Unusual Phonon Thermal Transport Mechanisms in Monolayer Beryllene

We compute the thermal conductivity of monolayer beryllene using the linearized phonon Boltzmann transport equation with interatomic force constants obtained from \textit{ab-initio} calculations. Monolayer beryllene exhibits an impressive thermal conductivity of 270 W/m$\cdot$K at room temperature, exceeding that of bulk beryllium by over 100%. Our study reveals a remarkable temperature-dependent behavior: $\kappa \sim T^{-2}$ at low temperatures, attributed to higher normal phonon-phonon scatterings, and $\kappa \sim T^{-1}$ at high temperatures, due to Umklapp phonon interactions. Mode-specific analysis reveals that flexural phonons with longer lifetimes are the primary contributors to thermal conductivity, accounting for approximately 80%. This dominance results from their lower scattering rates in the out-of-plane direction due to a restricted phase space for scattering processes. Additionally, our findings highlight suppressed Umklapp scattering and reduced phase space for flexural modes, providing a thorough understanding of the eased thermal conductivity in monolayer beryllene and its potential for advanced thermal management applications.