熔融织构YBa_2Cu_3O_(7-δ)晶体中磁通涡旋锁定转变反常行为研究

Abnormal behaviors in lock-in transition of the vortices in melt textured growth of YBa_2Cu_3O_(7-δ) crystals

通过改变磁场与c轴方向夹角测量了熔融织构YBa_2Cu_3O_(7-δ)(YBCO)晶体的磁力矩信号响应,观察到了磁通涡旋系统的锁定(lock-in)转变行为以及锁定转变角正比于外磁场强度的反常现象.基于GinzburgLandau理论和磁通涡旋线Kink结构模型,对上述锁定转变反常现象进行了分析讨论,提出了熔融织构YBCO晶体中存在平行于a-b面的延展性关联缺陷结构假设,导出了锁定转变临界角与温度和磁场之间的关系,理论分析模型结果与实验测量结果基本符合.

The magnetization behavior of the layered anisotropic high-Tc superconductor in the mixed state H_（c1） H H_（c2） has a feature that when the angle θ between the applied magnetic field H and the CuO plane（a-b plane） is less than a critical value（θ≤θ_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⊥（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 Cu O 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 YBa_2Cu_3O_（7-δ）（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.