Microwave surface impedance anisotropy of YBa$_2$Cu$_3$O$_x$ single crystals with different oxygen content

TL;DR

This study measures the microwave surface impedance anisotropy of high-quality YBa₂Cu₃Oₓ single crystals across various oxygen contents, revealing how the anisotropic conductivity components evolve with temperature and oxygen doping.

Contribution

It provides the first comprehensive analysis of the temperature dependence of surface impedance along different crystallographic axes in a wide oxygen content range.

Findings

Surface impedance anisotropy varies with oxygen content.

The c-axis conductivity's imaginary part shows similar temperature dependence to the ab-plane.

Oxygen doping affects the convexity of the conductivity temperature dependence.

Abstract

The linear microwave response of ultra high-quality YBa$_2$Cu$_3$O$_x$ single crystals grown in BaZrO$_3$ crucibles is measured at 9.4 GHz in rf magnetic fields parallel and perpendicular to the $ab$-plane in the temperature range 5$\leq T\leq 200$ K. Having found the analytic solution for the magnetic field distribution on the sample surface we determine both the surface impedance $Z_{ab}=R_{ab}+iX_{ab}$ in the $ab$-plane and $Z_c=R_c+iX_c$ along c-axis of the crystals. For the first time the evolution of the $Z_{ab}(T)$ and $Z_c(T)$ dependences on the same sample and in a wide range of oxygen content is obtained. For $x=6.95$ (optimum oxygen content) the temperature dependence of the imaginary part $\sigma_{c}''(T)/\sigma_{c}''(0)$ of the c-axis conductivity is found to be strikingly similar to that of $\sigma_{ab}''(T)/\sigma_{ab}''(0)$ and becomes more convex with $x$ lowering.