CPM Seminar

What is magnetic about superconductivity: An example of why “more is different”

Andrea Bianchi

Université de Montréal

I will start by giving an introduction to “conventional” superconductivity which was discovered almost 100 years ago in Mercury by Kamerlingh-Onnes and explained 50 years ago by Bardeen, Cooper, and Schrieffer. In this introduction I will point out what makes this macroscopic quantum state, which is coherent over large length scales and thus offers no resistance to the transport of electrons, so special. With the advances in chemistry in the 1950's, rare earth metals with a 4f electronic shell became available to experimentalists which consequently enabled them to open entirely new fields of research. In the following the application of solid state chemistry methods in the search of novel compounds lead to the discovery of the 4f or 5f elements containing heavy fermion superconductors. I will illustrate that as in the case of superconductivity, the heaviness of these compounds is a many body effect. Also, superconductivity in these compounds is intimately tied to their magnetic properties, making them “unconventional” superconductors. The discovery of superconductivity in these compounds came thus as quite a shock, as for “conventional” superconductors magnetism is detrimental to superconductivity, and one might say that antiferromagnetism is “tolerated” at best.