Field-modulated microwave surface resistance in a single-crystal ${\mathrm{Tl}}_2$${\mathrm{Ca}}_2$${\mathrm{BaCu}}_2$${\mathrm{O}}_8$ superconductor

We report magnetic-field-modulated microwave surface resistance in a single-crystal ${\mathrm{Tl}}_2$ ${\mathrm{Ca}}_2$ ${\mathrm{BaCu}}_2$ ${\mathrm{O}}_8$ superconductor. At a fixed temperature, as the applied magnetic field is varied, the observed signal has two features: an intense sharp peak at low field and a broad maximum at high field. The peak field defined for the broad maximum does not have the same temperature dependence as the irreversiblity line determined from dc magnetization measurement. The results are analyzed on the basis of recent theoretical models. It is found that the flux-flow resistivity ${\mathrm{\rho }}_{\mathit{f}}$∝B for B<${\mathit{B}}_{\mathrm{peak}}$ and ${\mathrm{\rho }}_{\mathit{f}}$=const for B>${\mathit{B}}_{\mathrm{peak}}$. Scaling of the data at different T and B gives a universal function for the surface resistance.