High Temperature Copper Oxide Superconductors: Properties, Theory and Applications in Society

Presented by Thomas Hines in partial fulfillment of Physics 335

Introduction

Superconductors are characterized by the complete absence of electrical resistivity below a given temperature.

Superconductors have a critical temperature, magnetic field and current, all of which must not be exceeded for superconductive behavior to be observed

Type I Superconductors

Usual composition is Metals and Metalloids Characterized by low critical temperatures Operate on BCS Theory, named after Bardeen, Cooper Schieffer

BCS Theory

Electrons couple to form Cooper pairs with energy close to the Fermi level, the higest energy an electron can have at 0 degrees Kelvin. These Cooper pairs act like bosons and propagate through the crystal by inducing resonance.

Type II Superconductors

Characterized by much higher critical temperatures than type I, often exceeding 77 K, the boiling point of nitrogen.

Many type II superconductors are metal oxides known as perovskites, containing two metal atoms for every three oxygen atoms Electrons form pairs, but the pairing mechanism is different than that described by BCS theory

Type II Superconductors continued Undergo a mixed state of superconductivity between upper and lower critical magnetic fields

Type II properties

Meisner Effect Exhibition of diamagnetic properties, defined as the expulsion of magnetic flux from penetrating the material Most superconductors are not completely diamagnetic

Type II Properties

Flux Pinning A phenomenon in which magnetic flux becomes trapped or pinned inside the superconducting material Requires defects to be present in the material Necessary to prevent the emergence of pseudo resistance and to keep critical current and critical magnetic field from dropping.

Type II Properties

Anisotropy Critical magnetic field dependence on which direction external magnetic field is applied relative to crystallographic orientation Results from the CuO layer (conducting layer) in the crystal spanning only two dimensions Generally speaking, the more anisotropic a superconductor is, the higher critical temperature it will have

Type II Properties

Vortices Swirls of electric current induced by the application of an external magnetic field Superconductive behavior is microscopically suppressed in vortices resulting in a mixed state behavior Complete superconductive behavior is lost when vortices overlap

Type II Properties

Theory – What is Generally Agreed Upon

Electron vacancies within the CuO layer of the crystal are thought to be charge carriers (although in Ln 2 X Ce X CuO 4-Y , experiments show electrons are the charge carriers).

During the Mott transition, holes from blocking layers dope the CuO layer to alter its conductivity.

Theory – Blackstead Dow

Chain layer (plane containing CuO is compressed by surrounding cuprate planes into the superconducting state.

Accounts for apparent unbalance of charge in ceramic superconducting compounds Supported by the experimental observation that the chain layer is sensitive to magnetism, and the cuprate plane is not.

Theory - Todura Takag Uchida

Theorizes that an electron factionalizes into a neutral spin half fermion called a “spinon” and a spinless “holon” or “chargon” with charge e.

Inspired by the observation that fermions do not carry heat in high temperature superconductors and attempts to fit this with Wiedmann- Franz Law.

Wiedmann- Franz Law predicts that both heat and charge a transported via a single “quasiparticle”.

Theory – Magnetic Resonance

When bombarded with neutrons, Cooper spins responded as a group by entering a magnetic resonance mode Shows that spins are strongly interacting and could hold cooper pairs together.

Applications

MAGLEV trains Magnetic Resonance Imaging SQUID – Superconducting Quantum Interface Device Particle accelerators Electric generators (up to 99% efficient) Distributed Superconducting Magnetic Energy Storage System (D-SMES) Microchips – up to 1000 teraflops E Bombs – create a strong EM pulse to disable electronics

Applications

Superconducting Cables Multifilament wire or tape composed of Cu or Ag matrix embedded with superconducting filaments Matrix impedes penetration of magnetic flux from one superconducting filament to another

Applications Projected worth of superconductor market at current growth rate

Questions?