Dynamic thermal relaxation in metallic films at sub-kelvin temperatures

TL;DR

This study investigates how hot electrons in copper and silver thin films relax over time at sub-kelvin temperatures, revealing multiple relaxation time constants influenced by film annealing and grain structure.

Contribution

It provides the first detailed experimental analysis of dynamic thermal relaxation in mesoscopic metallic films, introducing a model with an additional thermal reservoir.

Findings

Multiple relaxation time constants observed in copper and silver films

Annealing alters the relaxation time constants

Grain structure significantly affects electron thermal relaxation

Abstract

The performance of low temperature detectors utilizing thermal effects is determined by their energy relaxation properties. Usually, heat transport experiments in mesoscopic structures are carried out in the steady-state, where temperature gradients do not change in time. Here, we present an experimental study of dynamic thermal relaxation in a mesoscopic system -- thin metallic film. We find that the thermal relaxation of hot electrons in copper and silver films is characterized by several time constants, and that the annealing of the films changes them. In most cases, two time constants are observed, and we can model the system by introducing an additional thermal reservoir coupled to the film electrons. We determine the specific heat of this reservoir and its coupling to the electrons. The experiments point at the importance of grain structure on the thermal relaxation of electrons in metallic films.