# Crossover between electron-phonon and boundary resistance limited thermal relaxation in copper films

## TL;DR

This paper investigates how heat relaxation in copper films transitions from being dominated by electron-phonon interactions to boundary resistance effects at very low temperatures, providing quantitative insights and methods to control thermal resistance.

## Contribution

It presents a detailed experimental analysis of the crossover between electron-phonon and boundary resistance limited thermal relaxation in copper films, including a method to increase boundary resistance.

## Key findings

- Identified a crossover from ep to boundary resistance limited regimes at sub-kelvin temperatures.
- Quantified heat current dependencies in both regimes.
- Demonstrated that inserting a third layer increases boundary resistance fourfold.

## Abstract

We observe a crossover from electron-phonon (ep) coupling limited energy relaxation to that governed by thermal boundary resistance (pp) in copper films at sub-kelvin temperatures. Our measurement yields a quantitative picture of heat currents, in terms of temperature dependences and magnitudes, in both ep and pp limited regimes, respectively. We show that by adding a third layer in between the copper film and the substrate, the thermal boundary resistance is increased fourfold, consistent with an assumed series connection of thermal resistances.