Effect of flux flow on microwave losses in YBa2Cu3O7−x superconducting films

Abstract

Microwave losses in YBa2Cu3O7−x (YBCO) films carrying overcritical currents were measured at several temperatures between 74 and 85 K, and found to be in good agreement with the predictions of the vortex dynamics model for the magnetic flux flow. For the measurements a 2.3-GHz microstrip resonator made of an epitaxial double-sided YBCO film on CeO2 buffered sapphire was equipped with a Cu coil, which is usually used for ac (1.1 kHz) inductive measurements of the critical current density JC. The ac current density induced in the film, Jind controlled the vortex flow, while the microwave surface impedance (ZS=RS+iXS) of the film was measured as a function of Jind. Both RS and XS remained fairly constant for Jind<JC, indicating a weak contribution of the thermally activated flux flow to the microwave loss. The loss revealed a sharp increase at Jind≈JC. In the flux flow regime, Jind>~2JC, both ΔRS and ΔXS were linear with Jind. This behavior of the microwave losses agrees well with the dependence ΔZS=ρf/2λL predicted by the vortex dynamics model (λL is the London penetration depth and ρf is the complex flux flow resistivity of type-II superconductors). The ratio ΔXS/ΔRS in the flux flow regime was independent of Jind and increased with decreasing temperature from 2.7 at 85 K to 7.9 at 74 K, in agreement with the model.

Source-reported materials — not catalogue approval

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