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High T c Superconductors in Magnetic Fields

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T. P. Devereaux

Most successful many-body theory.

Quantum Coherent State

- “paired” electrons condense into coherent state -> no resistance.

- perfect diamagnetism – electrons circulate to screen magnetic field (Meissner effect).

- Fullerenes: Tc engineered to 117K.
- Iron becomes a superconductor under pressure.
- Plastic superconductor: polythiophene.
- DNA can be made superconducting.
- MgB2 changes our thinking (again).

Top speed: 552 km/hr

US Navy: 5,000 HP*

In-place in Detroit.*

*American Superconductor Corp.

- Transistors (RSFQ peta-flop supercomputer)?
- Filters?
- Nano-scale motors and devices?
- Superconducting DNA?
- Quantum computers!?

- OBSTACLES:
- cooling.
- architecture.
- ever-present magnetic fields destroy coherence.

- H. Safar et al (1993)

Resistance reappears!

<- Resistivity of Pure Copper

Electrons swirl in magnetic field – increased kinetic energy kills superconductivity.

SOLUTION: Magnetic field kills superconductivity in isolated places -> VORTICES (swirling “normal” electrons)

Apply current: Lorentz force causes vortices to move -> Resistance!

- Krusin-Elbaum et al (1996).

- Critical current enhanced by orders of magnitude over “virgin” material.
- Splayed defects better than straight ones.
- Optimal splaying angle ~ 5 degrees.

- High TC
- Elastic string under tension F:

Du2= kBTy(L-y)/FL~ kBT/F

String is floppier at higher T -> vortex “liquid”

2) Planar Structure

“pancake” vortices in layers weakly coupled

Decreased string tension -> vortex decoupling

- Widely used for a variety of problems:
- protein folding, weather simulation, cosmology, chaos, avalanches, marine pollution, other non-equilibrium phenomena.

- Solves equations of motion for each “particle”.
- Large scale simulations on pcs and supercomputers (parallel).

- Vortices = elastic strings under tension.
- Vortices strongly interact (repel each other).
- Temperature treated as Langevin noise.
- Solve equations of motion for each vortex.
- Calculate current versus applied Lorentz force, find what type of disorder gives maximum critical current.

At low T, lattice forms with “defects”.

At higher T, lattice “melts”.

At low T, a few pins can stop whole “lattice”.

At larger T, pieces of “lattice” shear away.

Columns of defects are effective at pinning vortices.

But “channels” of vortex flow proliferate at larger fields.

But too much splaying and vortices cannot accommodate to defects.

- All simulations performed by Dr. C. M. Palmer.
- Complex vortex dynamics.
- Future work to investigate
- Melting phenomena.
- Oscillatory motion of driven vortices.
- Onset of avalanches.
- Behavior as a qubit (quantum computing).
- Behavior of other dual systems (polymers, DNA,…).