "high temperature" is high only in the relative sense; these oxide materials remain superconductive above 30 degrees Kelvin, which corresponds to 243 degrees below zero on the more familiar Centigrade scale. However, some HTS materials superconduct at temperatures more than 50 degrees higher than the initial applications of superconductivity. That makes them easier to cool and easier to use, opening up a wide range of applications to take advantage of the superconductor's special properties: zero resistance, magnetic field exclusion, low noise and extremely low power loss for high frequency electronics.

With the advent of HTS materials, superconductors have begun to emerge from the laboratory and appear in practical applications.

A pioneer in this explosively advancing technology is Superconducting Technologies, Inc. (STI), Santa Barbara, California. STI's basic products are high quality thin films, sold to manufacturers of communications systems and military systems, who use them for in-house fabrication and component development. STI circuits and components - resonators, filters, oscillators,

microwave mixers, etc. - for manufacturers who prefer to have the superconducting fabrication and assembly done by specialists. STI uses thallium, the highest temperature material for making HTS. Thallium remains superconductive at temperatures above 77 degrees Kelvin and can be cooled to working temperature by a liquid nitrogen system instead of the more difficult and more expensive helium method.

Above, HTS is being produced by a laser ablation system.

Above, an STI scientist displays the result, a three-inch wafer of HTS material.

Above is a representative application: microwave circuits used in radars to reduce interference; the dark brown strips are made of HTS material.

Above is a closed cycle stirling cooler used to cool HTS subsystems to 77 degrees Kelvin; it has a one-quart capacity and the same electricity as an ordinary light bulb.

In addition to producing films, circuits and components, STI provides a wide range of foundry services, allowing access to HTS technology by company engineers who have not had previous experience in HTS. A number of government agencies and several large companies are using STI's foundry facility and personnel expertise to design and develop HTS circuits and components for specific applications.

STI credits NASA with a technological assist in the company's climb to a leadership position in the development and commercialization of superconducting products. Founded in 1986, STI has worked with two NASA centers - Lewis Research Center and Jet Propulsion Laboratory (JPL) - and advanced its own technology by adapting to NASA requirements. Says Joseph M. Madden, manager of sales and applications, "In fabricating antenna circuits for Lewis and other devices for JPL, we refined our standard production recipe for circuits. This now enables us to use the same standard production recipe for our production work."