Anomalous Temperature Induced Transition and Convergence of Thermal Conductivity in Germanene Monolayer

TL;DR

This study uncovers an unusual temperature-driven transition in germanene's thermal conductivity, shifting from a $T^{-2}$ to $T^{-1/2}$ scaling at 350 K, linked to phonon interactions and mode coupling.

Contribution

It reveals a novel temperature-induced transition in thermal conductivity scaling and elucidates the underlying phonon mechanisms in germanene.

Findings

Thermal conductivity scales as $T^{-2}$ below 350 K and $T^{-1/2}$ above.

A convergent thermal conductivity is observed due to high phonon scattering.

Phonon mode analysis links the transition to phonon overlap caused by ZO phonon redshift.

Abstract

We report an anomalous temperature-induced transition in thermal conductivity in germanene monolayer around a critical temperature $T_c = 350 \, \text{K}$. Equilibrium molecular dynamics simulations reveal a transition from $\kappa \sim T^{-2}$ scaling below $T_c$ to $\kappa \sim T^{-1/2}$ above, contrasting with conventional $\kappa \sim T^{-1}$ behavior. This anomalous scaling correlates with the long-scale characteristic timescale $\tau_2$ obtained from double exponential fitting of the heat current autocorrelation function. Phonon mode analysis using normal mode decomposition indicates that a redshift in ZO phonons reduces the acoustic-optical phonon gap, causing an overlap, enhances the phonon-phonon scattering, driving the anomalous scaling behavior. Moreover, nonequilibrium simulations find a convergent thermal conductivity of germanene with sample size, in agreement with mode coupling theory, owing to the high scattering of ZA phonons due to the inherent buckling of germanene.