arXiv subjects

K. Tsutsui

Publications and source records attributed to K. Tsutsui.

At least 19 recordsLinked to original sources

arXiv2025

Bimagnon dispersion of La2CuO4 probed by resonant inelastic X-ray scattering

We report on the study of the magnetic excitations of Mott insulator La2CuO4 by using resonant inelastic x-ray scattering (RIXS) and cluster calculations within the framework of exact diagonalization. Our results demonstrate experimentally that the bimagnon excitation in Cu L-edge RIXS is enhanced if the incident x-ray energy is slightly above the absorption edge. Through incident-energy-dependent momentum-resolved RIXS, we investigated the excitation of the bimagnon with predominantly A1 symmetry. The bimagnons of La2CuO4 exhibit a nearly flat dispersion with momentum along the Cu-O bond direction. This observation agrees with the bimagnon dispersion from the calculations on a single-band Hubbard model rather than a Heisenberg model with only the nearest neighbor exchange interaction. This means that the effect of the higher-order spin couplings such as the cyclic or ring exchange interactions caused by the coherent motion of electrons beyond nearest-neighbor sites is important for understanding the bimagnon dynamics of cuprates.

cond-mat.str-el↗

arXiv2012

Studying Superfluid Transition of a Dilute Bose Gas by Conserving Approximations

We consider the Bose-Einstein transition of homogeneous weakly interacting spin-0 particles based on the normal-state Phi-derivable approximation. Self-consistent calculations of Green's function and the chemical potential with several approximate Phi's are performed numerically as a function of temperature near Tc, which exhibit qualitatively different results. The ladder approximation apparently shows a continuous transition with the prefactor c=2.94 for the transition-temperature shift given in terms of the scattering length a and density n. In contrast, the second-order, particle-hole, and fluctuation-exchange approximations yield a first-order transition. The fact that some standard Phi's predict a first-order transition challenges us to clarify whether or not the transition is really continuous.

cond-mat.quant-gas↗

arXiv2011

Spin and Orbital Characters of Excitations in Iron Arsenide Superconductors Revealed by Simulated Fe L-Edge RIXS

We theoretically examine the orbital excitations coupled to the spin degree of freedom in the parent state of the iron-arsenide superconductor, based on the calculation in a five-band itinerant model. The calculated Fe $L_3$-edge resonant inelastic x-ray scattering (RIXS) spectra disclose the presence of spin-flip excitations involving several specific orbitals. Magnon excitations predominantly composed of a single orbital component can be seen in experiments, although its spectral weight is smaller than spin-flipped interorbital high-energy excitations. The detailed polarization and momentum dependence is also discussed with predictions for the experiments.

cond-mat.str-el↗

arXiv2009

Hole Binding around Ni Impurity in Cuprates

We examine the influence of Ni impurity in cuprates on the distribution of hole carriers by performing numerically exact diagonalization calculations for a model consisting of Cu3d, Ni3d, and O2p orbitals. Using realistic parameters for the system, we find that a hole is predominantly bound to O2p orbitals around the Ni impurity forming the Zhang-Rice doublet. This imposes strong restrictions on modeling Ni-substituted cuprates. We propose a resonant inelastic x-ray scattering experiment for Ni K-edge to confirm hole binding around the Ni impurity.

cond-mat.str-el↗

arXiv2007

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.

cond-mat.str-el↗

arXiv2007

Momentum Dependent Charge Excitations of 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.

cond-mat.str-el↗

arXiv2005

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.

cond-mat.str-el↗

arXiv2005

Theoretical study of angle-resolved two-photon photoemission in two-dimensional insulating cuprates

We propose angle-resolved two-photon photoemission spectroscopy (AR-2PPES) as a technique to detect the location of the bottom of the upper Hubbard band (UHB) in two-dimensional insulating cuprates. The AR-2PPES spectra are numerically calculated for small Hubbard clusters. When the pump photon excites an electron from the lower Hubbard band, the bottom of the UHB is less clear, but when an electron in the nonbonding oxygen band is excited, the bottom of the UHB can be identified clearly, accompanied with additional spectra originated from the spin-wave excitation at half filling.

cond-mat.str-el↗

arXiv2005

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

The optical conductivity σ(ω) 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.

cond-mat.str-el↗

arXiv2004

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.

cond-mat.str-el↗

arXiv2004

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.

cond-mat.str-el↗

arXiv2003

Mott Gap Excitations and Resonant Inelastic X-Ray Scattering in Doped Cuprates

Predictions are made for the momentum- and carrier-dependent degradation of the Mott gap upon doping in high-Tc cuprates as would be observed in Cu K-edge resonant inelastic x-ray scattering (RIXS). The two-dimensional Hubbard model with second- and third-nearest-neighbor hopping terms has been studied by numerical exact diagonalization. Special emphasis is placed on the particle-hole asymmetry of the Mott gap excitations. We argue that the Mott gap excitations observed by RIXS are significantly influenced by the interaction between charge carriers and antiferromagnetic correlations.

cond-mat.str-el↗

arXiv2002

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.

cond-mat.str-el↗

arXiv2001

Electronic Structure of Mott Insulators Studied by Inelastic X-ray Scattering

The electronic structure of Mott insulators continues to be a major unsolved problem in physics despite more than half-century of intense research efforts. Well-developed momentum-resolved spectroscopies such as photoemission or neutron scattering cannot probe the full Mott gap. We report observation of dispersive charge excitations across the Mott gap in a high Tc parent cuprate Ca2CuO2Cl2 using high resolution resonant inelastic x-ray scattering shedding light on the anisotropy of Mott-gap. The results provide direct support for the Hubbard model to describe charge excitations across the Mott gap.

cond-mat.str-el↗

arXiv2000

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 χ^(3) is calculated by using the exact diagonalization technique on small clusters. From the systematic study of the dependence of χ^(3) on dimensionality, we find that the spin-charge separation plays a crucial role in enhancing χ^(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.

cond-mat.str-el↗

arXiv2000

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.

cond-mat.str-el↗

arXiv1999

Momentum Dependence of Resonant Inelastic X-Ray Scattering Spectrum in Insulating Cuprates

The resonant inelastic x-ray scattering spectrum in insulating cuprates is examined by using the exact diagonalization technique on small clusters in the two-dimensional Hubbard model with second and third neighbor hopping terms. When the incident photon energy is tuned near the Cu K absorption edges, we find that the features of the unoccupied upper Hubbard band can be extracted from the spectrum through an anisotropic momentum dependence. They provide an opportunity for the understanding of the different behavior of hole- and electron-doped superconductors.

cond-mat.str-el↗

arXiv1999

Difference of optical conductivity between one- and two-dimensional doped nickelates

We study the optical conductivity in doped nickelates, and find the dramatic difference of the spectrum in the gap ($ω$$\alt$4 eV) between one- (1D) and two-dimensional (2D) nickelates. The difference is shown to be caused by the dependence of hopping integral on dimensionality. The theoretical results explain consistently the experimental data in 1D and 2D nickelates, Y$_{2-x}$Ca$_x$BaNiO$_5$ and La$_{2-x}$Sr$_x$NiO$_4$, respectively. The relation between the spectrum in the X-ray aborption experiments and the optical conductivity in La$_{2-x}$Sr$_x$NiO$_4$ is discussed.

cond-mat.str-el↗