arXiv subjects

T. Tohyama

Publications and source records attributed to T. Tohyama.

At least 55 records · Page 3Linked to original sources

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.

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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.

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arXiv2004

Asymmetry of the electronic states in hole- and electron-doped cuprates: Exact diagonalization study of the t-t'-t''-J model

We systematically examine the asymmetry of the electronic states in the hole- and electron-doped cuprates by using the t-t'-t''-J model. Numerically exact diagonalization method is employed for a 20-site square lattice. We impose twisted boundary conditions (BC) instead of standard periodic BC. For static and dynamical correlation functions, averaging procedure over the twisted BC is used to reduce the finite-size effect. We find that antiferromagnetic spin correlation remains strong in electron doping in contrast to the case of hole doping, being similar to the case of the periodic BC. This leads to a remarkable electron-hole asymmetry in the dynamical spin structure factor and two-magnon Raman scattering. By changing the twist, the single-particle spectral function is obtained for all momenta in the Brillouin zone. Examining the spectral function in detail, we find a gap opening at around the k=(pi,0) region for 10% doping of holes (the carrier concentration x=0.1), leading to a Fermi arc that is consistent with experiments. In electron doping, however, a gap opens at around k=(pi/2,pi/2) and persists up to x=0.2, being correlated with the strength of the antiferromagnetic correlation. We find that the magnitude of the gaps is sensitive to t' and t''. A pseudogap is also seen in the optical conductivity for electron doping, and its magnitude is found to be the same as that in the spectral function. We compare calculated quantities with corresponding experimental data, and discuss similarities and differences between them as well as their implications.

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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.

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arXiv2004

Excitonic effect on optical response in one-dimensional two-band Hubbard model

Motivated by the gigantic nonlinear optical response in the halogen-bridged Ni-compounds, the underlying electronic states of the compounds are examined in the one-dimensional two-band Hubbard model, by studying the current-current correlation function and the charge density in the ground state. The dynamical density matrix renormalization group method is employed. We find that the low-energy peak of the correlation function consists of a single Lorentzian component for a parameter range appropriate to the compounds. This is due to an excitonic state induced by the intersite Coulomb repulsion between holes on the metal and halogen ions. This is consistent with the optical absorption spectra of the compounds. We suggest that the localization of holes on the metal ions in the ground state brings about the formation of the excitonic state.

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arXiv2004

Temperature and Dimensionality Dependences of Optical Absorption Spectra in Mott Insulators

We investigate the temperature dependence of optical absorption spectra of one-dimensional (1D) and two-dimensional (2D) Mott insulators by using an effective model in the strong-coupling limit of a half-filed Hubbard model. In the numerically exact diagonalization calculations on finite-size clusters, we find that in 1D the energy position of the absorption edge is almost independent of temperature, while in 2D the edge position shifts to lower energy with increasing temperature. The different temperature dependence between 1D and 2D is attributed to the difference of the coupling of the charge and spin degrees of freedom. The implications of the results on experiments are discussed in terms of the dimensionality dependence.

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arXiv2004

Dynamical density matrix renormalization group study of photoexcited states in one-dimensional Mott insulators

One-dimensional Mott insulators exhibit giant nonlinear optical response. Based on the dynamical density matrix renormalization group method, photoexcited states and optical response in the insulators are studied as functions of the on-site and the nearest neighbor Coulomb interactions, $U$ and $V$, respectively. We find that the lowest optically-allowed and forbidden excited states, which have odd and even parities, respectively, are degenerate for $V/t\ltsim 2$ with $t$ being the hopping integral of an electron between nearest neighbor sites. For $V/t\gtsim 2$, the bound states with even and odd parities occur and are not degenerate. The nature of the degeneracy and its effect on the optical response are examined.

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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↗

arXiv2003

Friedel oscillations in a two-band Hubbard model for CuO chains

Friedel oscillations induced by open boundary conditions in a two-band Hubbard model for CuO chains are numerically studied. We find that for physically realistic parameters and close to quarter filling, these oscillations have a 2k_F modulation according with experimental results on YBa_2Cu_3O_{7-delta}. In addition, we predict that, for the same parameters, as hole doping is reduced from quarter filling to half filling, Friedel oscillations would acquire a 4k_F modulation, typical of a strongly correlated electrons regime. The 4k_F modulation dominates also in the electron doped region. The range of parameters varied is very broad, and hence the results reported could apply to other cuprates and other strongly correlated compounds with quasi-one dimensional structures. On a more theoretical side, we stress the fact that the copper and oxygen subsystems should be described by two different Luttinger liquid exponents.

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arXiv2003

Thermodynamic and thermoelectric properties of high-temperature cuprate superconductors in the stripe phase

We examine the thermodynamic and thermoelectric properties in the stripe phase of high-Tc cuprates, by using the finite-temperature Lanczos technique for the t-J model with a potential that stabilizes vertical charge stripes. When the stripe potential is turned on, the entropy is suppressed as a consequence of the formation of one-dimensional charge stripes accompanied by an enhancement of antiferromagnetic spin correlation in the spin domains. The stripe formation leads also to weak temperature dependence of the chemical potential, leading to the suppression of the thermoelectric power. The suppression of the entropy and thermoelectric power is consistent with experimental data in the stripe phase of La_{1.6-x}Nd_{0.4}Sr_xCuO_4.

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arXiv2003

Doping dependence of chemical potential and entropy in hole- and electron-doped high-Tc cuprates

We examine the thermodynamic properties of the hole- and electron-doped cuprates by using the t-t'-t''-J model. We find that the chemical potential shows different doping dependence between the hole and electron dopings. Recent experimental data of the chemical potential shift are reproduced except for lightly underdoped region in the hole doping where stripe and/or charge inhomogeneity are expected to be important. The entropy is also calculated as a function of the carrier concentration. It is found that the entropy of the electron-doped system is smaller than that of the hole-doped systems. This is related to strong antiferromagnetic short-range correlation that survives in the electron-doped system.

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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.

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arXiv2001

Electronic States in the Antiferromagnetic Phase of Electron-Doped High-Tc Cuprates

We investigate the electronic states in the antiferromagnetic (AF) phase of electron-doped cuprates by using numerically exact diagonalization technique for a t-t'-t''-J model. When AF correlation develops with decreasing temperature, a gaplike behavior emerges in the optical conductivity. Simultaneously, the coherent motion of carriers due to the same sublattice hoppings is enhanced. We propose that the phase is characterized as an AF state with small Fermi surface around the momentum k=(π,0) and (0,π). This is a remarkable contrast to the behavior of hole-doped cuprates.

cond-mat.str-el↗

arXiv2001

Temperature Dependence of Spin Correlation and Charge Dynamics in the Stripe Phase of High-T_c Superconductors

We examine the temperature dependence of the electronic states in the stripe phase of high-Tc cuprates by using the t-J model with a potential that stabilizes vertical charge stripes. Charge and spin-correlation functions and optical conductivity are calculated by using finite-temperature Lanczos method. At zero temperature, the antiferromagnetic correlation between a spin in a charge stripe and that in a spin domain adjacent to the stripe is weak, since the charge stripe and the spin domain are almost separated. With increasing temperature, the correlation increases and then decreases toward high temperature. This is in contrast to other correlations that decrease monotonically. From the examination of the charge dynamics, we find that this anomalous temperature dependence of the correlation is the consequence of a crossover from one-dimensional electronic states to two-dimensional ones.

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↗

arXiv2001

Vanishing Hall Constant in the Stripe Phase of Cuprates

The Hall constant R_H is considered for the stripe structures. In order to explain the vanishing of R_H in LNSCO at x = 1/8, we use the relation of R_H to the Drude weight D as well as direct numerical calculation, to obtain results within the t-J model, where the stripes are imposed via a charge potential and a staggered magnetic field. The origin of R_H ~ 0 is related to a maximum in D and the minimal kinetic energy in stripes with a hole filling ~ 1/2. The same argument indicates on a possibility of R_H ~ 0 in the whole range of static stripes for x < 1/8.

cond-mat.str-el↗

arXiv2000

Effect of nonmagnetic impurities on stripes in high-Tc cuprates

We perform the numerically exact diagonalization study of the t-J model with nonmagnetic impurities to clarify the relation between Zn impurities and the stripes. By examining the hole-hole correlation function for a two-hole \sqrt{18}x\sqrt{18} cluster with a single impurity, we find that the impurity has a tendency to stabilize vertical charge stripes. This tendency is caused by the gain of the kinetic energy of holes moving along the stripes that are formed avoiding the impurity.

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