SuperconductivitySuperconductivity

Allen M. GoldmanSchool of Physics and Astronomy

University of Minnesota

October 26, 2007

OUTLINE

•Introduction

• What is superconductivity? Phenomena • Mechanism

• Superconducting Materials

•Impacts of HTS and LTS • Challenges

Superconductivity is:Superconductivity is:Scientifically Challenging - offers new paradigms for our understanding

of solids & physics

Technologically Promising - may hold the key to the sustainable development of

the world

Traditional Path for IndustrializationTraditional Path for Industrialization

(Xu K. D., President, CAE)

GDP Per Capita

China?

(sustainable development is needed and HTS technology could play a key role)(sustainable development is needed and HTS technology could play a key role)

Resistance goes to zero at a characteristic temperature. This data is from Kammerlingh-Onnes in 1911, and shows the onset of super- conductivity in the element mercury. Data actually taken by Hulst.

Zero Resistance

Absolute Temperature Scale

0 Kelvin is -2730C

Applications: Power lines, magnets, motors, generators

What is Superconductivity?What is Superconductivity?

Flux ExclusionFlux Exclusion

Perfect conductor Superconductor

Superconductors exhibit nearly perfect diamagnetism below some characteristic field: Meissner-Ochsenfeld Effect-Type I Superconductors

LevitationLevitation

Levitation

Persistent Currents

Superconducting Magnets for MRI, accelerators, crystal growth, etc.

Flux QuantizationMagnetic Flux = Magnetic Field x Area

Flux is quantized in units of hc/2e

Planck’s constant: h

Electron charge: e

Velocity of Light: c

Vortex Lattice of a Type II SuperconductorVortex Lattice of a Type II Superconductor

For Type II superconductorsabove a certain value of mag-netic field, flux enters in the form of a lattice of quantizedvortices. Superconductivity canthen persist to extraordinarilyhigh magnetic fields.

Phase Diagram of a Type II SuperconductorPhase Diagram of a Type II Superconductor

TunnelingTunneling

Tunneling:- electrons transit an insulator that is sufficiently thin. This is an important quan- tum mechanical effect.

For sufficiently thin barriers have the Josephson Effect - the barrier has zero resistance

At nonzero voltage, V, the structure radiates:

frequency = 2eV/ h

Applications: detectors for Astronomy, high- performance computers, magnetometers.

SQUID (SuperconductingSQUID (SuperconductingQuantum Interference Device)Quantum Interference Device)

Detector of magnetic field that is based on the quantum properties of superconductors

Sensitivity: 10-12 to 10-14

of the earth’s magnetic field.

Prospecting for metallic oresMagnetoencephalographyMagnetocardiographyLow-field MRIMagnetic measurements

MechanismMechanism

The transition is an example of “spontaneousglobal symmetry breaking.”

Repulsive interaction of like charges is overcomeby “over-screening.”

Electrons pair up--Cooper Pairs. Theory: Bardeen, Cooper and Schrieffer (1957) Explains all “conventional” superconductors and

all of the exact properties discussed.

The Special Role of the BCS Theory in Science

Prediction and discovery of superfluidity in liquid 3He (Condensed Matter Physics)

Pairing in Nuclear Physics, explaining Nuclear Energy Levels (Nuclear Physics)

Neutron Stars are Superfluid, Pulsar rate glitches are evidence of vortices (Astronomy)

Pairing in Condensed Cold Atoms-Observation of Vortices(Atomic Physics)

Color Superconductivity in Compact Stars (Astronomy and Particle Physics)

The Higgs Particle (Particle Physics) (spontaneous symmetry breaking, mass)

EMERGENT RATHER THAN REDUCTIONIST

Examples of SuperconductorsExamples of Superconductors

Nonmagnetic metallic elements Binary and ternary compounds Organic compounds Alkali metal intercalated C60

Carbon Nanotubes Oxides

La-Ba-Cu-O

Bi-Sr-Ca-Cu-O Tl-Ba-Ca-Cu-O

Hg-Ba-Ca-Cu-O

Hg-Ba-Ca-Cu-O Hg-Ba-Ca-Cu-O##

#Under Pressure

La-Ba-Cu-O#

Y-Ba-Cu-O

La-Ba-Cu-O#

0

Year

1900 1920 1940 1960 1980 2000

Supe

rcon

duct

ing

Tran

sitio

n Te

mpe

ratu

re, T

C

20

40

60

80

100

120

140

160

180

(K)

Freon

Liquid Nitrogen

Liquid Helium

Nb3Ge

NbCV3Si Nb-Al-Ge

Nb3SnNbNNbPbHg

Ba-Ca-Cu-O

BaBa-Ca-Cu-O-Ca-Cu-O##

Mg-B Mg-B

?

Historic Evolution of Superconducting Transition Temperatures

LNG

From Paul Chu

Structures of High Temperature SuperconductorsStructures of High Temperature Superconductors

act

ive

bloc

k(C

uO )[

(R)(C

uO )]

22

n-1

char

ge r

eser

voir

blo

ck

(EO

)(A

O) (

EO

)m

CuO2

R

CuO2

R

CuO2

EO

AO

AO

EO

e.g. YBa2Cu3O7 = CuBa2YCu2O7

= Cu1212

Layered Structure AmE2Rn-1CunO2n+m+2 ⇒ [(EO)(AO)m(EO)] + (CuO2)[R(CuO2)]n-1

A – Bi, Tl, Pb,Cu…(AO)E – Ca, Sr, BaR – Ca, RE, (REO)

Am(n-1)n0m(n-1)n

Generic Crystal StructureGeneric Crystal Structure

Impacts of SuperconductivityImpacts of Superconductivity on S&T on S&T

Pre-High Temperature Superconductivity (SC) - Tc raised (to 23.2 K)

- new SC & non-SC compounds discovered.

- mystery of SC unraveled (BCS Theory)

- viability of SC technology demonstrated [MRI, Accelerators, SQUIDs,possible Quantum Computers and other high performance computers]

- new phenomena and physics discovered

- understanding non-SC phenomena improved [via interaction with SC]

- new material characterization and synthesis tools developed

Impacts (cont.)Impacts (cont.)

Post-High Temperature Superconductivity (HTS)

- Tc raised (to 134 K at ambient, 164 K under pressure)

- new SC & non-SC compounds discovered

- HTS prototypes constructed and demonstrated [cables, FCLs,Transformers, Filters, SQUIDs, MRI sensors]

- new phenomena and physics discovered

- understanding non-SC phenomena improved [via interaction with SC]

- new material characterization and synthesis tools developed also for non-SC

Current Status Current Status –– KnownKnown HTS - a new paradigm for condensed matter physics HTS - a new paradigm for condensed matter physics

More than 150 non-intermetallics withTc > 23 K (Cuprates, Bismuthates, and Fullerites, MgB2 )

All HTS with Tc > 77 K belong to Layered Cuprates

HgBa2Ca2Cu3O8+δ Has the Highest Tc = 134 K (at ambient), 164 K(at 30 GPa)

Hc2 ≥ 150 T (~ 3x106 Hearth)

Intriguing generic phase diagram and magnetic phase diagram

From Mike Norman

More structures - George Crabtree

Current Status - Unknown

No generally accepted microscopic theory after 20years!

HTS Considerations HTS Considerations for Sciencefor Science

Need intrinsic properties, but the materials arephysically intricate, chemically complex and chemically unstable

The grand challenge- higher Tc -- no new record since 1994!

HTS Considerations HTS Considerations for Technologyfor Technology

Need high performance at low cost andthe materials have to be in the proper forms for devices

-improved properties via processing & modification-cryogenic efficiency-tedious processing-auxiliary material compatibility-lifetime and reliability

A vision for the future!

Program at Minnesota

Tunneling and Transport in Ordered and Disordered Superconductors- Supported by NSF/DMR (quantum phase transitions)

Tunneling and Transport in One-Dimensional Systems-Supported by DOE Basic Energy Sciences (Luttinger Liquideffects, clusters)

Interfacial Phenomena in Superconducting Heterostructures-Supported by NSF/DMR and NSF/INT (The doping phasediagram, magnetic/superconducting structures)

US-India Collaborative Research -Supported by NSF/INT (penetration depth measurements)

University of Minnesota MRSEC-IRGs 2 and 3

Current Group: Sarwa Tan (postdoc), Yu Chen, Xiang Leng, MasayaNishioka, Alexey Kobrinskii, Yen-Hsiang Lin, Yeon Bae Lee

The Superconductor Plague-The Outsiders sold the Puppeteers the location of the Ringworld in 1733 a.d. Searching for a way to expand trade at no risk, the Experimentalists immediately sent robotic probes to determine the feasibility of an expedition.The Puppeteers mistook the City Builders to be the Ringworld's engineers and become suddenly afraid to encounter so powerful a race. The Puppeteers examined the Ringworld's superconductor material and created a technophytic bacterium to seed the Ringworld and destroy the superconductor. The strategy was to follow the probes with trading ships and come to the profitable rescue of the City Builders. However, the Puppeteers quickly realised that the City Builders could not possibly have created the Ringworld, and soon discovered several Pak artifacts. The Puppeteers feared the Pak above nearly all other threats.The Conservatives took control of the government and the Ringworld project was abandoned. The Experimentalists regained power under the threat of Kzinti expansion and decided that a manned expedition to the Ringworld might reveal treasures worth the risk (especially if they could convince others to take the risk for them). When the decision was made, the plague had reduced the Ringworld to barbarism, destroying thousands of years of civilization.