arXiv subjects

Sumio Ishihara

Publications and source records attributed to Sumio Ishihara.

At least 55 records · Page 3Linked to original sources

arXiv2012

Photoinduced magnetic bound state in itinerant correlated electron system with spin-state degree of freedom

Photo-excited state in correlated electron system with spin-state degree of freedom is studied. We start from the two-orbital extended Hubbard model where energy difference between the two orbitals is introduced. Photo-excited metastable state is examined based on the effective model Hamiltonian derived by the two-orbital Hubbard model. Spin-state change is induced by photo-irradiation in the low-spin band insulator near the phase boundary. High-spin state is stabilized by creating a ferromagnetic bound state with photo-doped hole carriers. An optical absorption occurs between the bonding and antibonding orbitals inside of the bound state. Time-evolution for photo-excited states is simulated in the time-dependent mean-field scheme. Pair-annihilations of the photo-doped electron and hole generate the high-spin state in a low-spin band insulator. We propose that this process is directly observed by the time-resolved photoemission experiments.

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arXiv2012

Ordering and Excitation in Orbital Compass Model on a Checkerboard Lattice

We study an orbital compass model on a checkerboard lattice where orbital degree of freedom is represented by the pseudo-spin operator. Competition arises from an Ising interaction for the $z$ component of pseudo-spins along the vertical/horizontal bonds and an Ising interaction for the $x$ component along diagonal bonds. Classical and quantum compass models are analyzed by utilizing several analytical methods and numerical simulations. At a fully frustrated point where the two Ising interactions compete with each other, a macroscopic number of orbital configurations are degenerate in a classical ground state. This degeneracy is lifted by thermal and quantum fluctuations, and a staggered long-range order of the $z$ component of the pseudo-spin is realized. A tricritical point for this order appears due to competition between the bond dependent Ising interactions. Roles of geometrical frustration on excitation dynamics are also examined.

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arXiv2010

Quantum Dielectric Fluctuation in an Electronic Ferroelectricity studied by Variational Monte-Carlo Method

Electronic structure and dielectric property in an electronic ferroelectricty, where electric polarization is driven by an electronic charge order without inversion symmetry, are studied. Motivated from layered iron oxides, roles of quantum fluctuation on ferroelectricity in a paired-triangular lattice are focused on. Three types of the extended V-t model are examined by the variational Monte-Carlo method with the Gutzwiller-type correlation factor. It is shown that electron transfer between the triangular layers corresponding to the inter-layer polarization fluctuation promotes the three-fold charge order associated with an electric polarization. This result is in highly contrast to the usual manner of quantum fluctuation in the hydrogen-bond type ferroelectricities and the quantum paraelectric oxides. Spin degree of freedom of electron and a realistic crystal structure for the layered iron oxides further stabilize the polar charge ordered state. Implications of the numerical results for layered iron oxides are discussed.

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arXiv2010

Electronic Ferroelectricity in a Dimer Mott Insulator

Motivated from recent experiments in kappa-(BEDT-TTF)_2Cu_2(CN)_3, dielectric and magnetic properties in a dimer-Mott insulator are studied. We derive the effective Hamiltonian where the number of electron per dimer is taken to be one. Charge and magnetic structures in finite temperature are obtained by using the mean-field and Monte-Carlo methods. Magnetic and ferroelectric phases are exclusive with each other. Dielectric fluctuation is remarkable near a phase boundary between dimer-Mott phase and ferroelectric phase. Implications of the present results for kappa-(BEDT-TTF)_2Cu_2(CN)_3 are discussed.

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arXiv2010

Ring-exchange interaction in doubly degenerate orbital system

Ring-exchange (RE) interaction in Mott insulator with doubly degenerate $e_g$ orbitals is studied. Hamiltonian for the RE interaction is derived by the perturbational calculation. A remarkably weak RE interaction destroys the orbital order caused by the order-by-fluctuation mechanism in the nearest-neighbor exchange-interaction model. A long range order of magnetic octupole moment, i.e. complex orbital wave functions, appears. Competition between octupole and quadrupole interactions strongly suppresses ordered moments.

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arXiv2009

Electronic Ferroelectricity and Frustration

Beyond a conventional classification of ferroelectricity, there is a class of materials where electronic degrees of freedom and electronic interactions are directly responsible for electric polarization and ferroelectric transition. This is termed electronic ferroelectricity. In this article, we review electronic ferroelectricity from a view point of frustration. Experimental and theoretical examinations in spin driven ferroelectric materials, recently termed multiferroics, are introduced. Spin frustration caused by competing magnetic interactions is of prime importance for this type of ferroelectricity. Charge driven ferroelectricity where electronic charge order induces electric polarization is reviewed. In particular, exotic dielectric and magneto-dielectric properties in layered iron oxides are focused on. Through a number of recent experimental and theoretical researches, charge fluctuation due to frustrated geometry plays essential roles on electronic ferroelectricity in this compound.

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arXiv2009

Spin-state transition and phase separation in multi-orbital Hubbard model

We study spin-state transition and phase separation involving this transition based on the milti-orbital Hubbard model. Multiple spin states are realized by changing the energy separation between the two orbitals and the on-site Hund coupling. By utilizing the variational Monte-Carlo simulation, we analyze the electronic and magnetic structures in hole doped and undoped states. Electronic phase separation occurs between the low-spin band insulating state and the high-spin ferromagnetic metallic one. Difference of the band widths in the two orbitals is of prime importance for the spin-state transition and the phase separation.

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arXiv2009

Quantum fluctuation and geometrical frustration effects on electric polarization

We examine theoretically a possibility of ferroelectricity caused by electronic charge order without inversion symmetry, motivated by layered iron oxides. Quantum electronic models in a paired-triangular lattice are analyzed by utilizing the variational Monte Carlo simulation. Our calculation demonstrates that combined effects of electron transfer between the layers, corresponding to quantum fluctuation between the potential minima, and geometrical frustration promote appearance of an electric polarization. Present results are in contrast to the conventional manner of quantum fluctuation in ferroelectricity.

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arXiv2008

Dilution effect in correlated electron system with orbital degeneracy

Theory of dilution effect in orbital ordered system is presented. The $e_g$ orbital model without spin degree of freedom and the spin-orbital coupled model in a three-dimensional simple-cubic lattice are analyzed by the Monte-Carlo simulation and the cluster expansion method. In the $e_g$ orbital model without spin degree of freedom, reduction of the orbital ordering temperature due to dilution is steeper than that in the dilute magnet. This is attributed to a modification of the orbital wave-function around vacant sites. In the spin-orbital coupled model, it is found that magnetic structure is changed from the A-type antiferromagnetic order into the ferromagnetic one. Orbital dependent exchange interaction and a sign change of this interaction around vacant sites bring about this novel phenomena. Present results explain the recent experiments in transition-metal compounds with orbital dilution.

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arXiv2008

Numerical study of t2g orbital system with ferromagnetic polarization

Finite temperature orbital state in a ferromagnetic Mott insulator with triply-degenerate $t_{2g}$ orbital is investigated numerically. We employ the quantum Monte Carlo simulation with the loop algorithm. Indications for conventional staggered-type orbital order are not remarkable down to the lowest temperature to which the present simulation can get access. Physical parameters monitoring the off-diagonal orbital order, which is characterized by a linear combination of the $(d_{yz}, d_{zx}, d_{xy})$ orbital-wave functions with equal weights, are not conspicuous. It is found that a orbital gap-like behavior appears in the uniform orbital susceptibility. This is supported by a threshold behavior in the staggered correlation function in a calculation with the additional Ising-type interaction. Some rigorous remarks for the long-range orbital order are also presented.

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arXiv2008

Magneto-Dielectric phenomena in charge and spin frustrated system of layered iron oxide

Dielectric and magnetic phenomena in spin and charge frustrated system RFe2O4 (R is a rare-earth metal ion) are studied. An electronic model for charge, spin and orbital degrees in a pair of triangular-lattice planes is derived. We analyze this model by utilizing the mean-field approximation and the Monte-Carlo simulation in a finite size cluster. A three fold-type charge ordered structure with charge imbalance between the planes is stabilized in finite temperatures. This polar charge order is reinforced by spin ordering of Fe ions. This novel magneto-dielectric phenomenon is caused by spin frustration and charge-spin coupling in the exchange interaction. We show cross-correlation effect in magnetic- and electric-field responses. Oxygen deficiency effect as an impurity effect in a frustrated charge-spin coupled system is also examined.

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arXiv2006

Photoinduced charge and spin dynamics in strongly correlated electron systems

Motivated by photoinduced phase transition in manganese oxides, charge and spin dynamics induced by photoirradiation are examined. We calculate the transient optical absorption spectra of the extended double-exchange model by the density matrix renormalization group (DMRG) method. A charge-ordered insulating (COI) state becomes metallic just after photoirradiation, and the system tends to recover the initial COI state. The recovery is accompanied with remarkable suppression of an antiferromagnetic correlation in the COI state. The DMRG results are consistent with recent pump-probe spectroscopy data.

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arXiv2006

Dilution Effects in Two-dimensional Quantum Orbital System

We study dilution effects in a Mott insulating state with quantum orbital degree of freedom, termed the two-dimensional orbital compass model. This is a quantum and two-dimensional version of the orbital model where the interactions along different bond directions cause frustration between different orbital configurations. A long-range correlation of a kind of orbital at each row or column, termed the directional order, is studied by means of the quantum Monte-Carlo method. It is shown that decrease of the ordering temperature due to dilution is much stronger than that in spin models. Quantum effect enhances the effective dimensionality in the system and makes the directional order robust against dilution. We discuss an essential mechanism of the dilute orbital systems.

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arXiv2004

Hole dynamics in spin and orbital ordered vanadium perovskites

Hole dynamics in spin and orbital ordered vanadates with perovskite structure is investigated. A mobile hole coupled to the spin excitation (magnon) in the spin G-type and orbital C-type (SG/OC) ordered phase, and that to the orbital excitation (orbiton) in the spin C-type and orbital G-type (SC/OG) one are formulated on an equal footing. The observed fragile character of the (SG/OC) order is attributed to the orbiton softening caused by a reduction of the taggered magnetic order parameter. It is proposed that the qualitatively different hole dynamics in the two spin-orbital ordered phases in vanadates can be probed by the optical spectra.

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arXiv2003

Interplay of Electron-Phonon Interaction and Electron Correlation in High Temperature Superconductivity

We study the electron-phonon interaction in the strongly correlated superconducting cuprates. Two types of the electron-phonon interactions are introduced in the $t-J$ model; the diagonal and off-diagonal interactions which modify the formation energy of the Zhang-Rice singlet and its transfer integral, respectively. The characteristic phonon-momentum $(\vec q)$ and electron-momentum $(\vec k)$ dependence resulted from the off-diagonal coupling can explain a variety of experiments. The vertex correction for the electron-phonon interaction is formulated in the SU(2) slave-boson theory by taking into account the collective modes in the superconducting ground states. It is shown that the vertex correction enhances the attractive potential for the d-wave paring mediated by phonon with $\vec q=(π(1-δ), 0)$ around $δ\cong 0.3$ which corresponds to the half-breathing mode of the oxygen motion.

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arXiv2003

Orbital Wave and its Observation in Orbital Ordered Titanates and Vanadates

We present a theory of the collective orbital excitation termed orbital wave in perovskite titanates and vanadates with the triply degenerate $t_{2g}$ orbitals. The dispersion relations of the orbital waves for the orbital ordered LaVO$_3$, YVO$_3$ and YTiO$_3$ are examined in the effective spin-orbital coupled Hamiltonians associated with the Jahn-Teller type couplings. We propose possible scattering processes for the Raman and inelastic neutron scatterings from the orbital wave and calculate the scattering spectra for titanates and vanadates. It is found that both the excitation spectra and the observation methods of the orbital wave are distinct qualitatively from those for the $e_g$ orbital ordered systems.

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arXiv2001

Resonant X-ray Scattering in Manganites - Study of Orbital Degree of Freedom -

Orbital degree of freedom of electrons and its interplay with spin, charge and lattice degrees of freedom are one of the central issues in colossal magnetoresistive manganites. The orbital degree of freedom has until recently remained hidden, since it does not couple directly to most of experimental probes. Development of synchrotron light sources has changed the situation; by the resonant x-ray scattering (RXS) technique the orbital ordering has successfully been observed . In this article, we review progress in the recent studies of RXS in manganites. We start with a detailed review of the RXS experiments applied to the orbital ordered manganites and other correlated electron systems. We derive the scattering cross section of RXS where the tensor character of the atomic scattering factor (ASF) with respect to the x-ray polarization is stressed. Microscopic mechanisms of the anisotropic tensor character of ASF is introduced and numerical results of ASF and the scattering intensity are presented. The azimuthal angle scan is a unique experimental method to identify RXS from the orbital degree of freedom. A theory of the azimuthal angle and polarization dependence of the RXS intensity is presented. The theoretical results show good agreement with the experiments in manganites. Apart from the microscopic description of ASF, a theoretical framework of RXS to relate directly to the 3d orbital is presented. The scattering cross section is represented by the correlation function of the pseudo-spin operator for the orbital degree of freedom. A theory is extended to the resonant inelastic x-ray scattering and methods to observe excitations of the orbital degree of freedom are proposed.

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arXiv2001

Re-examination of orbital ordering in LaMnO$_3$: Electron-electron and electron-lattice interactions

The interactions between $e_g$ orbitals in neighboring sites are investigated in LaMnO$_3$ by taking into account virtual exchange of electrons and phonons. The spin and orbital ordering temperatures and the spin wave dispersion relation are calculated. We find that the orbital ordering is mainly caused by the electronic interactions and that the Jahn-Teller coupling is much smaller than that reported previously. We propose that the elastic constant shows a characteristic change at the N{é}el temperature by the spin and orbital coupling and the higher-order Jahn-Teller coupling.

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