Exact solutions, spectrum properties, and hierarchical structures of the multiple temperature model

TL;DR

This paper derives exact analytical solutions for linear multiple temperature models used in ultrafast laser dynamics, revealing their spectral properties, hierarchical structures, and unique diffusion modes, thus advancing understanding of non-equilibrium thermal processes.

Contribution

It provides the first exact solutions for these models, analyzes their spectral and hierarchical properties, and uncovers diffusion modes specific to the three temperature model.

Findings

Exact analytical solutions for two and three temperature models.

Identification of eigenmodes with well-defined lifetimes.

Discovery of unique diffusion modes in the three temperature model.

Abstract

Recent developments of ultrafast laser pulse techniques enable us to study the subpicosecond scale dynamics out of thermal equilibrium. Multiple temperature models are frequently used to describe such dynamics where the total system is divided into subsystems each of which is in local thermal equilibrium. Typical examples include the electron-lattice two temperature model and electron-spin-phonon three temperature model. We present the exact analytical solutions of linear multiple temperature model, based on the Fourier series expansion, and discuss their properties for the case of the two and three temperature models. The solutions are linear combinations of "eigenmodes" characterized by the wave vector $\mathbf{q}$ and the well-defined mode lifetime. The eigenmode picture enables us to explore the hierarchical structure of models with respect to space, time and the coupling parameter. We also find diffusion modes unique to the three temperature model which unveils the rich physics in spite of the simplicity of the model. We prove that the eigensystem in this model is non-positive definite, which assures that the mode lifetime is always well-defined. This property clearly characterizes the model.