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High Temperature Copper Oxide Superconductors: Properties, Theory and Applications in Society
Presented by Thomas Hines
in partial fulfillment of Physics 335
Introduction
Superconductors have a critical temperature, magnetic field and current, all of which must not be exceeded for superconductive behavior to be observed
Superconductors are characterized by the complete absence of electrical resistivity below a given temperature.
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
Theory – What is Generally Agreed Upon
Electron vacancies within the CuO layer of the crystal are thought to be charge carriers (although in Ln2-
XCeXCuO4-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 trainsMagnetic Resonance ImagingSQUID – Superconducting Quantum Interface DeviceParticle acceleratorsElectric generators (up to 99% efficient)Distributed Superconducting Magnetic Energy Storage System (D-SMES)Microchips – up to 1000 teraflopsE 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