S. Nishimoto

Coexistence of distinct charge fluctuations in θ-(BEDT-TTF)2X

Using the Lanczos exact-diagonalization and density-matrix renormalization group methods, we study the extended Hubbard model at quarter filling defined on the anisotropic triangular lattice. We focus on charge ordering (CO) phenomena induced by onsite and intersite Coulomb interactions. We determine the ground-state phase diagram including three CO phases, i.e., diagonal, vertical, and three-fold CO phases, based on the calculated results of the hole density and double occupancy. We also calculate the dynamical density-density correlation functions and find possible coexistence of the diagonal and three-fold charge fluctuations in a certain parameter region where the onsite and intersite interactions compete. Furthermore, the characteristic features of the optical conductivity for each CO phase are discussed.

Low-lying excitations of the three-leg spin tube: A density-matrix renormalization group study

Using the (dynamical) density-matrix renormalization group method, we study the low-energy physics of three-leg antiferromagnetic Heisenberg model where the periodic boundary conditions are applied in the rung direction. We confirm that the spin excitations are always gapped as long as the intraring couplings form a regular triangle. From precise finite-size-scaling analyses of the spin gap and dimerization order parameter, we also find that the spin gap is collapsed by very small asymmetric modulation of the intraring couplings. Moreover, the dynamical spin structure factors on the intraleg and interleg correlations are calculated. It is demonstrated that the low-lying structure of the interleg spectra is particularly affected by the asymmetric modulation.

Ring-exchange mechanism for triplet superconductivity in a two-chain Hubbard model : Possible relevance to Bechgaard salts

The density-matrix renormalization group method is used to study the ground state of the two-chain zigzag-bond Hubbard model at quarter filling. We show that, with a proper choice of the signs of hopping integrals, the ring-exchange mechanism yields ferromagnetic spin correlations between interchain neighboring sites and produces an attractive interaction between electrons as well as long-range pair correlations in the spin-triplet channel, thereby leading the system to triplet superconductivity. We argue that this mechanism may have possible relevance to the superconductivity observed in Bechgaard salts.

Nonlocal interactions in doped cuprates : Correlated motion of Zhang-Rice polarons

In-plane, inter-carrier correlations in hole doped cuprates are investigated by ab initio multiconfiguration calculations. The dressed carriers display features that are reminiscent of both Zhang-Rice (ZR) CuO4 singlet states and Jahn-Teller polarons. The interaction between these quasiparticles is repulsive. At doping levels that are high enough, the interplay between long-range unscreened Coulomb interactions and long-range phase coherence among the O-ion half-breathing vibrations on the ZR plaquettes may lead to a strong reduction of the effective adiabatic energy barrier associated to each polaronic state. Tunneling effects cannot be neglected for a relatively flat, multi-well energy landscape. We suggest that the coherent, superconducting quantum state is the result of such coherent quantum lattice fluctuations involving the in-plane O ions. Our findings appear to support models where the superconductivity is related to a lowering of the in-plane kinetic energy.

Double exchange ferromagnetism in the Peierls insulator state.

We study the effects of opening the band gap on the double exchange ferromagnetism. Applying the density-matrix renormalization group method and an analytical expansion from the dimer limit to the one-dimensional double exchange model, we demonstrate for a relevant region of the exchange coupling that, in the weak dimerization regime, the Peierls gap opens in the fully spin-polarized conduction band without affecting its ferromagnetism, whereas in the strong dimerization regime, the ferromagnetism is destroyed, and the Mott gap opens instead, leading the system to the antiferromagnetic quasi-long-range order. An insulator version of double exchange ferromagnetism is thus established.

Mobile Zhang-Rice singlets: Electron correlations and bond-length fluctuations in copper oxides

We perform first principles, multiconfiguration calculations on clusters including several CuO

Renormalization of quasiparticle hopping integrals by spin interactions in layered copper oxides

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Phase diagram of the one-dimensional Hubbard model with next-nearest-neighbor hopping

octahedra and study the ground-state electron distribution and electron--lattice couplings when holes are added to the undoped

Near degeneracy and pseudo Jahn-Teller effects in mixed-valence ladders: The phase transition in Na V 2 O 5

d^9 p^6

Disorder and superconductivity in doped semiconductor nanotubes

configuration. We find that the so-called Zhang--Rice state on a single CuO