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- Measurements of microwave vortex response in dc magnetic fields in Tl2Ba2CaCu2O8+x films
Measurements of microwave vortex response in dc magnetic fields in Tl2Ba2CaCu2O8+x films
in: IEEE Transactions on Applied Superconductivity (2019)
There is a renewed interest in superconductors for high-frequency applications, leading to a reconsideration of already known low-Tc and high-T, materials. In this view, we present an experimental investigation of the millimeter-wave response in moderate magnetic fields of Tl2Ba2CaCu2O8+x superconducting films with the aim of identifying the mechanisms of the vortex-motion-induced response. We measure the dc magnetic-field-dependent change of the surface impedance, ΔZs(H) = ΔRs(H)+iΔXs(H) at 48 GHz by means of the dielectric resonator method. We find that the overall response is made up of several contributions, with different weights depending on the temperature and field: a possible contribution from Josephson or Abrikosov-Josephson fluxons at low fields; a seemingly conventional vortex dynamics at higher fields; a significant pair breaking in the temperature region close to Tc. We extract the vortex motion depinning frequency fp, which attains surprisingly high values. However, by exploiting the generalized model for relaxational dynamics we show that this result come from a combination of a pinning constant kp arising from moderate pinning, and a vortex viscosity η with anomalously small values. This latter fact, implying large dissipation, is likely a result from a peculiar microscopic structure and thus poses severe limits to the application of Tl2Ba2CaCu208+x in a magnetic field.