Microwave surface impedance anisotropy of YBa2Cu3Ox single crystals with different oxygen content

The linear microwave response of ultra high-quality YBa2Cu3Ox single crystals grown in BaZrO3 crucibles is measured at 9.4 GHz in rf magnetic fields parallel and perpendicular to the ab-plane in the temperature range 5≤T≤200 K. Having found the analytic solution for the magnetic field distribution on the sample surface we determine both the surface impedance Zab=Rab+iXab in the ab-plane and Zc=Rc+iXc along the c-axis of the crystals. For the first time the evolution of the Zab(T) and Zc(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 σc''(T)/σc''(0) of the c-axis conductivity is found to be strikingly similar to that of σab''(T)/σab''(0) and becomes more convex with x lowering