Phase transition and size effect in Josephson vortex state

为了了解高温超导体中的涡旋状态，必须对高质量的单晶样品进行测量。然而，在现实中，不可能有完美的样品，这完全排除了钉住的影响。为了使钉钉尽可能地逃离样品，至少应该避免表面屏障，因为表面屏障会掩盖和扭曲样品的体积特性。这种实验以前没有人做过。高温超导体中超导态的准二维行为等最具特征的性质对强磁场下的超导态影响很大。在强各向异性超导体Bi_2Sr_2CaCu_2O_<8+δ>中，效果显著。因此，高温超导体涡旋态的相图与传统的相图有很大的不同。此外，当磁场平行于二维平面施加时，约瑟夫森涡旋的形成与变化随温度、磁场、磁场相对于样品的角度等的变化而变化。当磁场稍微倾斜时，由于烙饼涡的附加钉钉效应，会出现交叉点阵，使现象变得更加复杂。在本研究中，为了避免表面钉住，采用Corbino盘法测量电阻率，采用微型线圈法测量交流磁响应，从而可以探索Bi_2Sr_2CaCu_2O<8+δ>在整个角度区域从烧饼到交格态，从交格到约瑟夫森态，从固态到液态的涡相图。其中的特点有：1。相对于磁场强度，约瑟夫森涡旋态有两个明显不同的相，这种相变似乎可能是二级相。峰效应出现在相变之前的交叉晶格态。有两个涡流液相。这些是在不同磁场条件下由于不同的钉钉效应而产生的诱导相。高场Josephson涡旋状态下的涡旋动力学与涡流煎饼液相完全不同。这些结果有助于理解高温超导体中的约瑟夫森涡旋态和固体涡旋相。少

It is inevitable to make measurement on high quality single crystal samples in order to understand the vortex state in high temperature superconductors. In reality, it is, however, not possible to have perfect samples, which completely exclude the effect of pinning. To make pinning flee sample as much as possible, one should avoid at least the surface barriers, which mask and distort the bulk properties. This sort of experiment has not been done before.The most characteristic properties such as the quasi two-dimensional behavior of the superconducting state in high temperature superconductors affect the superconducting state very much in strong magnetic fields. In strongly anisotropic superconductor Bi_2Sr_2CaCu_2O_<8+δ>, the effect is remarkable. Hence, the phase diagram of the vortex state in high temperature superconductors is very different from that of the conventional ones. Furthermore, when magnetic field is applied parallel to the two-dimensional plane, Josephson vortices are f … More ormed and variety of vortex states is expected as functions of temperature, magnetic field, angle of magnetic field with respect to the sample, etc. When magnetic field is tilted a little bit, the crossing lattice will appear and the phenomena become more complicated due to additional pinning effect of pancake vortices.In this study, to avoid the surface pinning the Corbino disk method is used for the electrical resistivity measurement and the miniature coil method is used for the ac magnetic response measurement As a result we could explore the vortex phase diagram of the Bi_2Sr_2CaCu_2O<8+δ>, from pancake to crossing lattice state, from crossing lattice to Josephson state, from solid to liquid state in the whole angle region. The characteristic features among them, are as follows :1. There are two distinctly different phases in the Josephson vortex state with respect to the magnetic field strength, and this transition seems to possibly be the second order phase transition.2. The peak effect is found in the crossing lattice state just prior to the phase transition.3. There are two vortex liquid phases. These are induced phase due to different pinning effects in the diferent regime of magnetic field4. The vortex dynamics in the Josephson vortex state in high fields is entirely different from the vortex pancake liquid phase.These results contributed to the understanding ofthe Josephson vortex state as well as solid vortex phase in high temperature superconductors. Less