Neutron Diffraction Study of Magnetic Flux-Line Lattice Structures in High Temperature Superconducting Oxides.

在本项目的前两年中，我们采用了小角中子散射方法，在后一年中采用了透射中子退极化测量方法，以研究超导氧化物YBa_2Cu_3O_7和Bi_2SrCaCu_2O_8的磁通线晶格结构。线晶格结构的超导金属铌，我们试图在类似的实验条件下测量布拉格峰从超导氧化物。然而，由于氧化物的Landau-Ginzburg参数kappai为100，而金属铌的Landau-Ginzburg参数kappai为4，我们未能观测到它。于是，我们改用第二种方法，测量了几乎100%极化中子穿过超导氧化物的去极化。我们发现，中子的退极化对磁通线晶格结构的状态非常敏感。从实验中我们观察到，中子的退极化与施加在样品上的磁场的变化、样品温度的变化以及样品温度变化后的恒温时间的关系。总之，我们已经发现必须满足几个实验条件以便从超导氧化物中的磁通线晶格结构测量布拉格峰，通过超导氧化物对极化中子的去极化的观察提供了一个非常合理的工具来检查和表征磁通线晶格结构的状态。

For the first 2 years of this project we have applied a method of small angle neutron scattering and for the last 1 year another method of depolarization measurement of transmitted neutrons in order to study the magnetic flux-line lattice structures in superconducting oxides YBa_2Cu_3O_7 or Bi_2SrCaCu_2O_8.In the first method after confirming that we can observe a Bragg peak from the magnetic tlux-line lattice structures in superconducting metal niobium, we tried to measure in similar experimental conditions a Bragg peak from those in superconducting oxides. However we failed to observe it, because the Landau-Ginzburg parameter kappais 100 for the oxides in comparison with that of 4 in niobium metal.Then we changed to the second method in which we measured the depolarization of almost 100% polarized neutrons that were transmitted through the superconducting oxides. We found that the depolarization of neutrons was very much sensitive to the state of the magnetic flux-line lattice structures.From this experiment we observed the dependence of the depolarization of neutrons on a change of the magnetic field applied to the samples, a change of the temperature of the samples and also an elapsed time at constant temperature after it was changed.In conclusion, we have found that several experimental conditions must be fulfilled in order to measure a Bragg peak from the magnetic flux-line lattice structure in the superconducting oxides, and that the observation of the depolarization of polarized neutrons through the superconducting oxides provides a very sensible tool to examine and characterize the state of the magnetic flux-line lattice structures.