Transcript How does Superconductivity Work?

How does
Superconductivity
Work?
Thomas A. Maier
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 out
Energy Crisis Solved
This photo shows
the Meissner effect,
the expulsion of a
magnetic field from
a superconductor in
super conducting
state.
Image courtesy Argonne
National Laboratory
Superconductivity = Cooper
dance
Cooper Pair Flash Mob
T
c
Electrons move
independently
Resistance is due to
scattering
T
c
Electrons form “Cooper”
pairs
Cooper pairs are
synchronized and are not
affected by scattering
But negative charges repel!
e-
e-
So how can electron pairs
form?
Everything you wanted to know about
pair formation
… in low-temperature superconductors
e-
+
+
+ +
e-
+
+
e-
+ +
1. First electron deforms lattice of metal
ions (ions shift their position due to
Coulomb interaction)
2. First electron moves away
3. Second electron is attracted by lattice
deformation and moves for former
position of first electron
→ Interaction is local in
space,
but delayed in time
→ Tc << Debye frequency
Bardeen
HgTlBaCuO 1995
Cooper
Schrieffer
HgBaCaCuO 1993
140
TIBaCaCuO 1988
High temperature
non-BCS
BiSrCaCuO 1988
100
YBa2Cu3O7 1987
Low-temperature (conventional)
superconductivity is a solved problem
Liquid N2
60
▻
La2-xBaxCuO4 1986
MgB2 2001
20
Liquid
He
Nb
NbC
Pb V3Si
NbN
Hg
1920
BCS Theory
1940
Nb3Su
1960
Nb3Ge
1980
2000
Low
temperature
BCS
Bednorz
and Müller
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
?
Why do electrons pair up into
Cooper pairs in the hightemperature superconductors?
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
Interlayer
tunneling Marginal Fermi liquid
Anyon superconductivity
d-density
wave
van Hove singularities
Spin fluctuations
Small q phonons Resonating
Flux
phases
SO(5)
Spin
liquids
Alexei
Abrikosov
valence bonds
Excitons
Charge Stripes
Interlayer
fluctuations
Coulomb
Bipolarons
BCS/BEC
Kinetic energy
Spin crossover
Orbital
bags
Plasmons currents
Gossamer
superconductivity
Phil
Anderson
Tony
Leggett
Bob
Schrieffer
Anisotropic
phonons
Bob
Laughlin
Karl
Müller
Example of a failed theory
Phil W. Anderson
(1997),
Interlayer tunneling
mechanism
A.A. Tsvetkov et al., Nature 395,
360 (1998)
“In the high-temperature
superconductor Tl2Ba2CuO6
[measurements provide evidence
for] a discrepancy of at least an
order of magnitude with deductions
based on the ILT model.”
(Incomplete) list of theories for high-Tc
Interlayer
tunneling Marginal Fermi liquid
Anyon superconductivity
d-density
wave
van Hove singularities
Spin fluctuations
Small q phonons Resonating
Flux
phases
SO(5)
Spin
liquids
Alexei
Abrikosov
valence bonds
Excitons
Charge Stripes
Interlayer
fluctuations
Coulomb
Bipolarons
BCS/BEC
Kinetic energy
Spin crossover
Orbital
bags
Plasmons currents
Gossamer
superconductivity
Phil
Anderson
Tony
Leggett
Bob
Schrieffer
Anisotropic
phonons
Bob
Laughlin
Karl
Electrons in the CuO2 layer
CuO2
layer
Antiferromagnetic CuO2 layer
Antiferromagnetic CuO2 layer
with doped holes
Antiferromagnetic CuO2 layer
with doped holes
Antiferromagnetic CuO2 layer
with doped holes
Antiferromagnetic CuO2 layer
with doped holes
Antiferromagnetic CuO2 layer
with doped holes
Hole motion creates “wake” in spin
density
Antiferromagnetic CuO2 layer
with doped holes
Second hole is attracted by wake in
spin density
Antiferromagnetic CuO2 layer
with doped holes
Second hole is attracted by wake in
spin density
Antiferromagnetic CuO2 layer
with doped holes
Second hole restores
antiferromagnetic structure when
paired with first hole
A successful theory should
explain the large differences in
transition temperatures
HgTlBaCuO 1995
HgBaCaCuO 1993
140
TIBaCaCuO 1988
High
temperature
non-BCS
BiSrCaCuO 1988
… or
even provide
a recipe for a
ROOMTEMPERATURE
SUPERCONDUCTOR!
100
YBa2Cu3O7 1987
Liquid N2
60
La2-xBaxCuO4 1986
MgB2 2001
20
Liquid
He
Nb
NbC
Nb3Ge
Pb V3Si
Nb
Su
3
NbN
Hg
1920
BCS Theory
1940
1960
1980
2000
Low
temperature
BCS
Bednorz
and Müller