Experiments on thermal relaxation of copper films for nano-calorimetry

Abstract

Hot-electron calorimeter is a device used to detect a small package of energy utilizing thermal effect. For detection, the incoming energy is converted to temperature changes by the absorber of the calorimeter and then it is detected by a thermometer. Normal metal is one of the candidates to function as the absorber of hot-electron calorimeter due to the decoupling of electrons from its phonons at low temperatures, which provides the natural thermal isolation for electrons in the normal metal. To improve the performance of a normal-metal based calorimeter, one needs to understand the thermal relaxation mechanism in the device. In this dissertation, we experimentally investigate thermal relaxation in evaporated copper films at sub-kelvin temperatures. We characterize the thermal transport properties of the copper film in both steady-state and dynamic regime.

First, we give a short introduction to the nano-calorimeter and theoretically describe the relevant thermal properties explored in the dissertation. We describe the working principle of proximity Josephson junction thermometer used in the experiments to measure the electron temperature of copper films.

Next, we show the steady-state measurement results where then thermal conductance between film electrons and the environment is measured. Electron-phonon and thermal boundary conductance limit to thermal relaxation are identified in the experiments. We report the crossover of the thermal relaxation mechanism as changing film thickness and temperature. The impact of reduced phonon dimensionality on thermal conductance is discussed.

Finally, we show the dynamic thermal relaxation of copper films. In contrary to what is expected, we find that the process shows several relaxation times for copper films. Experiments are explained with a thermal model considering an additional thermal reservoir coupled to the film electrons. One possible origin of the thermal reservoir is the electrically isolated grains in the film with a different texture, and the electron-phonon coupling strength varies for grains with different orientations.