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

Abstract

We present the results of a transport study of vortex dynamics in YBa2Cu3O6.87 crystals in magnetic field H∥c. At low magnetic fields, H<4kOe, the measurements were performed in the range of vortex velocities v=10−4–2m∕s, which covers the thermal creep and flux flow modes. The pinning force Fp depends nonmonotonically on magnetic field in both modes, though the low-field minimum in the Fp(H) curve shifts to higher fields with increasing velocity v, which is interpreted as partial ordering of the vortex lattice. The increase of the pinning force Fp with increasing field, which is observed in the flux flow mode in fields H⩾3kOe, is interpreted by the 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 the field variation of the pinning force in the deep creep mode has two maxima. The appearance of two maxima is associated with nonmonotonic field variation of the activation energy Upl, which corresponds to plastic vortex creep mediated by the motion of dislocations.