Abnormal behaviors in lock-in transition of the vortices in melt textured growth of YBa2Cu3O7- crystals

Abstract

The magnetization behavior of the layered anisotropic high-Tc superconductor in the mixed state Hc1 H Hc2 has a feature that when the angle between the applied magnetic field H and the CuO plane (a-b plane) is less than a critical value ( L), the vortex lattice is converted from three-dimensional structure into two-dimensional structure, forming a phenomenon so called the lock-in transition, where the flux lines are completely parallel to the a-b plane, and the vertical component of the magnetic induction B丄 (perpendicular to the a-b plane) is consequently zero. So far, there have still existed the differences in the physical explanation of the lock-in phenomenon. For the lock-in phenomenon occurring in the region between the CuO planes, it can be considered to be caused by the transverse Meissner effect. However, for the one occurring in other extended correlated defect areas, such as twin boundaries in YBa2Cu3O7- (YBCO) crystal, this phenomenon is believed to be the results of the energy linearization of the vortices trapped in the defect channels. Many theoretical and experimental studies have revealed the existence of the lock-in behaviors related to the microstructure properties of the superconductor crystals. Therefore, the research of the lock-in transition behavior will be helpful to understand the intrinsic pinning properties of the layered anisotropic superconductors, and the phase transition process in the vortex system. In this paper, we systematically measure the magnetic torque signal in melt texture growth YBCO (MTG-YBCO) bulk and observe an abnormal lock-in transition behavior in the vortex system. The critical angle of the lock-in transition is found to be directly proportional to the strength of the magnetic field, which is contrary to the observations in the common cases. According to the framework of the Ginzburg-Landau theory and the kink structure model of the vortex line, we discuss the abnormal phenomenon, and propose that there is a type of extend-correlated defect structure, which is parallel to the a-b plane, in the MTG-YBCO crystal. The relationship between the critical angle of the lock-in transition to the temperature and the magnetic field is established theoretically, and the theoretical results coincide well with the torque measurements.