Exact diagonalization study of optical conductivity in two-dimensional Hubbard model

The optical conductivity \sigma(\omega) in the two-dimensional Hubbard model is examined by applying the exact diagonalization technique to small square clusters with periodic boundary conditions up to \sqrt{20} X \sqrt{20} sites. Spectral-weight distributions at half filling and their doping dependence in the 20-site cluster are found to be similar to those in a \sqrt{18} X \sqrt{18} cluster, but different from 4 X 4 results. The results for the 20-site cluster enable us to perform a systematic study of the doping dependence of the spectral-weight transfer from the region of the Mott-gap excitation to lower-energy regions. We discuss the dependence of the Drude weight and the effective carrier number on the electron density at a large on-site Coulomb interaction.Comment: 5 pages, 5 figure

Resonant inelastic x-ray scattering in one-dimensional copper oxides

The Cu K-edge resonant inelastic x-ray scattering (RIXS) spectrum in one-dimensional insulating cuprates is theoretically examined by using the exact diagonalization technique for the extended one-dimensional Hubbard model with nearest neighbor Coulomb interaction. We find the following characteristic features that can be detectable by RIXS experiments: (i) The spectrum with large momentum transfer indicates the formation of excitons, i.e., bound states of holon and doublon. (ii) The spectrum with small momentum transfer depends on the incident photon energy. We propose that the RIXS provides a unique opportunity to study the upper Hubbard band in one-dimensional cuprates.Comment: 3 pages with 4 figures, minor changes, to appear in Phys.Rev.

Mott gap excitations in twin-free YBa2Cu3O7-d (Tc = 93 K) studied by RIXS

Mott gap excitations in the high-Tc superconductor of the optimal doped YBa2Cu3O7-d (Tc = 93 K) have been studied by the resonant inelastic x-ray scattering method. Anisotropic spectra in the ab-plane are observed in a twin-free crystal. The excitation from the one-dimensional CuO chain is enhanced at 2 eV near the zone boundary of the b* direction, while the excitation from the CuO2 plane is broad at 1.5-4 eV and almost independent of the momentum transfer. Theoretical calculation based on the one-dimensional and two-dimensional Hubbard model reproduces the observed spectra by taking the different parameters of the on-site Coulomb energy. The fact of the Mott gap of the CuO chain site is much smaller than that of CuO2 plane site is observed for the first time

Nucleon decay matrix elements with the Wilson quark action: an update

We present preliminary results of a new lattice computation of hadronic matrix elements of baryon number violating operators which appear in the low-energy effective Lagrangian of (SUSY-)Grand Unified Theories. The contribution of irrelevant form factor which has caused an underestimate of the matrix elements in previous studies is subtracted in this calculation. Our results are 2$\sim$4 times larger than the most conservative values often employed in phenomenological analyses of nucleon decay with specific GUT models.Comment: LATTICE99(matrixelements), 3 pages, 2 figure

Momentum Dependence of Charge Excitations in the Electron-Doped Superconductor Nd1.85Ce0.15CuO4: a RIXS Study

We report a resonant inelastic x-ray scattering (RIXS) study of charge excitations in the electron-doped high-Tc superconductor Nd1.85Ce0.15CuO4. The intraband and interband excitations across the Fermi energy are separated for the first time by tuning the experimental conditions properly to measure charge excitations at low energy. A dispersion relation with q-dependent width emerges clearly in the intraband excitation, while the intensity of the interband excitation is concentrated around 2 eV near the zone center. The experimental results are consistent with theoretical calculation of the RIXS spectra based on the Hubbard model

Momentum-resolved charge excitations in high-Tc cuprates studied by resonant inelastic x-ray scattering

We report a Cu K-edge resonant inelastic x-ray scattering (RIXS) study of high-Tc cuprates. Momentum-resolved charge excitations in the CuO2 plane are examined from parent Mott insulators to carrier-doped superconductors. The Mott gap excitation in undoped insulators is found to commonly show a larger dispersion along the [pi,pi] direction than the [pi,0] direction. On the other hand, the resonance condition displays material dependence. Upon hole doping, the dispersion of the Mott gap excitation becomes weaker and an intraband excitation appears as a continuum intensity below the gap at the same time. In the case of electron doping, the Mott gap excitation is prominent at the zone center and a dispersive intraband excitation is observed at finite momentum transfer

Nonlinear optical response and spin-charge separation in one-dimensional Mott insulators

We theoretically study the nonlinear optical response and photoexcited states of the Mott insulators. The nonlinear optical susceptibility \chi^(3) is calculated by using the exact diagonalization technique on small clusters. From the systematic study of the dependence of \chi^(3) on dimensionality, we find that the spin-charge separation plays a crucial role in enhancing \chi^(3) in the one-dimensional (1D) Mott insulators. Based on this result, we propose a holon-doublon model, which describes the nonlinear response in the 1D Mott insulators. These findings show that the spin-charge separation will become a key concept of optoelectronic devices.Comment: 5 pages with 3 figures, to appear in PRB RC, 15 August 200