Seminars Archive

Phonon Softening in Bi and Sb: Toward a Cubic Phase?

Abstract
When writing data to a dvd, your dvd-drive does something really interesting: it optically induces a phase transition in the active material of your dvd. This is an example of the much broader, rapidly developing field of optically induced changes in condensed matter. The present contribution focuses on optically induced phenomena in the semi-metals bismuth and antimony. The structure of these semi-metals is not the usual cubic one, as found in other simple metals, but rather a distorted one called the arsenic-A7 structure. The origin of this distortion lies in the electron-phonon interaction, and in particular depends on the charge density in the conduction band of these materials. At a high enough density this distortion should vanish, leading to a cubic phase, and one would expect that it is possible to induce a phase transition to this state by photo-doping. Recent experiments, using direct probing of the phonon excitation spectrum via time resolved optical pump/Raman probe spectroscopy, have shown that one can indeed reduce the distortion by photo-doping. Moreover, they yield strong indications for an optically induced phase transition in the semi-metals, albeit that it does not appear to be a transition to the cubic phase. Apart from introducing the novel experimental technique, the observed transient phonon spectra of Bi and Sb will be discussed in terms of thermal and non-thermal effects, and screening of the crystal potential by the photo induced charge density in the conduction band. Finally, the experimental evidence for, and the nature of the observed phase transition will be discussed. Ref. M. Ksikinova, F. Parmigiani