Numerical Solver for the out-of-equilibrium time dependent Boltzmann Collision operator: Application to 2D materials

TL;DR

This paper presents a robust and efficient numerical solver for the time-dependent Boltzmann equation's scattering integral, applied to 2D materials, revealing complex thermalization dynamics of out-of-equilibrium excitations.

Contribution

It extends previous 1D applications to 2D systems, enabling realistic, non-approximate simulations of thermalization processes in out-of-equilibrium conditions.

Findings

Higher energy excitations thermalize faster.

Thermalization follows a non-exponential decay.

Double exponential fit provides quantitative thermalization time scales.

Abstract

The Time Dependent Boltzmann equation (TDBE) is a viable option to study strongly out-of-equilibrium thermalization dynamics which are becoming increasingly critical for many novel physical applications like Ultrafast thermalization, Terahertz radiation etc. However its applicability is greatly limited by the impractical scaling of the solution to its scattering integral term. In our previous work\cite{Michael} we had proposed a numerical solver to calculate the scattering integral term in the TDBE and then improved on it\cite{1DPaper} to include second degree momentum discretisation and adaptive time stepping. Our solver requires no close-to-equilibrium assumptions and can work with realistic band structures and scattering amplitudes. Moreover, it is numerically efficient and extremely robust against inherent numerical instabilities. While in our previous work \cite{1DPaper} we showcased the application of our solver to 1D materials, here we showcase its applications to a simple 2D system and analyse thermalisations of the introduced out-of-equilibrium excitations. The excitations added at higher energies were found to thermalise faster than those introduced at relatively lower energies. Also, we conclude that the thermalisation of the out-of-equilibrium population to equilibrium values is not a simple exponential decay but rather a non-trivial function of time. Nonetheless, by fitting a double exponential function to the decay of the out-of-equilibrium population with time we were able to generate quantitative insights into the time scales involved in the thermalisations.