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

1)用微扰重整化群(PRG)方法分析了(TMTSF)_2PF_6和Sr_2Ca_(&lt)；12&gt；CU_&lt；24&gt；O_&lt；41&gt；等准一维系统的超导态。2)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(NRG)方法研究了它的杂质模型，表明轨道间相互作用可以主导轨道内相互作用。这为我们理解f^2基重费米子超导的微观机制提供了线索，如uPT_3.3)用双环PRG方法研究了金属中反铁磁量子临界点的近藤效应。结果表明，如果费米面存在与反铁磁性相对应的热点(S)或热点(S)，则多通道近藤效应的不动点是稳健存在的。这为所谓的局域量子…提供了微观基础在一些重费米子化合物的AFQCP附近观察到了更多的临界现象。4)我们提出了一个新的自旋三重态超导体Sr2RuO4的微观模型，在该模型中，Ru位的4d和O位的2p之间的有效杂化，使得2p电子在准粒子带中的重量是不可忽略的。结果表明，即使在配对相互作用的二阶微扰计算中，由O位排斥引起的短程铁磁关联也很容易引起自旋-三重态配对。5)为了研究Ce价的量子临界跃迁及其对重费米子化合物CeCu2(Si，Ge)2中观察到的非常规超导电性的影响，我们研究了一个扩展的Anderson模型。6)用名古屋1/N展开方法和Wilson的NRG方法研究了Ce基重费米子的晶态电场效应，发现晶场效应对压强下的电阻率的温度依赖性和Ce离子的价态跃迁有很大的影响。较少

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