Evidence for a low-gap component in compressively stressed niobium thin films — E8 Intelligence Research

We investigate the microwave response of sputtered Nb films as a function of biaxial film stress. Although transport measurements show nearly identical superconducting transition temperatures ($9.0\pm 0.2$K) and residual resistance ratios across the stress series, microwave resonators fabricated from highly compressive films exhibit enhanced temperature-dependent loss and larger frequency shifts. A conventional single-phase Mattis--Bardeen description yields an effective critical temperature of $2.4\pm0.2$K, approximately a factor of four below the value obtained from transport measurements. We show that the data are instead consistent with the presence of an additional low-gap component whose contribution increases with compressive stress. These results show that conventional transport measurements can overlook stress-dependent low-energy excitations that strongly influence microwave performance. Source: http://arxiv.org/abs/2609.26714v1 (arXiv:2609.26714v1) Author: Andrew Stewart Caldin, Independent Researcher, UK. Part of the E8 Intelligence Research series. Platform: e8intelligence.com