Shigeru Koikegami

Perturbation Theory on the Transition Temperature and Electronic Properties of Organic Superconductor

We study the superconducting transition temperature and the electronic properties of the metallic phase of κ-type (BEDT-TTF) 2 X which shows unconventional properties in experiments, on the basis o...

Electronic Structure and Transition Temperature of the d- p Model

The dressed d -electron Greens function of the d - p model is computed self-consistently based on the fluctuation-exchange approximation. Using this Greens function and assuming the d x 2 - y 2 symmetry for the momentum dependence of the gap function, we obtain the reasonable superconducting transition temperature T c in the several cases where the d -( p -)hole number is close to the experimental value of La 1.76 Sr 0.24 CuO 4 based on NMR/NQR analyses. Using this Greens function, we calculate the d -electron spin susceptibility, which has very sharp incommensurate peaks near T c for the low frequency as observed in neutron scattering data on La 1.86 Sr 0.14 CuO 4 . The quasi-particle excitation spectrum in the normal state near T c is also obtained from this Greens function. The energy dispersion of quasi-particles near the Fermi level is almost flat around the (π/ a ,0) point. This electronic structure is due to many-body effects, rather than being induced by band effects.

Strong Coupling Approach to the d- p Model on the Basis of Fermi Liquid Theory

We study the superconducting transition temperature T c of the bilayer d-p model with d x 2 - y 2 -wavelike attractive interaction based on the formalism first employed by Nozieres and Schmitt-Rink. In the strong coupling regime, T c obtained through this formalism are much suppressed, compared with those through Thouless criterion only. We also find that, whether the interlayer coupling exsists or not, T c is almost propotional to the Fermi energy E F in the strong coupling regime. Thus, we can reproduce the essential nature in the underdoped region.We study the superconducting transition temperature T c of the bilayer d-p model with d x 2 - y 2 -wavelike attractive interaction based on the formalism first employed by Nozieres and Schmitt-Rink. In the strong coupling regime, T c obtained through this formalism are much suppressed, compared with those through Thouless criterion only. We also find that, whether the interlayer coupling exsists or not, T c is almost propotional to the Fermi energy E F in the strong coupling regime. Thus, we can reproduce the essential nature in the underdoped region.

Uniaxial-Pressure Induced Ferromagnetism of Enhanced Paramagnetic Sr3Ru2O7

We report a uniaxial pressure-dependence of magnetism in layered perovskite strontium ruthenate Sr 3 Ru 2 O 7 . By applying a relatively small uniaxial pressure, greater than 0.1 GPa normal to the RuO 2 layer, ferromagnetic ordering manifests below 80 K from the enhanced-paramagnet. Magnetization at 1 kOe and 2 K becomes 100 times larger than that under ambient condition. Uniaxial pressure dependence of Curie temperature T C suggests the first order magnetic transition. Origin of this uniaxial-pressure induced ferromagnetism is discussed in terms of the rotation of RuO 6 octahedra within the RuO 2 plane.

Antiferromagnetic and Superconducting Correlations on the d-p Model

We study superconducting fluctuation effects on the quasi-two-dimensional d - p model, which can describe significant features of high- T c superconducting cuprates. We adopt the fluctuation exchange approximation in order to derive both normal and anomalous vertices of renormalized d -electron propagators, and then solve the Bethe-Salpeter equation for the anomalous vertex and derive t -matrix as a superconducting fluctuation propagator. We calculate the normal d -electron self-energy self-consistently, and get a self-consistent solution for the circumstance in which the system is close to both antiferromagnetic and superconducting instabilities. The solution exhibits the suppression of the density of states near (±π,0) and (0,±π), which reflects the experimental result on the normal state of underdoped cuprates.

Superconducting gap of the two-dimensional d-p model with small Ud

The superconductivity of the two-dimensional d–p model up to the second order of the Coulomb repulsion U d is investigated in the weak-coupling formalism. Near the half-filling, the superconducting gap function has b 1 g symmetry. The dependences of the gap magnitude on both the total hole density and the ratio of the number of d-holes to the number of p-holes are also studied. These dependences make it clear that the presence of the Van Hove singularity can make gap magnitudes large even in the case of small U d .

Superconductivity in multilayer perovskite : Weak coupling analysis

We investigate the superconductivity of a three-dimensional d–p model with a multilayer perovskite structure on the basis of the second-order perturabation theory within the weak coupling framework. Our model has been designed with multilayer high- T c superconducting cuprates in mind. In our model, multiple Fermi surfaces appear, and the component of a superconducting gap function develops on each band. We have found that the multilayer structure can stabilize the superconductivity in a wide doping range.

Enhancement of Hybridization between Two- and One-Dimensional Bands due to Coulomb and Spin–Orbit Interactions in Sr2RuO4

Phenomenological model calculations of Sr 2 RuO 4 show that the marked enhancement of the hybridization between the two-dimensional d x y band and one-dimensional d y z , z x bands is caused by the Coulomb and spin–orbit interactions. Owing to the enhanced hybridization effect, the van Hove singularity (vHS) of the γ band in the vicinity of M decreases to about 40 meV above the Fermi level ( E F ). Moreover, the vHS approaches E F owing to the electron–phonon interaction (EPI). A few peculiar phonon modes interacting with the one-dimensional orbital component via the vHS can mediate the p -wave paring in Sr 2 RuO 4 .

Kink structure in the electronic dispersion of high-Tcsuperconductors from the electron-phonon interaction

We investigate the electronic dispersion of high-Tc superconductor on the basis of the two-dimensional three-band Hubbard model with the electron-phonon interaction together with the strong electron-electron interaction. In our model, it is shown across the hole-doped region of high-Tc superconductor that the electron-phonon interaction makes a dispersion kink, observed along the nodal direction, and that the small isotope effect appears on the electronic dispersion.

Inhomogeneous Electronic Distribution in High-Tc Cuprates

We theoretically investigate the doping evolution of the electronic state of high-Tc cuprate on both sides of the half-filling on the basis of the three-dimensional three-band Hubbard model with a layered structure using the Hartree–Fock approximation. Once a small amount of holes or electrons are doped into the half-filled state, our model exhibits the charge-transfer insulator-to-metal transition along with a chemical potential jump. At the same time, the doped holes or electrons are inhomogeneously distributed, and they tend to form clusters in the vicinity of the half-filling. This suggests the possibility of microscopic phase separation with the separation between the metallic and the insulating regions.