Simon Fraser University

Superconductivity

Discovered in 1911, the most prominent property of a superconductor is the complete absence of electrical resistance.  The theory of Bardeen, Cooper and Schrieffer, published in 1957, provided the first complete explanation of this effect and others, such as the Meissner effect - the expulsion of magnet flux that leads to the levitation of a magnet above a superconductor.  A central feature of the BCS theory is the binding of electrons in Cooper pairs, which in some respects behave as composite bosons and can be Bose-condensed into a macroscopic condensate.  In conventional superconducting materials, quantized lattice vibrations (phonons) form the glue that binds electrons into Cooper pairs.  In more recently discovered materials, such as the high temperature superconductors and the so-called "heavy fermion" materials, the pairing glue is widely believed to be magnetic fluctuations - i.e. short-lived magnetic polarization of a nearly magnetic medium.  This opens the door to a profusion of new properties and exciting possibilities, such as unconventional pairing states in which the Cooper pairs have finite angular momentum.  Mnay of these materials and properties are actively investigated in the SFU Physics department.  The Sonier research group, for instance, uses nuclear probes, such as muon spin relaxation, to sensitively probe magnetism in superconducting materials, in particular high temperature superconductors.  The Dodge group studies these and other correlated electron materials using nonlinear optical techniques, that have the ability to take snapshots of the electron dynamics on femtosecond time scales.

On the theory side, SFU researchers have made a number of important contributions to our understanding of superconductivity under extreme conditions.  The Herbut group has shown that in many of the strange properties of high temperature superconductors can be understood when the phase of the superconducting wavefunction is a quantum-fluctuating field.  In this type of model, called the QED3 theory of superconductivity, whirlpool like vortices of supercurrent form inside the superconducting layers as quantum fluctuations, each vortex acting as a topological defect in the phase field of the condensate wavefunction.  The Kennett group also investigates high temperature superconductors, and has recently devised a number of methods for probing their electronic structure based on input from transport experiments.