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

1)2<12><24><41>）为了阐明基于f^2电子组态的重费米子准粒子相互作用的性质，我们用数值重整化群方法研究了它的杂质模型，发现轨道间的相互作用可以支配轨道内的相互作用.这为我们理解f^2基重费米子超导电性的微观机制，如UPt_3提供了线索。3）用双圈PRG方法研究了金属在反铁磁量子临界点（AFQCP）的近藤效应。研究发现，当费米面存在对应于反铁磁性的热点或热线时，多通道近藤效应的不动点是鲁棒存在的。这为所谓的局域量子提供了微观基础 关于我们 4）提出了一个新的自旋三重态超导体Sr_2RuO_4的微观模型，其中Ru位4d和O位2 p的有效杂化，使得2 p电子的质量在准粒子带中不可忽略。发现O位的在位排斥引起的短程铁磁关联，即使在对配对相互作用的二阶微扰计算中，也很容易产生自旋-三重态配对. 5）为了研究重费米子化合物CeCu_2（Si，Ge）_2中Ce的量子临界跃迁及其对非常规超导电性的影响，本文研究了一维系统中具有U_2<fc>，f-电子与传导电子间库仑排斥的推广的安德森模型，发现离位超导配对的趋势在参数区附近占主导地位，在该参数区，价态从近藤区到价态涨落区呈现出急剧的交叉。用a-la名古屋的1/N展开法和Wilson的NRG方法计算了Ce基重费米子中的场，发现它对压力下的电阻率和Ce离子的价态跃迁的温度依赖性有很大的影响。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