arXiv subjects

Sumio Ishihara

Publications and source records attributed to Sumio Ishihara.

At least 37 records · Page 2Linked to original sources

arXiv2016

Magnetic field effects in a correlated electron system with spin-state degree of freedom - Implication of an excitonic insulator -

Magnetic field (H) effects on a correlated electron system with the spin-state degree of freedom are examined. The effective Hamiltonian derived from the two-orbital Hubbard model is analyzed by the mean-field approximation. Applying H to the low-spin (LS) phase induces the excitonic insulating phase, as well as the spin-state ordered phase where the LS and high-spin (HS) states are ordered alternately. In the case where H is applied to the HS phase, a reentrant transition for the HS phase appears. A rich variety of the phase diagrams are attributed to the spin-state degree of freedom and their combinations in the wave function as well as in the real-space configuration.

cond-mat.str-el↗

arXiv2016

Phase diagram and collective excitation in excitonic insulator: from the orbital physics viewpoint

Excitonic insulating system is studied from the viewpoints of the orbital physics in strongly correlated electron systems. An effective model Hamiltonian for low-energy electronic states is derived from the two-orbital Hubbard model with a finite energy difference corresponding to the crystalline field splitting. The effective model is represented by the spin operators and the pseudo-spin operators for the spin-state degrees of freedom. The ground state phase diagram is analyzed by the mean-field approximation. In addition to the low-spin state and high-spin state phases, two kinds of the excitonic insulating phases emerge as a consequence of the competition between the crystalline field effect and the Hund coupling. The excitonic transition is classified to be an Ising-like transition reflecting a spontaneous breaking of the $Z_2$ symmetry. Magnetic structures in the two excitonic insulating phases are different from each other; an antiferromagnetic order and a spin nematic order. Collective excitations in each phase are examined by using the generalized spin-wave method. The Goldstone modes in the excitonic insulating phases appear in the dynamical correlation functions for the spins and pseudo-spin operators. Both the transverse and longitudinal spin excitation modes are active in the two excitonic insulating phases in contrast to the low-spin state and high-spin state phases. Connections of the present results to the perovskite cobalt oxides are discussed.

cond-mat.str-el↗

arXiv2016

Pulse excitation to continuous-wave excitation in a low-dimensional interacting quantum system

Real-time dynamics in one-dimensional transverse Ising model coupled with the time-dependent oscillating field is analyzed by using the infinite time-evolving block decimation algorithm and the Floquet theory. In particular, the transient dynamics induced by the pulse field and their connections to the dynamics by a continuous-wave field are focused on. During the pulse-field irradiation, the order parameter shows a characteristic oscillation, in which the frequency shifts from the pulse-field frequency. This is considered as a kind of the Rabi oscillation, but the frequency strongly depends on the intersite Ising interaction. After turning off the pulse field, the oscillation remains with a frequency $\mathitΩ$ and a damping constant $γ$. In the case of low fluence, both $\mathitΩ$ and $γ$ are scaled by the pulse amplitude in a wide range of the parameter values of the model. In the case of high fluence, $\mathitΩ$ and $γ$ are arranged by a product of the pulse amplitude and the pulse width. This implies that the dynamics after turning off the pulse field are decided by a population of the excited state when the pulse field is turned off.

cond-mat.str-el↗

arXiv2016

Quantum Melting of Magnetic Order in an Organic Dimer-Mott Insulating System

Quantum entanglement effects between the electronic spin and charge degrees of freedom are examined in an organic molecular solid, termed a dimer-Mott insulating system, in which molecular dimers are arranged in a crystal as fundamental units. A low energy effective model includes an antisymmetric exchange interaction, as one of the dominant magnetic interactions. This interaction favors a 90 degree spin configuration, and competes with the Heisenberg-type exchange interaction. Stabilities of the magnetic ordered phases are examined by using the spin-wave theory, as well as the Schwinger-boson theory. It is found that the spin-charge interaction promotes an instability of the long-range magnetic ordered state around a parameter region where two spin-spiral phases are merged. Implication for the quantum spin liquid state observed in $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ is discussed.

cond-mat.str-el↗

arXiv2015

Photoinduced correlated electron dynamics in a two-leg ladder Hubbard system

Photoinduced carrier dynamics in a correlated electron system on a coupled two-leg ladder lattice are studied. The two-leg ladder Hubbard model is analyzed by utilizing the exact diagonalization method based on the Lanczos algorithm in finite size clusters. In order to reveal the transient carrier dynamics after photoirradiation, we calculate the low-energy components of the hole kinetic energy, the pair-field correlation function, the optical conductivity spectra and others. It is shown that the photoinduced metallic-like state appears in a half filled Mott insulating state, while the low-energy carrier motion is suppressed by photoirradiation in hole doped metallic states. These photoinduced changes in electron dynamics are associated with changes in the carrier-pair coherence, and are not attributed to a naive thermalization but to a ladder-lattice effect. Based on the numerical results, optical controls of hole pairs by using the double-pulse pumping are demonstrated. Implications to the recent optical pump-probe experiments are presented.

cond-mat.str-el↗

arXiv2015

Magnetoelectric effect in organic molecular solids

The Magnetoelectric (ME) effect in solids is a prominent cross correlation phenomenon, in which the electric field (${\bm E}$) controls the magnetization (${\bm M}$) and the magnetic field (${\bm H}$) controls the electric polarization (${\bm P}$). A rich variety of ME effects and their potential in practical applications have been investigated so far within the transition-metal compounds. Here, we report a possible way to realize the ME effect in organic molecular solids, in which two molecules build a dimer unit aligned on a lattice site. The linear ME effect is predicted in a long-range ordered state of spins and electric dipoles, as well as in a disordered state. One key of the ME effect is a hidden ferroic order of the spin-charge composite object. We provide a new guiding principle of the ME effect in materials without transition-metal elements, which may lead to flexible and lightweight multifunctional materials.

cond-mat.str-el↗

arXiv2015

Photo-Induced Phase Transition in Charge Order Systems --Charge Frustration and Interplay with Lattice--

Lattice effects on photo-excited states in interacting charge frustrated system are examined. Real time dynamics in the interacting spinless fermion model on a triangular lattice coupled to lattice vibration are analyzed by applying the exact diagonalization method combined with the classical equation of motion. A photo-induced phase transition from the horizontal stripe-type charge order (CO) to the 3-fold CO occurs through a characteristic intermediate time domain. By analyzing the time evolution in detail, we find that this characteristic dynamics are seen when the electron and lattice sectors are not complementary to each other but show cooperative time evolutions. We also find that there are threshold values in the optical fluorescence and electron-lattice coupling for emergence of the photo-induced 3-fold CO. The dynamics are distinct from those from the vertical stripe-type CO, in which a monotonic CO melting occurs. A scenario of the photo-induced CO phase transition with lattice degree of freedom is presented from a view point of charge frustration.

cond-mat.str-el↗

arXiv2015

Optical freezing of charge motion in an organic conductor

Dynamical localization, i.e., reduction of the intersite electronic transfer integral t by an alternating electric field, E(omega) , is a promising strategy for controlling strongly correlated systems with a competing energy balance between t and the Coulomb repulsion energy. Here we describe a charge localization induced by the 9.3 MV/cm instantaneous electric field of a 1.5 cycle (7 fs) infrared pulse in an organic conductor alpha-(bis[ethylenedithio]-tetrathiafulvelene)_2I_3. A large reflectivity change of > 25% and a coherent charge oscillation along the time axis reflect the opening of the charge ordering gap in the metallic phase. This optical freezing of charges, which is the reverse of the photoinduced melting of electronic orders, is attributed to the 10% reduction of t driven by the strong, high-frequency (omega>t/h_bar) electric field.

cond-mat.str-el↗

arXiv2014

Charge Dynamics in a Correlated Fermion System on a Geometrically Frustrated Lattice

Charge dynamics in an interacting fermionic model on a geometrically frustrated lattice are examined. We analyze a spinless fermion model on a paired triangular lattice, an electronic model for layered iron oxides, in zero and finite temperatures by the exact diagonalization methods. We focus on the two charge ordered (CO) phases, termed CO_1/2 and CO_1/3, which, respectively, are realized by the inter-site Coulomb interaction and the quantum/thermal fluctuations. The optical spectra in CO_1/3 show multiple components and their low-energy weights are survived even below the ordering temperature (T_CO). Changes of the dynamical charge correlation below T_CO in CO_1/3 are weakly momentum dependent, in sharply contrast to CO_1/2. These characteristic dynamics in CO_1/3 are attributable to the charge frustration effects, and reproduce some aspects of the recent experimental optical and x-ray scattering spectra.

cond-mat.str-el↗

arXiv2014

Resonating Valence-Bond State in an Orbitally Degenerate Quantum Magnet with Dynamical Jahn-Teller Effect

Short-range resonating-valence bond states in an orbitally degenerate magnet on a honeycomb lattice is studied. A quantum-dimer model is derived from the Hamiltonian which represents the superexchange interaction and the dynamical Jahn-Teller (JT) effect. We introduce two local units termed "spin-orbital singlet dimer", where two spins in a nearest-neighbor bond form a singlet state associated with an orbital polarization along the bond, and "local JT singlet", where an orbital polarization is quenched due to the dynamical JT effect. A derived quantum-dimer model consists of the hopping of the spin-orbital singlet dimers and the JT singlets, and the chemical potential of the JT singlets. We analyze the model by the mean-field approximation, and find that a characteristic phase, termed "JT liquid phase", where both the spin-orbital singlet dimers and the JT singlets move quantum mechanically, is realized. Possible scenarios for the recently observed non magnetic-ordered state in Ba$_3$CuSb$_2$O$_9$ are discussed.

cond-mat.str-el↗

arXiv2014

Photo-Induced Dynamics in Charge-Frustrated Systems

Photo-excited charge dynamics of interacting charge-frustrated systems are studied using a spinless fermion model on an anisotropic triangular lattice. Real-time evolution of the system after irradiating a pump-photon pulse is analyzed by the exact diagonalization method. We focus on photo-excited states in the two canonical charge-ordered (CO) ground states, i.e., horizontal stripe-type and vertical stripe-type COs, which compete with each other owing to the charge frustration. We find that the photo-induced excited states from the two types of COs are distinct. From the horizontal stripe-type CO, a transition to another CO state called the three-fold CO phase occurs. In sharp contrast, the vertical stripe-type CO phase is only weakened by photo-irradiation. Our observations are attributable to the charge frustration effects occurring in the photo-excited states.

cond-mat.str-el↗

arXiv2013

Transient Carrier Dynamics in a Mott Insulator with Antiferromagnetic Order

We study transient dynamics of hole carriers injected at a certain time into a Mott insulator with antiferromagnetic long range order. This is termed ``dynamical hole doping" as contrast with chemical hole doping. Theoretical framework for the transient carrier dynamics are presented based on the two dimensional $t-J$ model. Time dependences of the optical conductivity spectra as well as the one-particle excitation spectra are calculated based on the Keldysh Green's function formalism at zero temperature combined with the self-consistent Born approximation. At early stage after dynamical hole doping, the Drude component appears, and then incoherent components originating from hole-magnon scatterings start to grow. Fast oscillatory behavior due to coherent magnon, and slow relaxation dynamics are confirmed in the spectra. Time profiles are interpreted as that doped bare holes are dressed by magnon clouds, and are relaxed into spin polaron quasi-particle states. Characteristic relaxation times for Drude and incoherent peaks strongly depend on momentum of a dynamically doped hole, and the exchange constant. Implications to the recent pump-probe experiments are discussed.

cond-mat.str-el↗

arXiv2013

Dynamical Jahn-Teller Effect in Spin-Orbital Coupled System

Dynamical Jahn-Teller (DJT) effect in a spin-orbital coupled system on a honeycomb lattice is examined, motivated from recently observed spin-liquid behavior in Ba$_3$CuSb$_2$O$_9$. An effective vibronic Hamiltonian, where the superexchange interaction and the DJT effect are taken into account, is derived. We find that the DJT effect induces a spin-orbital resonant state where local spin-singlet states and parallel orbital configurations are entangled with each other. This spin-orbital resonant state is realized in between an orbital ordered state, where spin-singlet pairs are localized, and an antiferromagnetic ordered state. Based on the theoretical results, a possible scenario for Ba$_3$CuSb$_2$O$_9$ is proposed.

cond-mat.str-el↗

arXiv2013

Vibronic Excitation Dynamics in Orbitally Degenerate Correlated Electron System

Orbital-lattice coupled excitation dynamics in orbitally degenerate correlated systems are examined. We present a theoretical framework, where both local vibronic excitations and superexchange-type inter-site interactions are dealt with on an equal footing. We generalize the spin-wave approximation so as to take local vibronic states into account. Present method is valid from weak to strong Jahn-Teller coupling magnitudes. Two characteristic excitation modes coexist; a low-energy dispersive mode and high-energy multi-peak mode. These are identified as a collective vibronic mode, and Flanck-Condon excitations in a single Jahn-Teller center modified by the inter-site interactions, respectively. Present formalism covers vibronic dynamics in several orbital-lattice coupled systems.

cond-mat.str-el↗

arXiv2013

Optical manipulation of magnetism in spin-charge coupled correlated electron system

Photoirradiation effects in correlated electrons coupled with localized spins are studied based on the extended double-exchange model. In particular, we examine melting of an antiferromagnetic (AFM) charge order insulating state by varying the light intensity. When intense light is irradiated, the AFM insulating characteristics are strengthened, rather than change into the ferromagnetic metallic characteristic, which are expected from the conventional double exchange interaction when carriers are introduced by weak light irradiation or chemical doping. This provides a new principle for optically manipulating magnetism.

cond-mat.str-el↗

arXiv2012

Polar Charge Fluctuation and Superconductivity in Organic Conductor

Superconductivity and polar charge fluctuation are studied in an organic conductor with the dimer-molecule degree of freedom. The extended Hubbard models, where the intra-dimer electronic structure and the inter-dimer Coulomb interactions are taken into account, are analyzed by the random-phase approximation and the fluctuation-exchange approximation. Superconductivity appears in a vicinity of the charge-density wave (CDW) phase where the electronic charge distributions are polarized inside of dimers. The extended s-wave type paring is favored and its competitive relations with the superconductivity due to the spin fluctuation depends on the triangular lattice geometry. Comparison between two superconductivities realized near the polar and non-polar CDW phases are also presented.

cond-mat.str-el↗

arXiv2012

Band and Mott Insulators and Superconductivity in Honeycomb-Lattice Ionic-Hubbard Model

Motivated by superconductivity (SC) in layered nitrides, we study an ionic-Hubbard model on a honeycomb lattice, which consists of two sublattices with an energy-level offset, by using an optimization variational Monte Carlo method. Parameter values for the Coulomb interaction and the energy-level offset, to realize a band insulator in non-doped state, are evaluated, and the SC state is investigated in a carrier-doped band insulator. It is found that, in a weakly doped band insulator, spin fluctuation remains and the spin-singlet d-wave SC state is realized. Present results support an unconventional spin-singlet SC state suggested by the experimental observations in layered nitrides.

cond-mat.supr-con↗

arXiv2012

Collective Charge Excitation in a Dimer Mott Insulating System

Charge dynamics in a dimer Mott insulating system, where a non-polar dimer-Mott (DM) phase and a polar charge-ordered (CO) phase compete with each other, are studied. In particular, collective charge excitations are analyzed in the three different models where the internal-degree of freedom in a dimer is taken into account. Collective charge excitation exists both in the non-polar DM phase and the polar CO phase, and softens in the phase boundary. This mode is observable by the optical conductivity spectra where the light polarization is parallel to the electric polarization in the polar CO phase. Connections between the present theory and the recent experimental results in kappa-(BEDT-TTF)2Cu2(CN)3 are discussed.

cond-mat.str-el↗