Unusual anisotropic thermal expansion in multilayer SnSe leads to positive-to-negative crossover of Poisson's ratio

Abstract

Monolayer SnSe has been predicted to exhibit an unusual anisotropic thermal expansion—when heated, the long lattice parameter contracts, while the short one expands, resulting in a rectangular-to-square-lattice phase transformation at a critical temperature (Tc). Here, we employ density-functional-theory calculations to demonstrate an even more notable thermal-expansion behavior of SnSe from monolayer to bulk. We find that the unusual thermal expansion persists in multilayers, while the coefficients of thermal expansion of different numbers of layers are almost identical. This behavior results from a delicate interplay between the elastic stiffness coefficient and Grüneisen parameters. Finally, we find that the Poisson's ratio of multilayer SnSe, which is positive at T=0 K, gets smaller with increasing temperature and even turns negative, signaling a zero Poisson's ratio at a particular temperature. Overall, the present results provide another perspective in understanding the unusual thermal properties of monochalcogenides.