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Microwave hysteretic losses in YBa 2 Cu 3 <mml:math …

1995; American Physical Society; Volume: 51; Issue: 10 Linguagem: Inglês

10.1103/physrevb.51.6686

1095-3795

P. P. Nguyen, D.E. Oates, G. Dresselhaus, M. S. Dresselhaus, Allan Anderson,

Advanced Condensed Matter Physics

Measurements of the nonlinear surface impedance of high-quality epitaxial YBCO and granular NbN films as a function of temperature (4.2--91 K) and frequency (1--13 GHz) are presented. The microwave-field-dependent surface impedance ${\mathit{Z}}_{\mathit{s}}$(${\mathit{H}}_{\mathrm{rf}}$) for both YBCO and NbN films increases quadratically with increasing ${\mathit{H}}_{\mathrm{rf}}$ in the low- and intermediate-rf-field region (${\mathit{H}}_{\mathrm{rf}}$50 Oe for YBCO at 77 K and for NbN at 13.4 K). In the high-rf-field region, ${\mathit{Z}}_{\mathit{s}}$(${\mathit{H}}_{\mathrm{rf}}$) changes to a different functional dependence of ${\mathit{H}}_{\mathrm{rf}}$. Through the use of a modified Bean critical-state model, the results for the high-rf-field region are explained quantitatively by hysteresis losses due to penetration of microwave vortices. The value of the microwave-vortex penetration field ${\mathit{H}}_{\mathit{p}}$(T) for YBCO thin films, which is nearly frequency independent, is low compared with the dc lower critical fields. The critical- current density values obtained from fits to the hysteresis model are consistent with those measured via dc transport.