| Research Abstract |
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
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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 field
4. 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
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