Speaker
Description
Superconducting thin films are central to many quantum sensors being developed for low-threshold particle detection, where device performance and backgrounds can be strongly influenced by quasiparticle dynamics and other low-energy excitations. Understanding how material properties control these excitations is therefore increasingly important as detectors approach the quantum limit. We present microwave measurements of niobium resonators fabricated from films spanning a wide range of intrinsic stress. Despite nearly identical superconducting transition temperatures and normal-state transport properties, the devices exhibit a strong stress dependence in both microwave loss and resonant frequency response that cannot be described by a conventional single-gap Mattis-Bardeen model. Instead, the response is consistent with an additional low-gap superconducting component whose contribution increases with compressive stress. These results demonstrate that material properties largely invisible to conventional characterization metrics (e.g. $T_c$, RRR) can strongly modify the low-energy electrodynamics of superconducting devices. We discuss how such low-gap regions may influence quasiparticle trapping and transport, and their potential relevance to understanding nonequilibrium quasiparticle populations and low-energy backgrounds in superconducting quantum sensors.