Extreme Suppression of Thermal Conductivity in Monolayer Beryllene

We demonstrate an extreme suppression of thermal conductivity in monolayer beryllene through first-principles solutions of the phonon Boltzmann transport equation. While the fully converged three-phonon calculation predicts a room-temperature lattice thermal conductivity of 154.3 W/(m times K), inclusion of four-phonon interactions suppresses it to 10.7 W/(m times K), a reduction of more than 90%. This exceptionally large reduction of the thermal conductivity is amongst the highest reported in two-dimensional materials. The microscopic origin of this breakdown is traced to the strong sensitivity of the low-frequency flexural ZA phonons to four-phonon scattering: mode-resolved scattering rates reveal a pronounced enhancement of ZA scattering, which substantially shortens their lifetimes and suppresses their dominant contribution to heat conduction. Moreover, the temperature dependence of the thermal conductivity changes from κ proportional to T-0.86 to κ proportional to T-1.38 upon inclusion of four-phonon processes. Our results establish α-beryllene as an unusually strong case of four-phonon-controlled thermal transport amongst two-dimensional materials. These findings identify selective higher-order scattering of flexural phonons as a key mechanism for controlling thermal transport in strongly anharmonic low-dimensional systems.