Quasiparticle spectroscopy in technologically-relevant niobium using London penetration depth measurements

TL;DR

This study measures the London penetration depth in various niobium samples to understand low-energy quasiparticle behavior and distinguish pair-breaking mechanisms, aiding the improvement of superconducting devices.

Contribution

It demonstrates how penetration depth measurements can differentiate pair-breaking mechanisms in niobium, relevant for superconducting technology.

Findings

Different low-temperature behaviors linked to pair-breaking mechanisms

Penetration depth measurements can identify niobium hydrides and TLS effects

Results applicable to optimizing SRF cavity performance

Abstract

London penetration depth was measured in niobium foils, thin films, single crystals, and superconducting radio-frequency (SRF) cavity pieces cut out from different places. The low-temperature (T<Tc/3) variation, sensitive to the low-energy quasiparticles with states inside the superconducting gap, differs dramatically between different types of samples. With the help of phenomenological modeling, we correlate these different behaviors with known pair-breaking mechanisms and show that such measurements may help distinguish between different pair-breaking mechanisms, such as niobium hydrides and two-level systems (TLS). The conclusions also apply to SRF cavities when tracking the temperature-dependent quality factor and the resonant frequency.