K. Tsutsui

Momentum Dependence of Charge Excitations in the Electron-Doped Superconductor Nd1:85Ce0:15CuO4: A Resonant Inelastic X-Ray Scattering Study

We report a resonant inelastic x-ray scattering (RIXS) study of charge excitations in the electron-doped high-T(c) superconductor Nd1.85 Ce0.15 CuO4. 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.

Mott gap excitations in twin-free YBa2Cu3O7-delta (Tc=93 K) studied by resonant inelastic x-ray scattering.

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.

Momentum-dependent charge excitations of a two-leg ladder: Resonant inelastic x-ray scattering of(La,Sr,Ca)14Cu24O41

Momentum dependent charge excitations of a two-leg ladder are investigated by resonant inelastic x-ray scattering of (La,Sr,Ca)14Cu24O41. In contrast to the case of a square lattice, momentum dependence of the Mott gap excitation of the ladder exhibits little change upon hole-doping, indicating the formation of hole pairs. Theoretical calculation based on a Hubbard model qualitatively explains this feature. In addition, experimental data shows intraband excitation as continuum intensity below the Mott gap and it appears at all the momentum transfers simultaneously. The intensity of the intraband excitation is proportional to the hole concentration of the ladder, which is consistent with optical conductivity measurements.

Exact diagonalization study of optical conductivity in the 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.

Resonant two-magnon Raman scattering and photoexcited States in two-dimensional mott insulators.

We investigate the resonant two-magnon Raman scattering in two-dimensional (2D) Mott insulators by using a half-filled 2D Hubbard model in the strong coupling limit. By performing numerical diagonalization calculations for small clusters, we find that the Raman intensity is enhanced when the incoming photon energy is not near the optical absorption edge but well above it, being consistent with experimental data. The absence of resonance near the gap edge is associated with the presence of background spins, while photoexcited states for resonance are found to be characterized by the charge degree of freedom. The resonance mechanism is different from those proposed previously.

Theory of RIXS in strongly correlated electron systems: Mott gap excitations in cuprates

We theoretically examine the momentum dependence of resonant inelastic X-ray scattering (RIXS) spectrum for one-dimensional and two-dimensional cuprates based on the single-band Hubbard model with realistic parameter values. The spectrum is calculated by using the numerical diagonalization technique for finite-size clusters. We focus on excitations across the Mott gap and clarify spectral features coming from the excitations as well as the physics behind them. Good agreement between the theoretical and existing experimental results clearly demonstrates that the RIXS is a potential tool to study the momentum-dependent charge excitations in strongly correlated electron systems.

Electron and phonon dynamics in copper oxides

Abstract We examine the phonon and electron dynamics in the strongly correlated electron systems coupled with phonons by using the numerically exact diagonalization techniques. The total number of phonons in the model is truncated to treat the phonon Hilbert space with finite dimensions. We calculate the phonon excitation spectra and the single-particle excitation spectra in one-dimensional half-filled Hubbard model coupled with phonons. We find that the phonon excitation is strongly influenced by the electron–phonon interaction via hopping of electrons, compared with that via density of electrons. We also find that the character of the spin-charge separation is destroyed by phonons in the single-particle excitation spectra.

Doping and Momentum Dependence of Charge Dynamics in Nd2−xCexCuO4 (x = 0, 0.075, and 0.15) Studied by Resonant Inelastic X‐ray Scattering

We report a resonant x‐ray scattering (RIXS) study at Cu K‐edge in the n‐type cuprate superconductor Nd2−xCexCuO4 from the half‐filled Mott insulator to the Tc = 25 K superconductor. The Mott gap is clearly seen in the spectra of x = 0, while both interband excitation across the Mott gap and intraband excitation in the upper Hubbard band are observed at electron‐doped x = 0.075 and 0.15. The momentum dependence of x = 0.075 is presented and the dispersion relation of x = 0.075 is qualitatively similar to that of x = 0.15. We find in the doping dependence that the intraband excitation at low energy increase in intensity with carrier concentration (x).

Momentum Dependence of Mott Gap Excitations in Optimally Doped YBa2Cu3O7−δ Studied by Resonant Inelastic X‐ray Scattering

Momentum dependence of Mott gap excitations in the twin‐free YBa2Cu33O7−δ at optimal doping has been studied by the resonant inelastic x‐ray scattering at the Cu K‐edge. We have succeeded to distinguish between the excitations from CuO chain and that from the CuO2 plane. The excitation from the chain is enhanced at 2 eV near the zone boundary of the b* direction, while the excitation from the 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 models reproduces these features when we assume a smaller value of on‐site Coulomb repulsion of the chain site than that of the plane site.

Resonant inelastic X-ray scattering in copper oxides

Momentum dependence of resonant inelastic x-ray scattering (RIXS) in one- (1D) and two-dimensional (2D) copper oxides is examined theoretically. Cu K-edge RIXS spectrum is calculated by using the exact diagonalization techniques on small clusters in 1D and 2D Hubbard models with 1s-core bands. We find that the spectral line shape around the zone boundary is sensitive to the nearest-neighbor Coulomb interaction V. Whereas the anisotropic momentum dependence in the 2D case stressed in the previous work [Phys. Rev. Lett. 80 (1999) 3705] remains irrespective of V.