Theory for New Mechanism of Superconductivity with Repulsive Origin and Local Quantum Critical Phenomena

1)利用微扰重整化群（PRG）方法分析了（TMTSF）_2PF_6和Sr_2Ca__<12>CU_<24>O_<41>等准一维体系的超导态。2)为了阐明基于f^2-电子构型的重费米子准粒子相互作用的性质，采用数值重整化群（NRG）方法研究了重费米子的杂质模型，结果表明轨道间相互作用优于轨道内相互作用。这为理解f^2基重费米子（如UPt_3.3）超导的微观机制提供了线索。本文采用双环PRG方法研究了金属在反铁磁量子临界点（AFQCP）上的近田效应。发现如果费米表面存在与反铁磁性相对应的热点或热线，则多通道近藤效应的不动点牢固存在。这为所谓的局部量子提供了一个微观基础……在一些重费米子化合物的AFQCP周围观察到更多的临界现象。4)我们提出了一种新的自旋三重态超导体Sr_2RuO_4的微观模型，其中ru位的4d和o位的2p有效杂化，使得2p电子的质量在准粒子带中不可忽略。在对相互作用进行二阶微扰计算时，发现由o位斥力引起的短程铁磁相关很容易产生自旋三重态配对。5)为了研究Ce价态的量子临界跃迁及其对重费米子化合物CeCu_2(Si, Ge)_2中观察到的非常规超导性的影响，我们研究了一维系统中U_<fc>， f-和传导电子之间的库仑排斥的扩展Anderson模型。在参数区附近，价态从近藤区向价态涨落区明显交叉，场外超导配对趋势占主导地位。6)采用a la Nagoya的1/ n展开法和Wilson的NRG法研究了Ce基重费米子中晶体电场的影响，发现它对Ce离子在压力下电阻率的温度依赖性和价态跃迁有很大的影响。少

1) We succeeded in understanding the superconducting state of quasi-one dimensional systems, such as (TMTSF)_2PF_6 and Sr_2Ca__<12>CU_<24>O_<41> by analyzing the pairing interaction by perturbation renormalization group (PRG) method.2) In order to clarify the properties of quasi-particle interaction of heavy fermion based on f^2- electronic configuration, we studied its impurity model by the numerical renormalization group (NRG) method, and showed that the inter-orbital interaction can dominate over the intra-orbital one. This gives us a clue to understand a microscopic mechanism of superconductivity of f^2-based heavy fermions, such as UPt_3.3) Kondo effect in a metal at antiferromagnetic quantum critical point (AFQCP) is studied by two-loop PRG method. It is found that the fixed point of multichannel Kondo effect exists robustly if the Fermi surface has hot-spot (s) or hot-line (s) corresponding to the antiferromagnetism. This gives a microscopic basis for the so-called local quantum … More critical phenomena observed around AFQCP of some heavy fermion compounds.4) We proposed a new microscopic model for spin-triplet superconductor Sr_2RuO_4 in which the effective hybridization between 4d at Ru-site and 2p at O-site, making the weight of 2p electrons non-negligible in the quasiparticle band. It is found that the short-ranged ferromagnetic correlations caused by on-site repulsion at O-site can easily give rise to spin-triplet pairing even in 2nd-order perturbation calculation for the paring interaction.5) In order to investigate the quantum critical transition of valence of Ce and its effect on the unconventional superconductivity observed in heavy fermion compounds CeCu_2(Si, Ge)_2, we studied an extended Anderson model with U_<fc>, Coulomb repulsion between f-and conduction electrons in one dimensional system, and found that the tendency of off-site superconducting pairing becomes dominant around parameter region where the valence exhibits sharp crossover from the Kondo to the valence fluctuation regime.6) We investigated the effect of crystalline-electric-field in Ce-based heavy fermion by means of 1/N-expansion method of a la Nagoya and NRG method of Wilson, and found that it causes great influences on temperature dependences of the resistivity under pressure and the valence transition of Ce ion. Less