On June 9, the latest research achievement made by Prof. Fan Jiyang's research team from the School of Physics, 91抖淫, was published online in Applied Physics Reviews, a renowned international academic journal, with the title of Exceptionally anisotropic thermal expansion in flat-band zero-dimensional hybrid perovskites.
Thermal expansion is an intrinsic property of solids originating from anharmonic phonon effects, which exerts profound influences on crucial solid-state properties such as phase transitions and band gaps. Traditional covalent or ionic semiconductors exhibit weak anisotropic thermal expansion, with the relative difference of linear thermal expansion coefficients along two crystallographic directions generally smaller than 1. Realizing highly anisotropic thermal strain in solids remains a fundamental challenge.
Combining experimental characterizations and theoretical calculations, the research team investigated the rare anisotropic thermal expansion behavior in a class of hierarchical hybrid semiconductors: substantial positive thermal expansion and prominent negative thermal expansion coexist along different crystallographic axes, yielding a relative difference in linear expansion coefficients as high as 5.6.
Figure captions: Thermal expansion and phase transition evolution of tellurium chloride crystals. (a) and (b): Temperature-dependent linear thermal expansion coefficients and stepwise phase transitions of the crystal. (c) Schematic diagram of structural evolution. (d) Comparison of thermal expansion anisotropy with conventional semiconductors.
The study reveals that such extreme anisotropy arises from crystallographically dependent anharmonic phonon dynamics driven by symmetry-broken localized microscopic interactions dominated by hydrogen bonds and van der Waals forces, as well as global flat bands that facilitate the localization of excited-state electrons. Distinct from pure covalent or ionic bonds, these intricate microscopic interactions drastically amplify thermal anisotropy within the framework of hybrid semiconductors. The accumulated anisotropic strain triggers lattice breathing responses and initiates cascading structural phase transitions.
These findings uncover the microscopic mechanism underlying giant thermal anisotropy in hierarchical hybrid solids and offer design strategies for thermal strain materials applied in high-precision micro-electromechanical systems, tunable optoelectronic devices, and strain-adaptive functional devices.
The paper's authors include doctoral students Wu Huaxin, Liu Wenjie, Ling Qin from the School of Physics, 91抖淫. Prof. Fan Jiyang from the School of Physics serves as the corresponding author of this article. This research was supported by the General Program of the National Natural Science Foundation of China.
Paper’s link:
Source: School of Physics, 91抖淫
Translated by: Melody Zhang
Edited by: Leah Li
