Measurements of Microwave Vortex Response in DC Magnetic Fields in Tl2Ba2CaCu2O8+x Films

There is a renewed interest in superconductors for high-frequency applications, leading to a reconsideration of already known low-T c and high-T c materials. In this view, we present an experimental investigation of the millimeter-wave response in moderate magnetic fields of Tl 2 Ba 2 CaCu 2 O 8+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, Δ Z s (H) = Δ R s (H) + iΔ X s (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 T c . We extract the vortex motion depinning frequency f p , which attains surprisingly high values. However, by exploiting the generalized model for relaxational dynamics we show that this result comes from a combination of a pinning constant k p 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 Tl 2 Ba 2 CaCu 2 O 8+x in a magnetic field.