Magnetic and Thermodynamic Properties of Layered and Thin Film Superconductors
Doctoral thesis
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Date
2004Metadata
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- Institutt for fysikk [2707]
Abstract
A large variety of measurements, most of magnetic nature, performed on single crystals of high temperature superconductors and thin films of lead, are presented. Furthermore, numerical calculations of the magnetic properties of thin films and thin cylinders have been carried out within the framework of the Ginzburg-Landau theory. The results of these calculations are compared with the experimental results.
Observation of diamagnetic response in measurements of ac-susceptibility with low amplitude on high temperature superconductors are explained by Cooper pairs, which are formed above the critical temperature. In a region above the critical temperature these superconductor are characterised by phase fluctuations in the order parameter.
Near the transition temperature we observe critical behaviour in the specific heat and the longitudinal elastic stiffness along the c-axis of the crystal. We show that the critical behaviour of these two quantities is similar. The results appear to be consistent with the 3D-XY model.
For thin films both experimental and numerical result show that the critical field at zero temperature is inversely proportional to the thickness. Furthermore, the numerical results for thin cylinders show that the critical field is inversely proportional to the diameter.
Our numerical results also reveal a change in the order of the Meissner state to normal metal phase transition. We show that this phase transition is first order for thick films and cylinders and second order for thin films and cylinders.
Furthermore, we present preliminary results of a paramagnetic response for fields perpendicular to the c-axis in single crystals of the high temperature superconductor La2-xSrxCuO4. These results are compared to previous experimental result for paramagnetism in superconductors.