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The results of substitution of A (with A = Ni and Zn) for Cu are presented in the high Tc superconductors, YBa₂(Cu_{1 - x}A_x)₃O_{6 + δ} where x < 0.03 for Zn and x< 0.10 for Ni. We have observed that the superconducting temperature in the high Tc YBa₂Cu₃O_{6 + δ} as system decreases if copper is substituted by Ni and Zn and the substitution of Zn affects Tc much more than Ni. We discuss our results in terms of pair breaking by magnetic impurities and in terms of the Resonance Valence Bond model proposed by Anderson. / Master of Science

The objective of this thesis is to reach theoretical understanding of the unusual relationship between charge-ordering and superconductivity in correlated-electron systems. The competition between these broken symmetries and magnetism in the cuprate high temperature superconductors has been extensively discussed, but exists also in many other correlated-electron superconductors, including quasi-two-dimensional organic charge-transfer solids. It has been suggested that the same attractive interaction is responsible for both charge-order and superconductivity. We propose that the specific interaction is the tendency in correlated-electron systems to form spin-singlet bonds, which is strongly enhanced at the commensurate carrier density p of ½ a charge carrier per site, characteristic of all superconducting charge-transfer solids. To probe superconductivity driven by electron correlations, a necessary condition is that electron-electron interactions enhance superconducting pair-pair correlations, relative to the non-interacting limit. We have performed state of the art numerical calculations on the two-dimensional Hubbard model on different triangular lattices, as well as other lattices corresponding to K-BEDT-TTF based organic charge transfer solids, for the complete range of carrier densities per site p (0 ≤ p ≤ 1). We have shown that pair-pair correlation for each cluster is enhanced by electron-electron interaction only for p ≃ 0.5, far away from the density range thought to be important for superconductivity. Although initial focus is on charge-transfer solids, the results of the research will impact the field of correlated electrons as a whole. We believe our calculations will provide fundamental and fresh insight to the theory of superconductivity in strongly correlated systems.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第21555号 / 理博第4462号 / 新制||理||1640(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 前野 悦輝, 教授 石田 憲二, 教授 寺嶋 孝仁 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

京都大学 / 新制・課程博士 / 博士(理学) / 甲第23688号 / 理博第4778号 / 新制||理||1684(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 柳瀬 陽一, 教授 川上 則雄, 教授 松田 祐司 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

In this thesis we study the edge currents and the multi-band superconductivity in the unconventional superconductor Sr2RuO4. Numerous measurements have given strong support for a topologically non-trivial time-reversal symmetry breaking chiral p-wave state in this material. However, the spontaneous edge current expected for this order has eluded experimental detection. In this thesis, we present a general theoretical description of the edge currents in chiral superconductors. Our results elucidate the connection between the edge currents and the topological property of the chiral pairing. On this basis, we argue that superconducting gap anisotropy, combined with surface disorder, may provide an explanation for the absence of observable edge currents in Sr2RuO4. In addition, contrary to intuitive expectations, the integrated edge current is found to identically vanish for any non-p-wave chiral superconductor in the continuum limit-- a result which may be connected with the orbital angular momentum problem in chiral superfluids, such as the A phase of He-3. In lattice models, the integrated edge current may not vanish in non-p-wave superconductors but, in general, is substantially smaller compared to that of a simple chiral p-wave. In a separate study, we investigate the multi-band nature of the superconductivity in Sr2RuO4, via explicit microscopic calculations of the multi-band interactions. Our results indicate comparable pairing correlations on all of the bands and the existence of soft collective phase fluctuations--a Leggett mode. We also examine the possibility of alternative time-reversal symmetry breaking multi-band superconductivity which does not necessarily require chiral p-wave pairing. / Thesis / Doctor of Philosophy (PhD)

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