太田 幸則

Using a proposed numerical technique for calculating anomalous Green's functions characteristic of superconductivity, we show that the low-lying excitations in a wide parameter and doping region of the two-dimensional t-J model are well described by the picture of dressed Bogoliubov quasiparticles in the BCS pairing theory. The pairing occurs predominantly in the dx2-y2-wave channel and the energy gap has a size DELTAd congruent-to 15J-0.27J between quarter and half fillings. Opening of the superconducting gap in the photoemission and inverse-photoemission spectra is demonstrated.

The effective Hamiltonian which describes the magnetic properties of La2CuO4-type crystals is derived by taking into account the spin-orbit interaction. It is shown that the weak ferromagnetism is caused by the superexchange interaction through the multi-orbitals on the ligand ions. The Hamiltonian is applied to the analysis of the magnetic properties of La2CuO4-type crystals.

An exact diagonalization technique for small clusters is used to study the electronic structure of the Hubbard model with nearest-neighbor repulsive interaction. The phase diagram of the model is extracted from the calculated charge and spin correlation functions and single-particle excitation spectra. It is shown that the small Fermi surface is realized in a parameter and doping region, which may provide a complementary understanding of the large Fermi surface believed to exist in the Hubbard model.

A Comment on the Letter by W. Stephan and P. Horsch, Phys. Rev. Lett. 66, 2258 (1991). © 1994 The American Physical Society.

Effects of long-range Coulomb interaction in one-dimensional electron systems are studied. By using a quantum Monte Carlo method, we obtain that the zero-frequency wave-number dependent charge susceptibility at quarter-filling shows both 2kF and 4kF singularities, kF being the Fermi wave number, in contrast with the Hubbard model with the on-site and nearest-neighbor interactions. The results are in accord with the X-ray experiments in (NMP)x(Phen)1-x(TCNQ). The power-law exponent α in the spectral function ρ{variant}(ω) ∼ |ω|α observed in the high-resolution photoemission experiments is calculated by using the exact diagonalization method and found to increase with increasing the range of the interaction. © 1994.

An exact-diagonalization technique on small clusters is used to study excitation spectra and superconductivity in the two-dimensional t-J model. We demonstrate that the low-lying excitations in a wide parameter (J/t) and doping regions of the model are described well by the picture of dressed Bogoliubov quasiparticles within the BCS pairing theory. The pairing occurs predominantly in dx2-y2-wave channel and the energy gap has a size Δd{reversed tilde equals}0.15J-0.27J between quarter and half fillings. © 1994.

An exact-diagonalization technique on small clusters is used to study excitations and superconductivity in the two-dimensional negative-U Hubbard model. We calculate the Bogoliubov-quasiparticle spectrum, condensation amplitude, and coherence length as a function of the coupling strength ( U t), thereby working out how the picture of Bogoliubov quasiparticles in the BCS superconductors is affected by increasing the attraction U t. © 1994.

The effective spin Hamiltonians for the distorted NiO2 and CuO2 planes with low-temperature orthorhombic (LTO) and low-temperature tetragonal (LTT) structures are derived by taking into account the spin-orbit coupling and structural distortion as perturbations. We explain why the weak ferromagnetism occurs in both LTO and LTT phases for cuprates whereas it does only in the LTT phase for nickelates. The spin-wave gaps observed by neutron scattering experiments are analyzed. We find that the gaps of the in-plane and out-of-plane modes in La1.65Nd0.35CuO4 are caused, respectively, by anisotropic superexchange and direct-exchange interactions. © 1994.

Moriya's perturbation theory is applied to the distorted NiO2 and CuO2 planes, and the effective spin Hamiltonians are derived. Spin-wave theory is explored for the Hamiltonians, and the spin-wave excitations are studied for nickelates and cuprates with low-temperature orthorhombic and low-temperature tetragonal (LTT) structures. It is shown that in the cuprates the origin of the anisotropy responsible for the spin-wave gaps is different for in-plane and out-of-plane modes: i.e., anisotropic superexchange for the in-plane mode, and anisotropic direct exchange for the out-of-plane mode. This difference explains the observed structural dependence of the spin-wave gaps in La1.65Nd0.35CuO4. In nickelates, the single-ion anisotropy is solely responsible for the spin-wave gaps, which are observed in La2NiO4. We discuss the mechanism of the weak ferromagnetism; in the nickelates the Dzyaloshinski-Moriya (DM) interaction causes the weak ferromagnetism in the LTT phase where an appropriate spin configuration is prepared by the single-ion anisotropy. In the cuprates, where the single-ion anisotropy for preparing the spin configuration is absent, the DM and pseudodipolar interactions themselves determine the spin configuration: the weak ferromagnetism originates only from the competition between these two interactions. It is proposed that a multiorbital effect is essential to explain the observed weak ferromagnetism in the LTT phase of La1.65Nd0.35CuO4. © 1994 The American Physical Society.

An exact-diagonalization technique on small clusters is used to calculate the single-particle spectral function for the one-band and three-band Hubbard models. We show that the strongly momentum-dependent spectral-weight transfer induced by carrier doping leads to the evolution of the dispersion of low-energy states. The quasiparticle-like band narrowing is examined.

The dispersive electronic state emerging upon carrier-doping in cuprates, i.e., the so-called ingap state, is derived in the three-band and one-band Hubbard models by means of the exact diagonalization of finite-size clusters. The dispersive state near the charge transfer gap at half filling undergoes a strongly momentum-dependent transfer of the spectral weight by carrier doping, and evolves into the new ingap state at the edge of the gap with a free-electron-like dispersion characteristic of a large Fermi surface consistent with Luttinger's sum rule. It is shown that the extended Hubbard model with strong nearest-neighbor repulsive interaction, on the other hand, forms a small Fermi surface by carrier doping and provides a counter-example of the doped cuprates. © 1993.

The effective Hamiltonian for the CuO2 plane of high-Tc cuprates is derived by taking into account the spin-orbit interaction. A finite-size exact-diagonalization technique is applied to the Hamiltonian, and the magnetic and electronic structures of the CuO2 plane in the low-temperature orthorhombic (LTO) and low-temperature tetragonal (LTT) phases of the La2CuO4-type crystals are examined. It is shown that the contribution from the in-plane oxygen 2pz orbital perpendicular to the plane is essential for the emergence of the weak ferromagnetism in the LTO-phase La2CuO4. In the LTT phase, either the uniaxial antiferromagnetism or the weak ferromagnetism is induced, depending on how the 2pz orbital contributes to the anisotropic superexchange interaction of the single Cu-O-Cu bond. We study the dynamics of a hole in an extended t-J model, and show that the lattice distortion works to change the symmetry of the electronic ground state via the spin-orbit coupling. The effect is, however, small and seems irrelevant to the anomalous properties of La1.88Ba0.12CuO4. © 1993 The American Physical Society.

An exact diagonalization technique on small lattices is used to calculate the single-particle spectral function A (k,omega) for one- and three-band Hubbard models at various parameter values and doping rates, thereby examining the low-energy electronic structure (or in-gap state) of the doped CuO2 plane in high-T(c) superconducting cuprates. It is illustrated that, by carrier doping, the dispersive state of the charge excitation gap at half filling undergoes a strongly momentum-dependent spectral-weight transfer, and evolves into the new state with a free-electron-like dispersion that forms the in-gap state characteristic of a large Fermi surface consistent with Luttinger's sum rule. The quasiparticlelike band narrowing observed is shown to depend sensitively on the parameter values and doping rate.

層状ペロブスカイト構造を持つ銅酸化物にキャリアを導入すると絶縁体から金属に変化する. そして, 絶縁体相と金属相の中間領域で高温超伝導状態が出現する. このような銅酸化物の特長は電子間に強いクーロン相互作用が働いていることであり, 固体物理学の多くの難問を包含している. まだ高温超伝導発現機構の解明までには距離があるものの, 多彩な物質群の発見に助けられて高温超伝導体の電子構造の姿がかなり明確に浮かび上がってきた. 高温超伝導発現機構解明のための条件が整備されつつある. 銅酸化物の電子構造に対する最近の研究を概観する.

The 63Cu nuclear quadrupole resonance (NQR) frequency of a variety of high-Tc cuprates is analyzed by a cluster model to examine the origin of the electronic field gradient and its implications on the electronic ground state of the CuO2 plane. It is shown that a primary contribution from the Cu 3d(x2-y2) hole is cancelled largely by the contribution from the Cu 4p(x,y) electrons, and this cancellation explains both the large material dependence of the frequency in the insulating phase and an extremely small frequency observed in the electron-doped cuprates. Measured NQR frequencies thus provide rich information on the ground-state charge distribution in the CuO2 plane. © 1992, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

A combined ionic- and cluster-model approach is used to examine the electronic structure of the CuO2 plane in a variety of insulating cuprates. The role of Madelung potential in the charge-transfer type insulator is clarified through the analysis of the optical energy gap and superexchange interaction. The implication for the HighTc superconductivity is discussed. © 1991.

The electronic structure of cuprates consists of the orbitals of both copper and oxygen and is dependent sensitively on the crystal structure. Correlation between the energy level structure and superconducting transition temperature Tc is examined in the ionic and cluster models. It is found that the energy level of the apex oxygen is of crucial importance for the electronic state of the CuO2 plane and controls the optimum Tc's as well as carrier doping. A cluster model analysis is made on the properties of doped carriers. © 1991.

A finite-size exact diagonalization technique is applied for the two-dimensional spin- 1 2 Heisenberg Hamiltonian with the Dzyaloshinski-Moriya(DM) interaction to examine the ground state magnetic structure of the insulating CuO2 plane in the low-temperature orthorhombic (LTO) and tetragonal (LTT) phases of the La2CuO4-type crystal. It is demonstrated that the weak ferromagnetism induced in the LTO phase is accompanied by a stabilization of the antiferromagnetism with the planar anisotropy. It is predicted that there are two possibilities in the magnetic ground states in the LTT phase, depending on the direction of the DM vector: the weak ferromagnetic state similar to that of the LTO phase and the antiferromagnetic state characterized by the strong uniaxial anisttropy of the quantum-mechanical origin. A fluctuation for the chiral outlet state is suppressed in the former but is enchanged in the latter. © 1991.

A cluster-model analysis is made on the material dependence of the optical charge-transfer gap and antiferromagnetic superexchange interaction of a variety of insulating cuprates. It is shown that the electronic structure of cuprates typically of the charge-transfer type is characterized by the unique energy-level separation that reflects the three dimensionality of the crystal via the long-range Madelung potential; such characteristics are absent in the Mott-Hubbard regime.

It is proposed that ΔVA, the difference in Madelung site potential between apex and in-plane-oxygens defined independently to the doping rate, is a primary parameter for the electronic states of the CuO2plane and governs the optimum Tc's throughout all the families of Cu-oxide superconductors. Microscopic origin of the Tcversus ΔVAcorrelation is discussed in relation to the stability of Zhang-Rice singlet. © 1990, Elsevier Science Publishers B.V. (North-Holland). All rights reserved.

Electronic configurations in the Pb2Sr2Y1-xCaxCu3O8+ superconducting layered compounds are examined through the calculation of the Madelung energy in the ionic model. It is shown that the carrier distributions in the compounds exhibit strong contrast with those in the well-understood Ba2YCu3O6+ compounds despite their structural similarity. We demonstrate how the charge transfer between structural building blocks, i.e., the copper-oxygen planes and the charge reservoir, controls the region (x,) where the superconductivity emerges. The existence of new superconducting regions separable from the known one is predicted. © 1990 The American Physical Society.

We examine the electronic states in the electron-doped copper oxide superconductors, Nd2-xCexCuO4, with the cluster model approach. In the oxides, the Cu-4s orbital plays an important role under electron-doping. We find that in a possible parameter range the doping causes a composite state with spin-singlet, which is made of Cu-3d, Cu-4s and O-2p orbitals. The Cu-3d valence-band photoemission spectra are calculated. © 1989.