Surface resistance of grain-aligned YBa2Cu3Ox bulk: Evidence for two kinds of weak link.

Abstract

We have measured the magnetic-field and temperature dependence of the surface resistance ${\mathit{R}}_{\mathit{s}}$ of grain-aligned ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{x}}$. Experiments were conducted on ab-plane- and ac-plane-oriented samples, with the dc magnetic field parallel and perpendicular to the sample surface. We explain the nonlinear behavior of ${\mathit{R}}_{\mathit{s}}$ at low field using a model with an array of Josephson weak links and the linear dependence on magnetic field at high field using an oscillatory-vortex-motion-loss model. The model allows a quantitative fit to the experimental data using the lower critical field of the grain, the entry field ${\mathit{H}}_{\mathit{c}1\mathit{J}}$ of weak links, the junction decoupling parameter ${\mathit{H}}_{\mathit{d}}$, and the vortex viscosity as fitting parameters. Two different sets of parameters ${\mathit{H}}_{\mathit{d}}$ and ${\mathit{H}}_{\mathit{c}1\mathit{J}}$ were used to fit the data, suggesting that in grain-aligned ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{x}}$ bulk material at least two kinds of weak link are responsible for low-field losses. An estimation of the Josephson-junction area was obtained from field-modulated microwave absorption. In addition, thermal-excitation and radiation-activation mechanisms for rf losses are compared.