Suguru Yoshida

Lundi 3 février à 14h00 - Atomic displacements in crystals: what occurs and why

Suguru YOSHIDA
Associate Professor
Kyoto University

Website: http://www.ehcc.kyoto-u.ac.jp/eh10/

Abstract

Collective changes in atom positions often have drastic influences on the physical properties of solids. An example is the lifting of spatial inversion symmetry caused by atomic displacement, from which a lot of intriguing states of matter emerge, including ferroelectricity, Weyl semimetals, spin-valley locked state, and so on. Predicting physical property requires knowledge of the underlying atomic structure, or at least of the space group symmetry, provided by experiments or computations. Finding the most stable structure of a given crystal is, however, difficult even in silico due to the lack of a priori knowledge on what distortion can reduce total energy.

In this talk, I will introduce the case studies where the combined experimental/computational/group-theoretical approaches revealed a hidden true structure, which has been missed despite previous experimental attempts or does not appear in related systems, and thus showed that such "new material" unlocks new physical properties. Examples include 2D layered perovskite materials exhibiting hybrid improper ferroelectricity and a high-entropy layered antimonide system with Dirac electrons. The second part will discuss the quantum-mechanical origin of such atomic displacements, based on the second-order Jahn-Teller effect. After introducing this idea, I would like to think of the following open question together through discussion among the audience: is it possible to induce an intended atomic displacement in solids by tuning electronic states through chemical substitution (like the mixed-anion approach)? If so, what kind of anion-combination or anion-ordering is promising?

Contact : Shunsuke Sasaki (PMN)