Theoretical Studies on Electronic State and Superconductivity in Oxide Superconductors

氧化物超导体中的超导电性只出现在金属绝缘体相变附近的金属区域，弄清该区域附近电子态的性质是理解高T_c超导电性机制的关键之一。我们用d-p混合模型计算了包括自旋和电荷涨落引起的多体效应在内的铜轨道和O轨道的电子态密度。我们发现，由于多种体效应，载流子掺杂使电荷转移绝缘体的带隙内形成了新的能级。所得结果定性地解释了光电子能谱强度和光学电导的实验结果。我们在14篇论文中发表了这些结果。理论上在费米能级上得到的新态有望包含大的电荷涨落。这种涨落在超导体…机制中起引力作用的可能性铜氧化物超导体由于具有层状结构，在超导态下也表现出各种反常的电磁性质。我们提出了高T_c超导电性主要出现在CuO_2面上的模型，而其他层上的超导电性是由邻近效应引起的。从该模型出发，我们计算了各层离子的隧道电导、光学电导各向异性、奈特位移和核磁弛豫率。利用我们的模型可以统一地理解实验观测中物理量出现的变化。上述模式中的通量线结构具有很强的各向异性。我们预测了本征钉扎效应和磁化曲线的反常行为。这些结果发表在14篇论文中。上述结果的一部分已在英国布莱顿第19届国际低温物理会议的应邀演讲中发表。较少

The superconductivity in the oxide superconductors appears only in the metallic region close to the metalinsulator transition, and to clarify the properties of electronic states around this region is considered one as of the keys to understand the mechanism of the high-T_c superconductivity. We calculated the electronic density of states selfconsistently by use of the d-p mixing model among Cu-orbits and O-orbits, including many body effects induced by spin and charge fluctuations. We have found that new states are formed inside the charge transfer insulator gap with carrier doping due to many body effects. The obtained results explain qualitatively the experimental results of spectral intensity of photoemission and optical conductivity. We published these results in 14 papers. The theoretically obtained new states at the Fermi level are expected to include large charge fluctuation. A possibility for such fluctuation to work as an attractive force in the mechanism of the superconductiv … More ity is under investigation at present.Due to their layered structures, cuprate oxide superconductors also show various anomalous electronic and magnetic properties in the superconducting state. We have proposed the model in which the high-T_c superconductivity appears primarily in the CuO_2 planes by certain attractive force and superconductivity on the other layers is induced by the proximity effect. From this model, we calculated tunneling conductance, anisotropy of the optical conductivity, Knight shift and nuclear magnetic relaxation rates of ions on various layers. Variety in appearance of physical quantities in the experimental observation can be understood in a unified way by use of our model. Structure of flux line in the above mentioned model are expected to be very anisotropic. We predicted the intrinsic pinning effect and anomalous behaviors of magnetization curves. These results are published in 14 papers. A part of the above results was presented in the invited talks at the 19-th international conference of low temperature physics in Brighton, England. Less