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

通过测量各种热力学和动力学量，我们研究了层状超导体中涡旋的性质，特别是Bi_2Sr_2CaCu_2O_y。在热力学测量方面，我们观察到在一阶涡熔化转变时的磁化跳变。通过对不同氧含量样品的测量，我们发现熔点线受体系各向异性的影响。然而，由于跃迁处的熵变太小，我们无法检测到跃迁处的比热异常。在动态测量方面，我们同时进行了局部磁化率和电阻率的测量。我们发现涡流液相的电阻率在熔点上方是非欧姆的。这一结果表明，由于液体的高粘度，即使在液相中，收缩仍然是相关的。此外，我们还开发了一种利用微型霍尔探头和FFT分析仪测量局部涡旋密度噪声的新技术。当涡旋开始移动时，我们发现有很大的噪音。即使在涡旋液相中也观察到这种噪声，这进一步表明涡旋液相中存在钉住现象。为了检验涡旋的层间相干性，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.