Theoretical Study of Exotic Superconductivity and Magnetism

在分析超导跃迁对核磁共振弛豫速率影响的基础上，发现铜酸盐超导体对的对称性有利于d&lt；xy&gt；在d对称性之间的配对。我们考虑了电子与铜和氧之间的离位斥力U&lt；Pd&gt；，计算了转变温度与空穴数的关系。在计算中，我们遵循U&lt；Pd&gt；的RPA处理，在强的现场斥力U_d被重整化为准粒子状态后。计算结果与实验结果定性一致。在对二维金属中反铁磁嵌套自旋涨落性质研究的基础上，发现在正常状态下观察到的反常金属相，如“Spin Gap…More“和”自旋-电荷分离“，可以统一地解释为当重整化费米面处于技术嵌套状态时，自旋涨落的模模耦合对巡回定域对偶图像的影响。为了研究超导态下的de Haas-van Alphen(DHvA)效应的性质，我们采用Landau轨道作为单粒子态的基础进行了计算。结果表明，如果对的对称性是各向同性的S波，则dHvA振荡只能在靠近上临界场的很窄的区域内观察到。这反过来又表明，当dHvA振荡远低于上临界场时，费米面上的点(S)或线(S)上的带隙消失的各向异性超导体应该被考虑。我们讨论了波数空间配对和实空间配对之间的交叉问题，这是因为铜酸盐超导体的相干长度很短，是晶格常数的几倍，应该位于交叉区。批判性地分析了Nozieres&Schmitt-Rink理论在二维情况下应用的困难。结果表明，用Kosterlitz&Thouless形式主义来处理超计数转变是可以避免困难的。较少

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 … More " 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