Nonlinear flux creep resistivity and anisotropy of transport critical current in YBaCuO single crystals with unidirected twins

Abstract

Taking into account a partial pinning of vortices by twin planes (TPs) in titled magnetic field, we derive new formulas for angular dependence of the critical current density and nonlinear flux creep resistivity of superconductor with unidirected twins in the case when both transport current and magnetic field vectors are in the a− b plane of crystal and arbitrary values of angles θ j and θ between them and TP are considered. We also compare our theoretical results with the earlier published experimental voltage-angle characteristics of YBaCuO single crystals with unidirected twins measured at different current densities at fixed temperature in the presence of magnetic field H = 1.5 T. In this way we have obtained the activation energy of flux creep, the predicted transport critical current angle dependence, the deppining angle θ ∗ (for θ > θ ∗ there is no partial pinning of vortices by TPs), and the value of critical current anisotropy α= J // / J⊥ for H // TP and H ⊥ TP respectively.