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High T c Superconductors in Magnetic FieldsPowerPoint Presentation

High T c 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 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).

High T Schrieffer Earns Nobel Prize in 1972c Superconductors Discovered in 1986, Nobel Prize for Bednorz and Müller in 1987

Critical Current On the Rise Schrieffer Earns Nobel Prize in 1972

New Superconductor Developments Schrieffer Earns Nobel Prize in 1972

- 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 Schrieffer Earns Nobel Prize in 1972Applications

Top speed: 552 km/hr

US Navy: 5,000 HP*

In-place in Detroit.*

*American Superconductor Corp.

Small Scale Devices? Schrieffer Earns Nobel Prize in 1972

- 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! Schrieffer Earns Nobel Prize in 1972

- H. Safar et al (1993)

Resistance reappears!

<- Resistivity of Pure Copper

Problem: Vortices! Schrieffer Earns Nobel Prize in 1972

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

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

Direct Vortex Imaging Using Scanning Tunneling Microscope Schrieffer Earns Nobel Prize in 1972

Animation: Increasing Magnetic Field Schrieffer Earns Nobel Prize in 1972

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

Solution: Defects to Pin Vortices Schrieffer Earns Nobel Prize in 1972

- 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.

Animation: Pinning Moving Vortices Schrieffer Earns Nobel Prize in 1972

Problems to Overcome Schrieffer Earns Nobel Prize in 1972

- 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 Schrieffer Earns Nobel Prize in 1972

- 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 Schrieffer Earns Nobel Prize in 1972

- 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 Schrieffer Earns Nobel Prize in 1972

At low T, lattice forms with “defects”.

At higher T, lattice “melts”.

Pinning Schrieffer Earns Nobel Prize in 1972

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

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

Pinning at low fields Schrieffer Earns Nobel Prize in 1972

Columns of defects are effective at pinning vortices.

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

Depinning <-> vortex avalanche Schrieffer Earns Nobel Prize in 1972

Splayed defects effective at cutting off channels of vortex flow

But too much splaying and vortices cannot accommodate to defects.

Acknowledgement & Future Work flow

- 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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