Resonant ultrasound spectroscopy (RUS) has been used as a probe to determine the ultrasonic velocity and attenuation in high transition temperature superconductors, in order to study the gap anisotropy which is a signature of unconventional superconductivity. The RUS technique involves exciting the resonant modes in a sample and measuring the natural frequencies; from these and the geometry of the sample, one can determine the elastic constants using available computing algorithms. A major advantage of this technique, over pulse-echo ultrasonic techniques which have been utilized for conventional superconductors, is that very tiny samples can be used, so that experiments on untwinned and homogeneous high temperature superconducting crystals are feasible.

Electrons in Metals

Fundamental aspects of electronic properties of transition metals are deduced from measurements of Landau quantum oscillations and corresponding theoretical calculations of spin-splitting g factors of the conduction electrons. While the experiments can in principle yield information on the anisotropy of the spin-splitting over the complete set of extremal orbits on each sheet of the Fermi surface, they are impossible in practice at many orientations because of interference between different oscillations and suffer from the fundamental disadvantage that the results are indeterminate to the extent of an additive integer because of the inversion of a periodic (cosine) function in the analysis. The most effective approach is to develop theoretical calculations of the spin-splitting, and to use strategic experiments to validate the theory or point out its deficiencies; e.g., recent measurements in Mo have shown the inadequacy of a scalar-relativistic LMTO calculations in a situation where spin-orbit coupling lifts a degeneracy.

Amorphous Semiconductors