Microscopic examination of hot spots giving rise to nonlinearity in superconducting resonators

TL;DR

This paper explores the microscopic origins of nonlinearity in superconducting resonators, linking hot spot formation in Nb films to nonlinear microwave responses through direct microscopic imaging.

Contribution

It provides direct microscopic evidence of hot spot formation in superconducting resonators and relates these to nonlinear microwave behavior.

Findings

Hot spots cause nonlinear response in superconducting resonators.

Microscopic hot spot formation correlates with transmission spectrum features.

Direct imaging confirms the link between microscopic defects and macroscopic nonlinearity.

Abstract

We investigate the microscopic origins of nonlinear rf response in superconducting electromagnetic resonators. Strong nonlinearity appearing in the transmission spectra at high input powers manifests itself through the emergence of jumplike features near the resonant frequency that evolve toward lower quality factor with higher insertion loss as the rf input power is increased. We directly relate these characteristics to the dynamics of localized normal regions (hot spots) caused by microscopic features in the superconducting material making up the resonator. A clear observation of hot-spot formation inside a Nb thin film self-resonant structure is presented by employing the microwave laser scanning microscope, and a direct link between microscopic and macroscopic manifestations of nonlinearity is established.