Magnetic Field Dependent Microwave Losses in Superconducting Niobium Microstrip Resonators

TL;DR

This study presents a comprehensive experimental protocol to analyze magnetic field dependent microwave losses in superconducting niobium microstrip resonators, highlighting the dominant loss mechanisms and their dependence on magnetic field orientation and cooling procedures.

Contribution

It introduces a unified method to distinguish between quasiparticle generation and vortex motion as loss mechanisms, including a novel plot of quality factor versus resonance frequency for analysis.

Findings

Quasiparticle generation dominates under parallel magnetic fields.

Vortex motion is the primary loss mechanism under perpendicular fields.

A niobium resonator with thickness similar to penetration depth is optimal for X-band ESR.

Abstract

We describe an experimental protocol to characterize magnetic field dependent microwave losses in superconducting niobium microstrip resonators. Our approach provides a unified view that covers two well-known magnetic field dependent loss mechanisms: quasiparticle generation and vortex motion. We find that quasiparticle generation is the dominant loss mechanism for parallel magnetic fields. For perpendicular fields, the dominant loss mechanism is vortex motion or switches from quasiparticle generation to vortex motion, depending on cooling procedures. In particular, we introduce a plot of the quality factor versus the resonance frequency as a general method for identifying the dominant loss mechanism. We calculate the expected resonance frequency and the quality factor as a function of the magnetic field by modeling the complex resistivity. Key parameters characterizing microwave loss are estimated from comparisons of the observed and expected resonator properties. Based on these key parameters, we find a niobium resonator whose thickness is similar to its penetration depth is the best choice for X-band electron spin resonance applications. Finally, we detect partial release of the Meissner current at the vortex penetration field, suggesting that the interaction between vortices and the Meissner current near the edges is essential to understand the magnetic field dependence of the resonator properties.