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

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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.