how does superconductivity work?
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How does Superconductivity Work?. Thomas A. Maier. Why are we interested in Superconductors?. - PowerPoint PPT PresentationTRANSCRIPT
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 outEnergy 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
Tc
Electrons move independentlyResistance is due to scattering
Tc
Electrons form “Cooper” pairsCooper pairs are synchronized and are not affected by scattering
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- +
++
+
→ Interaction is local in space, but delayed in time
→ Tc << Debye frequency
+
++
+
e-
e-
1. First electron deforms lattice of metal ions (ions shift their position due to Coulomb interaction)
2. First electron moves away3. Second electron is attracted by
lattice deformation and moves for former position of first electron
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
NbHgPb
NbCV3Si
Low temperature BCS
BCS Theory
Bednorzand Müller
TIBaCaCuO 1988
HgBaCaCuO 1993HgTlBaCuO 1995
BiSrCaCuO 1988
La2-xBaxCuO4 1986
YBa2Cu3O7 1987
High temperaturenon-BCS
Liquid N2
?
Why do electrons pair up into Cooper pairs in the
high-temperature superconductors?
Complexity in high-temperature superconducting cuprates
Low-temperature superconductors behave like normal metals above the transition (to superconductor) temperatureHigh-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 bondsSpin
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 liquidAnyon
superconductivityPhil
Anderson Alexei
Abrikosov
Bob Schrieffe
r
Tony Leggett
Bob Laughlin Karl
Müller
Example of a failed theoryPhil 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
Resonating valence bondsSpin
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 liquidAnyon
superconductivityPhil
Anderson Alexei
Abrikosov
Bob Schrieffe
r
Tony Leggett
Bob Laughlin
Karl Müller
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
140
100
60
20
LiquidHe
Tem
pera
ture
[K]
1920 1960 19801940 2000
Nb3Ge
MgB2 2001
Nb3SuNbN
NbHgPb
NbCV3Si
Low temperature BCS
BCS Theory
Bednorzand Müller
TIBaCaCuO 1988
HgBaCaCuO 1993HgTlBaCuO 1995
BiSrCaCuO 1988
La2-xBaxCuO4 1986
YBa2Cu3O7 1987
High temperaturenon-BCS
Liquid N2
A successful theory should explain the large differences in transition temperatures… or even provide a recipe for a ROOM-TEMPERATURE SUPERCONDUCTOR!