On 4 April 2022, Professor Jie Chen's group at the Centre for Research in Acoustics and Thermal Energy in the School of Physical Sciences and Engineering published a paper entitled "Enhancement of the lattice thermal conductivity of two-dimensional functionalized MXenes by inversion symmetry breaking" was published in Physical Review B. (Shuang Lu, a 2022 PhD student, is the first author of the paper, and Prof. Jie Chen is the corresponding author）

Crystal symmetry plays a critical role in thermal transport in solids. One example is mirror symmetry in monolayer graphene that promotes thermal transport by forbidding certain phonon scattering channels. Here, we show an interesting counterexample in monolayer Ta2CS2 that inversion symmetry breaking can abnormally enhance lattice thermal conductivity of two-dimensional (2D) functionalized MXenes based on the phonon Boltzmann transport theory combined with first-principles calculations. We find inversion symmetry breaking not only affects scattering channels but also redistributes the charge and changes the anharmonic phonon properties. Both atomic vibrations in real space and phonon scattering rates in the reciprocal space confirm the significantly reduced lattice anharmonicity in an asymmetric Ta2CS2 sheet, leading to substantially enhanced thermal conductivity after inversion symmetry breaking. The physical origin for the variation of anharmonic properties induced by symmetry is discussed in detail. In this paper, we provide insight into the complex role of symmetry on thermal transport in 2D functionalized MXenes.

This project is supported in part by grants from the National Natural Science Foundation of China, the Science and Technology Commission of Shanghai Municipality, and the Fundamental Research Funds for the Central Universities.

Link: https://journals.aps.org/prb/abstract/10.1103/PhysRevB.105.165301

(a) The side and top views of asymmetric Ta2CS2 and (b)symmetric Ta2CS2. (c) Thermal conductivity of asymmetric Ta2CS2 and symmetric Ta2CS2.（d）Anharmonic scattering rates in symmetric Ta2CS2 (filled symbols) and asymmetric Ta2CS2 (empty symbols).