Direct measurement of the electron-phonon relaxation rate in thin copper films

TL;DR

This paper introduces a novel ac-technique using SINIS thermometers to directly measure the electron-phonon relaxation rate in thin copper films at sub-Kelvin temperatures, revealing a T^4 dependence.

Contribution

The study presents a new method for direct measurement of electron-phonon scattering rates using overheating and frequency response, independent of material or geometry.

Findings

Electron-phonon scattering rate follows a T^4 temperature dependence.

The technique accurately measures the relaxation rate without relying on indirect methods.

Results align with previous indirect measurements and numerical models.

Abstract

We have used normal metal-insulator-superconductor (NIS) tunnel junction pairs, known as SINIS structures, for ultrasensitive thermometry at sub-Kelvin temperatures. With the help of these thermometers, we have developed an ac-technique to measure the electron-phonon (e-p) scattering rate directly, without any other material or geometry dependent parameters, based on overheating the electron gas. The technique is based on Joule heating the electrons in the frequency range DC-10 MHz, and measuring the electron temperature in DC. Because of the nonlinearity of the electron-phonon coupling with respect to temperature, even the DC response will be affected, when the heating frequency reaches the natural cut-off determined by the e-p scattering rate. Results on thin Cu films show a $T^{4}$ behavior for the scattering rate, in agreement with indirect measurement of similar samples and numerical modeling of the non-linear response.