Anomalous electronic heat capacity of copper nanowires at sub-kelvin temperatures

TL;DR

This study measures the electronic heat capacity of copper and silver nanowires at sub-kelvin temperatures, revealing an unexpected high heat capacity in copper films that challenges existing understanding.

Contribution

It provides the first measurements of electronic heat capacity in copper nanowires at millikelvin temperatures, highlighting an anomalous increase not explained by current theories.

Findings

Silver films match bulk and free-electron estimates

Copper films show tenfold higher heat capacity

Silver films are promising for microwave photon calorimetry

Abstract

We have measured the electronic heat capacity of thin film nanowires of copper and silver at temperatures 0.1 - 0.3 K; the films were deposited by standard electron-beam evaporation. The specific heat of the Ag films of sub-100 nm thickness agrees with the bulk value and the free-electron estimate, whereas that of similar Cu films exceeds the corresponding reference values by one order of magnitude. The origin of the anomalously high heat capacity of copper films remains unknown for the moment. Based on the low heat capacity and the possibility to devise a tunnel probe thermometer on it, the Ag films form a promising absorber material, e.g., for micro-wave photon calorimetry.