Why Do We Need Superconductors?
And How Are The Made?
Developed from the talks of: Dr. Athena S. Sefat
AndDr. Thomas Maier
This photo shows the Meissner
effect, the expulsion of a magnetic field
from a superconductor in super conducting
state.
Image courtesy Argonne National Laboratory
Why are we interested in Superconductors?
Power plants must increase their current to high voltages when transmitting it across country: to overcome energy lost due to resistance. Resistance is to electrons moving down a wire as rocks are in a stream. Imagine if we could find a way to remove resistance;
Energy in would equal Energy outEnergy Crisis Solved
.
Compact, powerful motors/generators!
Efficient/condensed underground transmission lines!
Why superconductors?
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The Facts• Copper wire can carry 5
Amps/mm2 • Superconductors conduct
electricity with significantly less resistance, but they must be cooled for the effect.
• NbTi can carry 2500 Amps/mm2
• A “low temperature” superconductor because it must be cooled to 4.2 Kelvin using liquid Helium (very expensive)
• YBCO can carry 10,000 Amps/mm2
• A “high temperature” superconductor because it must be cooled to 92 Kelvin using liquid Nitrogen (relatively inexpensive to produce)
Other PossibilitiesFrom
Superconductors
https://www.youtube.com/watch?v=Z4XEQVnIFmQ
- Element Properties are important: reactivity, toxicity, general properties
metal radius(pm) Tmelt(°C) Tboil (°C) D20°C(g/cm3)
Creating a Superconductor
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Ca 197 842 1494 1.55Sr 215 769 1382 2.63Ba 222 729 1805 3.59
Eutectic pts
Congruent reaction
Peritectic reaction
- Phase Diagrams Help to Target the Proper Elements
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** Tetragonal structures may be crucial
** ThCr2Si2-type structure may be important
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. . .
One can explore other Fe-based compounds with this structure:
- The Secret Appears to be in the Crystalline Structure
Fe-based superconductorCuprates
B. R. Pamplin, Crystal Growth, Pergamonpress
(1980).9 Managed by UT-Battelle
for the U.S. Department of Energy
- Preparation Methods are Critical
platinum alumina (Al2O3) zirconia (ZrO2)
1720 3452 ° F 177019002000
20722700
Tmax (°C) Tmelting (°C)
Alkali & alkaline-earth metals Al, Ga
Ta, steel Al2O3, MgO, BeO
MgCu, Ag,
Au Fe, Co, Ni Zn, Cd,
Hg InRare-earth metals
Bi, SnSb
MgO, Ta, graphite or steel graphite, MgO,
Al2O3, TaAl2O3, ZrO2
Al2O3
Al2O3, TaTa, Mo,
W, BeOAl2O3, SiO2, graphite
SiO2, graphite
Elements Container & tube choices
- Know how to confine reactions!
magnesia (MgO) tantalum
24001400
28523017
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borosilicate glass (Pyrex) 515 820gold 1013 1064silica (quartz) 1200 1853 3370 ° F
1111RFeAsO
TC = 56 K
111, 11AFeAs, FeSeTC ≈ 33, 15 K
Li or Fe
A or BR
O
Sr
T O
42622Sr4T2O6Fe2As2
TC ≈ 37 K
122
BFe2As2, AFe2Se2
TC ≈ 38 K, 45 K
La-214 La2CuO4
TC = 40 KHg-1223
HgBa2Ca2Cu3O9+δ
TC = 133 K
Ba
Y
Ba
Ca
Tl
Tl-2223Tl2Ba2Ca2Cu3O10
TC = 125 K
Y-123YBa2Cu3O7-δ
TC = 92 K
Ca
BaHg
La
FeCu
Superconducting Materials have Complex Structures
Sefat, et al. MRS Bulletin 36 (2011), 614.
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Normal Conductors
Superconductors
LaAs + x/3 Co3O4 + (3-4x)/9 Fe2O3 + (3-x)/9 Fe LaFe1-xCoxAsO
e.g. 1220 °C (2230 °F)!
Example:
Weigh accurately
Seal into a silica tubeHeat strongly
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fc
zfc
0
T (K)
χ (c
m3 /m
ol)
0T (K)
ρ (
mO
hm c
m)
A research workflow!
wires
mm to cm
Our Superconductivity Research
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(a) Decision on a crystal structure
(b) Synthesis
(c) Characterization
The Science of Low Temperature
Superconductors
Low Temperature Superconductivity
Tc
At Normal TemperaturesElectrons in Conductors move independentlyResistance within the conductor is due to the scattering.
Superconductivity is enabled by the Cooper
dance
Cooper Pair Flash Mob
Superconductivity = Cooper dance
Tc
Electrons form “Cooper” pairs. Cooper pairs synchronize electrons eliminating the scattering affect and removing resistance.
But negative charges repel!
e- e-
So how can electron pairs form?
+e-
Everything you wanted to know about pair formation
… in low-temperature superconductors
++
+
e- +
++
+
+
++
+
e-
e-
1. A lattice structure is formed as a part of the creation of the superconducting material.
2. The first electron deforms the lattice of metal ions (ions shift their position due to Coulomb interaction)
3. First electron moves away4. Second electron is
attracted by lattice deformation and moves for former position of first electron
Duplicating a Nobel Prize Winning Experiment
Low-temperature (conventional) superconductivity is a solved problem
▻ We know that ion vibrations cause the electrons to pair
High-temperature superconductivity is an unsolved problem▻ We know that ion vibrations play no
role in superconductivity
▻ We don’t know (agree) what causes the electrons to pair
Bardeen
Cooper
Schrieffer
140
100
60
20
LiquidHe
Tem
pera
ture
[K]
1920 1960 19801940 2000
Nb3Ge
MgB2 2001
Nb3SuNbN
Nb
HgPb
NbCV3Si
Low temperature BCS
BCS Theory
Bednorzand Müller
TIBaCaCuO 1988
HgBaCaCuO 1993
HgTlBaCuO 1995
BiSrCaCuO 1988
La2-xBaxCuO4 1986
YBa2Cu3O7 1987
High temperaturenon-BCS
Liquid N2
?
The Science of Low Temperature
Superconductors is a SOLVED Problem
High Temperature Superconductors are still
Mysterious.
Complexity in high-temperature superconducting cuprates
Low-temperature superconductors behave like normal metals above the transition (to superconductor) temperature
High-temperature superconductors display very strange behavior in their normal state▻ Stripes▻ Charge density waves▻ Spin density waves▻ Inhomogeneities▻ Nematic behavior▻…Many theories have been proposed,most of them are refuted by experiments
“If one looks hard enough, one can find in the cuprates something that is reminiscent of almost any interesting phenomenon in solid state physics.” (Kivelson & Yao, Nature Mat. ’08)
(Incomplete) list of theories for high-Tc
Resonating valence bonds
Spin fluctuations
Stripes
Small q phonons
Anisotropic phonons
Bipolarons
Excitons
Kinetic energy
d-density wave
Orbital currents
Flux phases
Charge fluctuations
Spin bags
Gossamer superconductivity
SO(5)
BCS/BEC crossoverPlasmon
s
Spin liquids
Interlayer Coulomb
Interlayer tunneling
van Hove singularities
Marginal Fermi liquid
Anyon superconductivity
Phil Anderso
n Alexei Abrikosov
Bob Schrieffe
r
Tony Leggett
Bob Laughlin Karl
Müller
If we can solve Room Temperature
Superconductors• We can generate power from Solar Energy in the Deserts and use it anywhere.• We can trim our energy budget while increasing our thirst for energy.• Energy generation methods that are not viable become viable.