Unexpected Nonlinear Dynamics in NbN Superconducting Microwave Resonators

TL;DR

This study investigates the unusual nonlinear resonance behaviors in NbN superconducting microwave resonators at low power levels, revealing phenomena like bifurcations and hysteresis, and suggests Josephson junctions as a possible cause.

Contribution

It provides a detailed characterization of nonlinear dynamics in NbN resonators and proposes a hypothesis linking observed behaviors to Josephson junctions at grain boundaries.

Findings

Nonlinear effects occur at low input powers (2-4 orders lower than Nb).

Resonance response shows bifurcations, hysteresis, and frequency shifts.

Experimental results align with the hypothesis of Josephson junctions causing the nonlinearities.

Abstract

In this work we characterize the unusual nonlinear dynamics of the resonance response, exhibited by our NbN superconducting microwave resonators, using different operating conditions. The nonlinear dynamics, occurring at relatively low input powers (2-4 orders of magnitude lower than Nb), and which include among others, bifurcations in the resonance curve, hysteresis loops and resonance frequency shift, are measured herein using varying temperature, applied magnetic field, white noise and rapid frequency sweeps. Based on these measurement results, we consider a hypothesis according to which Josephson junctions forming weak links at the boundaries of the NbN grains are responsible for the observed behavior, and we show that most of the experimental results are qualitatively consistent with such hypothesis.