Congratulations to Andrew Dane, who won the best poster award at the 706th W.E. Heraeus Seminar, titled Superconducting Kinetic Inductances for his work on high-Q superconducting niobium resonators incorporating nanoscale quasiparticle traps.

Details regarding his poster abstract may be found below:

Superconducting Nb Resonators with Gold Nanodot Decorations

Andrew Dane, Omid Noroozian, Emily Barrentine, Di Zhu, Thomas Stevenson, Harvey Moseley, Karl Berggren

While the operation of kinetic inductance detectors (KIDs) is well understood, reports of anomalous behaviors in KIDs fabricated from highly-disordered, thin-film superconductors indicate that a detailed understanding of the superconducting density of states and behavior of excited quasiparticles is necessary to reconcile experimental findings with theory. Anomalous behaviors include: (1) differences in the inductive and dissipative photo-response times, such that the shift in the frequency of the resonator outlasts the reduction in Q by a factor of four, (2) resonant frequency vs temperature that cannot be explained by Mattis-Bardeen theory without ad-hoc modifications. It has been observed experimentally that in highly-disordered thin-film superconductors, spontaneous fluctuations of the superconducting gap can arise. The interaction between excited quasiparticles and a spatially varying gap may explain the anomalous behaviors observed in KIDs, for instance if quasiparticle trap states exist that can temporarily localize excited quasiparticles.

In this work, we report on the fabrication and measurement of Nb CPW resonators, as well as progress towards a physical simulation of the case where the superconducting gap varies along the resonator body. The Nb used for fabrication was 50 nm thick, sputtered onto r-plane sapphire at 500 °C, and was not expected to have significant variation in the gap along the film. In order to produce a spatially varying gap, we lithographically defined arrays of gold nano-dots, on top of the CPW centerline, in an attempt to locally reduce the superconducting gap in the Nb due to the proximity effect. In undecorated resonators at high photon number, an internal Q of greater than one million was measured at 7 mK, while the internal Q of the decorated resonators was substantially less. The resonant behavior of these devices vs temperature and power, and their relevance to high-kinetic inductance materials will be discussed.