Thermal conductance of Nb thin films at sub-kelvin temperatures

TL;DR

This study measures the thermal conductance of niobium thin films at sub-kelvin temperatures, revealing significantly lower conductance than aluminum and deviations from BCS theory, with implications for nanoscale thermal insulation.

Contribution

It provides the first detailed measurement of Nb thin film thermal conductance at sub-kelvin temperatures, showing a power law dependence and potential for thermal insulation applications.

Findings

Nb's thermal conductance is two orders of magnitude lower than Al at 0.6 K.

Measured conductance follows a $T^{4.5}$ power law, not exponential.

Nb can serve as effective thermal insulator while maintaining electrical connectivity.

Abstract

We determine the thermal conductance of thin niobium (Nb) wires on a silica substrate in the temperature range of 0.1 - 0.6 K using electron thermometry based on normal metal-insulator-superconductor tunnel junctions. We find that at 0.6 K, the thermal conductance of Nb is two orders of magnitude lower than that of Al in the superconducting state, and two orders of magnitude below the Wiedemann-Franz conductance calculated with the normal state resistance of the wire. The measured thermal conductance exceeds the prediction of the Bardeen-Cooper-Schrieffer theory, and demonstrates a power law dependence on temperature as $T^{4.5}$, instead of an exponential one. At the same time, we monitor the temperature profile of the substrate along the Nb wire to observe possible overheating of the phonon bath. We show that Nb can be successfully used for thermal insulation in a nanoscale circuit while simultaneously providing an electrical connection.