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High T c Superconductors in Magnetic Fields. T. P. Devereaux. Kamerlingh Onnes, 1913 Nobel Prize for Discovery of Superconductivity in Mercury. Theory of Superconductivity by Bardeen, Cooper, and Schrieffer Earns Nobel Prize in 1972. Most successful many-body theory. Quantum Coherent State.

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theory of superconductivity by bardeen cooper and schrieffer earns nobel prize in 1972
Theory of Superconductivity by Bardeen, Cooper, and Schrieffer Earns Nobel Prize in 1972

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).
new superconductor developments
New Superconductor Developments
  • Fullerenes: Tc engineered to 117K.
  • Iron becomes a superconductor under pressure.
  • Plastic superconductor: polythiophene.
  • DNA can be made superconducting.
  • MgB2 changes our thinking (again).
large scale applications
Large ScaleApplications

Top speed: 552 km/hr

US Navy: 5,000 HP*

In-place in Detroit.*

*American Superconductor Corp.

small scale devices
Small Scale Devices?
  • Transistors (RSFQ peta-flop supercomputer)?
  • Filters?
  • Nano-scale motors and devices?
  • Superconducting DNA?
  • Quantum computers!?
  • OBSTACLES:
  • cooling.
  • architecture.
  • ever-present magnetic fields destroy coherence.
small devices magnetic fields
Small Devices? Magnetic Fields!
  • H. Safar et al (1993)

Resistance reappears!

<- Resistivity of Pure Copper

problem vortices
Problem: Vortices!

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

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

animation increasing magnetic field
Animation: Increasing Magnetic Field

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

solution defects to pin vortices
Solution: Defects to Pin Vortices
  • 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.
problems to overcome
Problems to Overcome
  • 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

molecular dynamics simulations
Molecular Dynamics Simulations
  • 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).
molecular dynamics simulations for vortices
Molecular Dynamics Simulations for Vortices
  • 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.
abrikosov lattice melting vortex liquid
Abrikosov Lattice Melting - > Vortex Liquid

At low T, lattice forms with “defects”.

At higher T, lattice “melts”.

pinning
Pinning

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

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

pinning at low fields
Pinning at low fields

Columns of defects are effective at pinning vortices.

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

splayed defects effective at cutting off channels of vortex flow
Splayed defects effective at cutting off channels of vortex flow

But too much splaying and vortices cannot accommodate to defects.

acknowledgement future work
Acknowledgement & Future Work
  • 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,…).
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