Anisotropic superconductivity of niobium based on its response to non-magnetic disorder

TL;DR

This study investigates how non-magnetic disorder affects the anisotropic superconducting properties of niobium, confirming theoretical predictions through proton irradiation experiments on thin films.

Contribution

It provides experimental validation of a microscopic theory describing the impact of disorder on niobium's anisotropic superconductivity.

Findings

Superconducting transition temperature $T_c$ decreases with non-magnetic disorder.

The suppression of $T_c$ aligns closely with theoretical predictions.

Disorder affects the anisotropic order parameter of niobium.

Abstract

Niobium is one of the most studied superconductors, both theoretically and experimentally. It is tremendously important for applications, and it has the highest superconducting transition temperature, $T_{c}=9.33$ K, of all pure metals. In addition to power applications in alloys, pure niobium is used for sensitive magneto-sensing, radio-frequency cavities, and, more recently, as circuit metallization layers in superconducting qubits. A detailed understanding of its electronic and superconducting structure, especially its normal and superconducting state anisotropies, is crucial for mitigating the loss of quantum coherence in such devices. Recently, a microscopic theory of the anisotropic properties of niobium with the disorder was put forward. To verify theoretical predictions, we studied the effect of disorder produced by 3.5 MeV proton irradiation of thin Nb films grown by the same team and using the same protocols as those used in transmon qubits. By measuring the superconducting transition temperature and upper critical fields, we show a clear suppression of $T_{c}$ by potential (non-magnetic) scattering, which is directly related to the anisotropic order parameter. We obtain a very close quantitative agreement between the theory and the experiment.