Creep and depinning of vortices in nontwinned YBa2Cu3O6.87 single crystal

Анотація

We present the results of transport study of vortex dynamics in YBa₂Cu₃O_6.87 crystals in the magnetic field H||c. In low magnetic fields, H < 4 kOe, the measurements were performed in the range of vortex velocities n=10^-4–2 m/s, which covers thermal creep and flux flow modes. The pinning force F_p nonmonotonically depends on magnetic field in both modes, though low-field minimum in the F_p(H) curve shifts to higher fields with increased velocity n, that is interpreted as partial ordering the vortex lattice. The increase of the pinning force F_p upon increasing the field, which is observed in the flux flow mode in fields H ≥ 3 kOe, is interpreted by presence of finite transverse barriers. The barriers result in preserving the entangled vortex solid phase for the above-barrier vortex motion along the action of the Lorentz force. We also show that field variation of the depinning current has a single maximum, while field variation of the pinning force inside deep creep mode has two maxima. Appearance of two maxima is associated with nonmonotonous field variation of the activation energy U_pl, which corresponds to plastic vortex creep mediated by motion of the dislocations.