Non-linear Microwave Surface Impedance of Epitaxial HTS Thin Films in Low DC Magnetic Fields

TL;DR

This study investigates the non-linear microwave surface impedance of epitaxial YBaCuO thin films under low dc magnetic fields, revealing diverse field-dependent behaviors and universal trends in reactance.

Contribution

It provides new insights into how low dc magnetic fields influence the microwave surface impedance of epitaxial HTS thin films, highlighting both universal and sample-specific responses.

Findings

Different films show varied residual surface resistance and penetration depth.

Application of dc magnetic field alters both the magnitude and functional dependence of surface impedance.

Some films exhibit a decrease in surface resistance below zero-field values under dc magnetic fields.

Abstract

We have carried out non-linear microwave (8 GHz) surface impedance measurements of three YBaCuO thin films in dc magnetic fields $H_{dc}$ (parallel to c axis) up to 12 mT using a coplanar resonator technique. In zero dc field the three films, deposited by the same method, show a spread of low-power residual surface resistance, $R_{res}$ and penetration depth, $\lambda$ (T=15 K) within a factor of 1.9. However, they exhibit dramatically different microwave field, $H_{rf}$ dependences of the surface resistance, $R_s$, but universal $X_s(H_{rf})$ dependence. Application of a dc field was found to affect not only absolute values of $R_s$ and $X_s$, but the functional dependences $R_s(H_{rf})$ and $X_s(H_{rf})$ as well. For some of the samples the dc field was found to decrease $R_s$ below its zero-field low-power value.