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High Tc Superconductors in Magnetic Fields

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High Tc Superconductors in Magnetic Fields T. P. Devereaux Kamerlingh Onnes, 1913 Nobel Prize for Discovery of Superconductivity in Mercury Theory of ... – PowerPoint PPT presentation

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Title: High Tc Superconductors in Magnetic Fields


1
High Tc Superconductors in Magnetic Fields
T. P. Devereaux
2
Kamerlingh Onnes, 1913 Nobel Prize for Discovery
of Superconductivity in Mercury
3
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
    -gt no resistance.
  • perfect diamagnetism electrons circulate to
    screen magnetic field (Meissner effect).

4
High Tc Superconductors Discovered in 1986, Nobel
Prize for Bednorz and Müller in 1987
5
Critical Current On the Rise
6
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).

7
Large Scale Applications
Top speed 552 km/hr
US Navy 5,000 HP
In-place in Detroit.
American Superconductor Corp.
8
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.

9
Small Devices? Magnetic Fields!
Resistance reappears!
lt- Resistivity of Pure Copper
  • H. Safar et al (1993)

10
Problem Vortices!
Electrons swirl in magnetic field increased
kinetic energy kills superconductivity.
SOLUTION Magnetic field kills superconductivity
in isolated places -gt VORTICES (swirling normal
electrons)
11
Direct Vortex Imaging Using Scanning Tunneling
Microscope
12
Animation Increasing Magnetic Field
Apply current Lorentz force causes vortices to
move -gt Resistance!
13
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.

14
Animation Pinning Moving Vortices
15
Problems to Overcome
  • High TC
  • Elastic string under tension F

Du2 kBTy(L-y)/FL kBT/F
String is floppier at higher T -gt vortex liquid
2) Planar Structure pancake vortices in layers
weakly coupled
Decreased string tension -gt vortex decoupling
16
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).

17
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.

18
Abrikosov Lattice Melting - gt Vortex Liquid
At low T, lattice forms with defects.
At higher T, lattice melts.
19
Pinning
At low T, a few pins can stop whole lattice.
At larger T, pieces of lattice shear away.
20
Pinning at low fields
Columns of defects are effective at pinning
vortices.
But channels of vortex flow proliferate at
larger fields.
21
Depinning lt-gt vortex avalanche
22
Splayed defects effective at cutting off channels
of vortex flow
But too much splaying and vortices cannot
accommodate to defects.
23
Resistivity is smaller for splayed defects
24
Optimal angle for splaying
25
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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