MgCNi3, MgB2, La1.87Y0.13CuO4 초전도체들에서 보이는 자기 소용돌이 동역학 연구

Abstract

The vortex dynamics of superconducting materials MgCNi3, MgB2 and La1.87Y0.13CuO4 was studied. The La1.87Y0.13CuO4 (LYCO) superconductor was expectedto show an antiferromagnetic insulating behavior at a first glance but superconductivity of Tc = 17 K has been reported when Y3+ was in the La site. In spite of the urgent need to identify the origin of the superconductivity,this compound was not studied in detail because of the limited sample availability. One of the first questions to be answered was about the superconducting dimensionality. Analysis of I-V characteristics of LYCO thin film showed three-dimensional (3D) superconducting scaling behavior, although this material has a quasi-2D crystal structure. This scaling behavior of this material was very similar to other 3D superconductors, such as Mo3Si, MgB2,and YBa2Cu3O7, but not that of 2D Bi2Sr2Ca2Cu3Ox. From these results, 3D scaling behavior was confirmed.While the scaling behavior of the vortex matter is readily predictable from the theory based on elastic manifolds in vortex lattice in a random pinning circumstance, the peak effect (PE: sudden increase of critical current nearthe end of the superconductivity) has been a puzzling problem in vortex physics. Recently I have successfully grown MgCNi3 single crystals which aresuitable for study of the PE. With this new superconductor, the PE by using DC and AC excitation currents in a strip geometry is investigated. Remarkably, critical current (Ic) measured by using DC showed very sharp transition incontrast to previously reported results based on edge contamination scheme in anisotropic vortex system in NbSe2. Furthermore, Ic determined from ACresponse of the system exhibits collapsing peak as the frequency of driving current increases. Current-voltage characteristics at different frequencies and transient behavior of vortex matter supports existence of AC induced flux creep. I attribute the AC induced creep to isotropic nature of vortices in MgCNi3 single crystals. Since it becomes clear that the PE originates from dynamical reason, transient behavior of vortex matter in the vicinity of the PE has to be understood. In principle, to see the real space flow pattern of vortex matter is the ultimate task. However, any experimental tool now at hand to visualizemoving vortices is not available. So, noise investigation from transport measurement is still one of the best ways to deduce vortex follow pattern. In this thesis, new result from MgCNi3 showing that chaotic intermittency of vortex flow is presented together with random telegram type noise. Then, the result will be related to chaotic theory. In the last chapter of the thesis, I present temperature dependent surface superconductivity of MgB2. The temperature dependence of the ratio Hc3/Hc2 at different angles was qualitatively well described by a recent theory based onthe two-gap nature. The angular dependence of this ratio shows that it can be both enhanced and reduced, which can be explained by the two-band theory. Theslight deviations of Hc3/Hc2 from the theory of the half-infinite geometry are to be accommodated by effects due to the finite geometry and to the orientation of the applied field relative to the crystal plane.