Studies of Electron Transitions Using Solid He Pressure Techniques

Abstract

Studies of the pressure dependence of the Fermi surface of metals have proven useful in simplifying the details of the electronic structure of a large number of metals and semimetals in the past few years. Efforts have centered on: (1) the testing of band theoretical models or descriptions of the metal, and (2) studies of electron transitions or changes in the topology of the Fermi surface (FS), the subject of this discussion. Low-temperature pressure studies provide the best method for probing these topology changes because either temperature or alloying tends to smear out effects of these purely electronic transitions. There is a hierarchy of electron transitions of which the simplest Situation is the electron or Lifshitz transition [1]. This is a change in FS topology with no change in the crystal structure and no change in the interatomic spacing. The next step in the hierarchy is a change in FS topology accompanied by a change in volume, but no change in structure. This class includes the “mixed valence” and “electron promotion” situations found in an increasing number of narrow band systems which are currently receiving an enormous amount of attention. Included in this class are many of the metal-insulator and metal-semiconductor transitions. Finally, there is the third tier in the hierarchy where the topology change in the Fermi surface is accompanied by (or results from, depending on what is thought to be driving the transition) a change in both structure and volume. In this discussion, emphasis will be placed on the “simplest” Situation, the Lifshitz transition, where it is possible in several cases to follow directly the changes in the Fermi surface by use of deHaas-van Alphen or high field magnetoresistance techniques.