On the properties of superconducting planar resonators at mK temperatures

TL;DR

This paper investigates the temperature-dependent properties of niobium superconducting planar resonators at millikelvin temperatures, revealing insights into their interaction with substrate two-level fluctuators.

Contribution

It provides experimental analysis of three types of niobium resonators, highlighting their non-trivial temperature dependence and interaction mechanisms.

Findings

Resonators show non-trivial temperature dependence of frequency and quality factor.

Interaction between electromagnetic waves and substrate two-level fluctuators is characterized.

Results offer new understanding of superconductor-environment interactions at mK temperatures.

Abstract

Planar superconducting resonators are now being increasingly used at mK temperatures in a number of novel applications. They are also interesting devices in their own right since they allow us to probe the properties of both the superconductor and its environment. We have experimentally investigated three types of niobium resonators - including a lumped element design - fabricated on sapphire and SiO_2/Si substrates. They all exhibit a non-trivial temperature dependence of their centre frequency and quality factor. Our results shed new light on the interaction between the electromagnetic waves in the resonator and two-level fluctuators in the substrate.