Born effective charge removed anomalous temperature dependence of lattice thermal conductivity in monolayer GeC.

ABSTRACT

Due to potential applications in nano- and opto-electronics, two-dimensional (2D) materials have attracted tremendous interest. Their thermal transport properties are closely related to the performance of 2D materials-based devices. Here, the phonon transports of monolayer GeC with a perfect planar hexagonal honeycomb structure are investigated by solving the linearized phonon Boltzmann equation within the single-mode relaxation time approximation (RTA). Without inclusion of Born effective charges (Z *) and dielectric constants ([Formula see text]), the lattice thermal conductivity ([Formula see text]) decreases almost linearly above 350 K, deviating from the usual [Formula see text] law. The underlying mechanism is because the contribution to [Formula see text] from high-frequency optical phonon modes increases with increasing temperature, and the contribution exceeds one from acoustic branches at high temperature. These can be understood by huge phonon band gap caused by large difference in atom mass between Ge and C atoms, which produces important effects on scattering process involving high-frequency optical phonon. When considering Z * and [Formula see text], the phonon group velocities and phonon lifetimes of high-frequency optical phonon modes are obviously reduced with respect to ones without Z * and [Formula see text]. The reduced group velocities and phonon lifetimes give rise to small contribution to [Formula see text] from high-frequency optical phonon modes, which produces the the traditional [Formula see text] relation in monolayer GeC. Our works highlight the importance of Z * and [Formula see text] to investigate phonon transports of monolayer GeC, and motivate further theoretical or experimental efforts to investigate thermal transports of other 2D materials.