High Tc Superconductors in Magnetic Fields

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High Tc Superconductors in Magnetic Fields
T. P. Devereaux
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Kamerlingh Onnes, 1913 Nobel Prize for Discovery
of Superconductivity in Mercury
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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).

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

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

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Small Devices? Magnetic Fields!
Resistance reappears!
lt- Resistivity of Pure Copper

• H. Safar et al (1993)

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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)
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Direct Vortex Imaging Using Scanning Tunneling
Microscope
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Animation Increasing Magnetic Field
Apply current Lorentz force causes vortices to
move -gt Resistance!
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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.

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Animation Pinning Moving Vortices
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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
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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).

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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.

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