Thermodynamic study of mixed states of quasi-two-dimensional superconductors

通过测量各种热力学和动力学量，研究了层状超导体，特别是Bi_2Sr_2CaCu_2O_y中涡旋的性质。在热力学测量中，我们观察到了一级涡旋熔化转变时的磁化跳变。通过对不同氧含量的样品的测量，我们发现系统的各向异性对熔化线有影响。然而，由于相变点的熵变太小，我们无法探测到相变点的比热异常。在动态测量方面，我们进行了局部磁化强度和电阻率的同时测量。我们发现，在涡旋液相的电阻率是非欧姆以上的熔化转变。该结果表明，由于液体的高粘度，即使在液相中，钉扎仍然是相关的。此外，我们开发了一种新的技术来测量噪声的局部涡密度使用一个微型霍尔探头和FFT分析仪。我们发现当漩涡开始移动时会有很大的噪音。甚至在涡旋液相中也观察到这种噪声，这进一步表明涡旋液体中的钉扎。为了研究涡旋的层间相干性，还研究了约瑟夫森等离子体共振。

By measuring various thermodynamic and dynamic quantities, we studied the nature of vortices in layred superconductors, especially in Bi_2Sr_2CaCu_2O_y. As for the thermodynamic measurements, we observed the magnetization jump at the first-order-vortex-melting transition. From the measurements on the samples with different oxygen contents, we revealed that the melting line is affected by the anisotropy of the system. However, we could not detect the specific heat anomalies at the transition because the entropy change at the transition was too small.As for the dynamic measurements, we performed the simultaneous measurements of the local magnetization and resistivity. We found that the resistivity in the vortex liquid phase is non-ohmic just above the melting transition. This result suggests that pining is still relevant even in the liquid phase because of the high viscosity of the liquid. Moreover, we developed a novel technique to measure the noise of the local vortex density using a miniature Hall probe and an FFT analyzer. We found a large noise when the vortices start moving. Such a noise was observed even in the vortex liquid phase, which is a further indication of the pinning in the vortex liquid. To examine the interlayr coherence of the vortices, Josephson plasma resonance was also investigated.