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(Seminar) local excitations in amorphous solids
2021-05-18     Text Size:  A

CAS Key Laboratory of Theoretical Physics

Institute of Theoretical Physics

Chinese Academy of Sciences

 Seminar

Title

题目

local excitations in amorphous solids

Speaker

报告人

吉文成

Affiliation

所在单位

EPFL

Date

日期

5月18日周二上午10点

Venue

地点

南楼6620

Contact Person

所内联系人

金瑜亮

Abstract

摘要

 Amorphous solids are structurally disordered. They are very common and examples include glasses, colloids, and granular materials. However, they are far less understood than crystalline solids. For amorphous solids key aspects of these materials are controlled by the presence of excitations in which a group of particles rearranges. This motion can be triggered by either: (a) quantum fluctuations associated with two-level systems (TLS); (b) thermal fluctuations associated with activations; (c) exerting an external stress or strain associated with shear transformations. It is thus important to understand how temperature and system preparation determine the density and geometry of these excitations. The possible unification of these excitations into a common description is also a fundamental problem. All these local excitations are thought to have a close relationship with Quasi-localised modes (QLMs), local normal modes of the Hessian (Stiffness Matrix), that are present in the low-frequency vibrational spectrum of amorphous solids. To study the local excitations we thus proceed by understanding the properties of QLMs and clarifying their relation to the local excitations. In this talk I focus on our recent published work. We propose a unification of theories predicting a gap in the spectrum of QLMs that grows upon cooling, with others that predict a pseudo-gap DL(ω) ~ ωα. Specifically, we generate glassy configurations of controlled gap magnitude ωc at zero temperature (T = 0), using our ‘breathing’ particles, and study how such gapped states respond to thermal fluctuations. We find that (i) the gap always fills up at finite T with DL(ω)≈A4(T)ω4 and A4 ~ exp(?Ea/T) at low T, (ii) E a rapidly grows with ωc , in reasonable agreement with a simple scaling prediction Ea ~ ω3c and (iii) at larger ωc excitations involve fewer particles, as we rationalise, and eventually become string-like. Following these observations, we propose an interpretation of mean-field theories of the glass transition, in which the modes beyond the gap act as an excitation reservoir, from which a pseudo-gap distribution is populated with its magnitude rapidly decreasing at lower T . We discuss how this picture unifies the rarefaction as well as the decreasing size of excitations upon cooling, together with a string-like relaxation occurring near the glass transition.
  Appendix:
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