Two temperature model

In statistical mechanics, the two-temperature model (TTM) is a mathematical model that describes how materials respond to intense electric currents or ultrashort optical pulses, such as those produced by ultrafast lasers. It describes a transient partial equilibrium state between electronic and phononic populations within the material. In the context of material science and solid state physics, this model is used to describe the process of ultrafast carrier relaxation following excitation. Such dynamics is so fast that, in order to probe it, it is necessary to set up experiments that use ultrafast lasers as the sources of excitation. This kind of experiments (and related simulations) fall under the subjects of ultrafast spectroscopy and ultrafast laser physics.

The two-temperature model was first introduced by Moisey Isaakovich Kaganov and colleagues in 1957. Then, in 1974 it was extended to the case of metals exposed to ultrashort laser pulses. Although originally developed for metals, it also applies to semiconductors. Nowadays, the TTM is used in the context of ultrafast spectroscopy, ultrafast laser ablation and can be applied (with additional constraints) to micro and nano systems. It can also be used to study widespread technologies, such as electronic circuits, where the passage of an electric current in the metallic conductor can create a state of non-equilibrium between electrons and phonons.

Experimental evidence supports this model. Also, its application for the description of emerging systems such as low dimensional ones has been a subject of investigation.