Unusual features in the nonlinear microwave surface impedance of Y-Ba-Cu-O thin films

TL;DR

This paper investigates the nonlinear microwave surface impedance of high-quality YBaCuO thin films, revealing complex behaviors in resistance and consistent reactance patterns, with implications for understanding high-temperature superconductor properties.

Contribution

It reports novel observations of nonlinear surface impedance features in YBaCuO thin films and discusses potential mechanisms behind these behaviors.

Findings

Surface resistance varies unpredictably with microwave field at different temperatures.

Surface reactance consistently follows a specific functional form.

Findings suggest complex underlying mechanisms affecting nonlinear microwave response.

Abstract

Striking features have been found in the nonlinear microwave (8 GHz) surface impedance $Z_s=R_s + jX_s$ of high-quality YBaCuO thin films with comparable low power characteristics [$R_{res}\sim 35--60 \mu\Omega$ and $\lambda_L(15 K)\sim 130--260 nm$]. The surface resistance $R_s$ is found to increase, decrease, or remain independent of the microwave field $H_{rf}$ (up to 60 mT) at different temperatures and for different samples. However, the surface reactance $X_s$ always follows the same functional form. Mechanisms which may be responsible for the observed variations in $R_s$ and $X_s$ are briefly discussed.