An outline is given of the effects of the various processes occurring at a structural phase transformation on the quantities measured in an ultrasonic experiment: sound velocity and attenuation. Various theoretical approaches are discussed as well as experimental results.

The study of structural phase transitions by means of ultrasonic experiments

Superconducting A15-type compounds were discovered 25 years ago. Several thousand original publications have appeared on this growing subject. The author reviews the essential structural aspects together with the characteristic superconducting properties and information on electron and phonon spectra.

https://iopscience.iop.org/article/10.1088/0034-4885/43/5/003/pdf

A15-type superconductors

Microstructure, composition and critical current density of superconducting Nb3Sn wires

Publisher Summary This chapter describes the work that has already been done, and also mentions work that is in progress, but is not as yet properly understood, in β-Tungsten research. Regarding work that has already been done, it is seen that elastic anomalies and martensitic phase transformations appear to be quite a general phenomenon; they require a large deformation potential, a large electronic density of states at the Fermi level (that may result from Van Hove singularities), and a degeneracy of the electronic states (with the same value of k, or with different ones). To a certain extent, they can be described by the Landau theory of order-disorder transformation, but most commonly they may be expected to be first-order transitions. The various microscopic models proposed so far to account for these anomalies; all predict a Landau type behavior. Therefore, this behavior is not a test for the applicability of one or another of these models. Lattice instabilities due to optical phonons should also be present, but they have not been observed so far in the β-tungstens, except for Nb3Sn, probably because they have not been looked for (except in V3Si, where they are not present). They may be associated with sharp peaks in the ultrasonic attenuation as a function of temperature, such as those observed in V3Ge.

Some Lattice and Electronic Properties of the β-Tungstens

http://venus.ifca.unican.es/~xray/XEUS/archivopapers/Koch_Cryo17_391.pdf

A review of mechanical behaviour and stress effects in hard superconductors

The elastic moduli ${c}_{11}$, ${c}_{12}$, and ${c}_{44}$ and the magnetic susceptibility $\ensuremath{\chi}$ of single-crystal ${\mathrm{Nb}}_{3}$Sn have been measured as a function of temperature below 300 K. Detailed comparison is made between the experimental results and the predictions of simple one-dimensional band models. It is found that the behavior of the elastic moduli ${c}_{11}$ and ${c}_{12}$ above and below the cubic-tetragonal transformation at 45 K is well accounted for by the band model with an effective Fermi temperature of 80 K. Unlike the case of ${\mathrm{V}}_{3}$Si, the modulus ${c}_{44}$ is observed to undergo a considerable softening at low temperatures. This softening is not predicted by the theory. The susceptibility displays the predicted maximum near the lattice-transformation temperature. However, the decrease of $\ensuremath{\chi}$ in the tetragonal state, associated with a drop in the electronic density of states, is not nearly as large as expected. Furthermore, the cubic-state $\ensuremath{\chi}$ data indicate a much larger Fermi temperature (230 K) than is obtained from the ${c}_{11}$ and ${c}_{12}$ data. We review these anomalies in terms of available band-structure calculations. The implications of our experimental results for superconductivity in ${\mathrm{Nb}}_{3}$Sn are discussed.

W. Rehwald

Papers

ELASTIC MODULI AND MAGNETIC SUSCEPTIBILITY OF MONOCRYSTALLINE Nb$sub 3$Sn.