| Research Abstract |
On the basis of analysis of the superconducting flusctuation effect on the NMR relaxation rate, which shows drastic decrease just above the transition temperature, it was found that the symmetry of the pairing of cuprate superconductor is favorable for d_<xy>pairing among the d-symmetry. We have performed a calculation for hole-number dependence of the transition temperature by taking into account the effect of the off-site repulsion U_<pd> between electrons on copper and oxygen. In the calculation, we follow the RPA treatment of U_<pd> after the strong on-site repulsion U_d is renormalized into the quasi-particle state. the results are in qualitative agreement with experiments. It was also found that the off-site repulsion U_<pd> easily destroy the d_<x2-y2> pairing.
On the basis of research on the properties of antiferromagnetic nested spin fluctuations in 2D metals, it was found that the anomalous metallic phase observed in the normal state of cuprates, with aspects such as "spin gap
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" and "spin-charge separation", can be explained in a unified way as an effect of the mode-mode coupling of spin fluctuations on the itinerant-localized duality picuture when the renormalized Fermi surface is technically nested.
In order to investigate the property of the de Haas-van Alphen (dHvA) effect in superconducting state, we have carried out the calculations adopting the Landau orbit as a basis of the single particle state. As a result, it turned out that the dHvA oscillations can be observed only in a very narrow region near the upper critical field if the symmetry of the pairing is isotropic s wave. This in turn shows that the anisotropic superconductivity, with the gap vanishing on the point (s) or line (s) on the Fermi surface, should be considered in the case the dHvA oscillations are observed well below the upper critical field.
We have discussed the problem of the crossover between wavenumber-space pairing and real-space pairing being motivated by the fact that the cuprate superconductors, with very short coherence length of a few times the lattice constant, are expected to locate in the crossover region. The difficulty of the Nozieres & Schmitt-Rink theory applied to 2D case was analyzed critically. It was shown that its difficulty can be avoided when the supercounducting transition is treated in the Kosterlitz & Thouless formalism. Less
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