Search

This work reviews the experimental studies on photo-induced cooperative phenomena (photo-induced phase transitions, PIPT), which are related to ultrafast structural and electronic-state dynamics. The review includes a brief history of this research field. The current growth of PIPT research is facilitated by deep collaboration between quantum beam science and materials science. Meanwhile, owing to developments in ultrafast quantum beam technology, the concept of PIPT itself has largely expanded. In the initial stage of PIPT research, the dynamical PIPT process was thought to be governed by energy relaxation from the photoexcited state, which is a classical and incoherent process. Therefore, the main research target was realizing the quasi-stable state on the energy surface of the ground state. Using ultrashort pulsed light and quantum beam technologies (including X-ray and electron technologies), one can now directly probe the ultrafast and coherent electronic dynamics of PIPT materials coupled with changes in their structural properties. This new technology has realized a new ordered phase called the hidden state, which is unique to the photoexcited state. Modern laser technology has also enabled a controlled PIPT process utilizing coherent coupling between the light field and electronic states in materials. Exploiting these leaps in experimental techniques, we have expanded the way of controlling PIPT dynamics from classical (incoherent) to coherent/quantum processes. This review discusses the recent developments in PIPT tuning via quantum (coherent) control methods based on charge–lattice (orbital)–spin coupled materials.

We investigate resonant inelastic X-ray scattering (RIXS) at the O $K$-edge in insulating cuprates by means of three methods: cluster perturbation theory (CPT), Hartree--Fock approximation (HFA) and exact diagonalization (ED) method. We consider the three-band Hubbard model and show the overall momentum-dependence of the Zhang--Rice singlet (ZRS) excitation and charge-transfer excitation by the CPT combining with the perturbation scheme. A comparison of the RIXS spectra calculated using CPT and HFA reveals different momentum-dependencies through the changes in the properties of the upper Hubbard band and the ZRS band. These findings are supported by analyses using the ED method on the RIXS spectra and dynamical charge structure factor., Comment: 9 pages, 9 figures

Antiferromagnets exhibit distinctive characteristics such as ultrafast dynamics and robustness against perturbative fields, thereby attracting considerable interest in fundamental physics and technological applications. Recently, it was revealed that the N\'eel vector can be switched by a current-induced staggered (N\'eel) spin-orbit torque in antiferromagnets with the parity-time symmetry, and furthermore, a nonsymmorphic symmetry enables the control of Dirac fermions. However, the real-time dynamics of the magnetic and electronic structures remain largely unexplored. Here, we propose a theory of the ultrafast dynamics in antiferromagnetic Dirac semimetals and show that the N\'eel vector is rotated in the picosecond timescale by the terahertz-pulse-induced N\'eel spin-orbit torque and other torques originating from magnetic anisotropies. This reorientation accompanies the modulation of the mass of Dirac fermions and can be observed in real time by the magneto-optical effects. Our results provide a theoretical basis for emerging ultrafast antiferromagnetic spintronics combined with the topological aspects of materials., Comment: 8 pages, 4 figures

Charge acceleration during an intense light field application to solids attracts much attention as elementary processes in high-harmonic generation and photoelectron emission. For manipulating such attosecond dynamics of charge, carrier-envelope-phase (CEP: relative phase between carrier oscillation of light field and its envelope function) control has been employed in insulators, nanometal and graphene. In superconducting materials, collective control of charge motion is expected because of its strongly coherent nature of quasi-particles. Here we report that, in a layered organic superconductor, a non-linear petahertz current driven by a single-cycle 6 femtosecond near infrared field shows up as second harmonic generation (SHG), which is in contrast to the common belief that even harmonics are forbidden in the centrosymmetric system. The SHG represents a CEP sensitive nature and an enhancement near the superconducting temperature. The result and its quantum many-body analysis indicate that a polarized current is induced by non-linear acceleration of charge, which is amplified by superconducting fluctuations. This will lead to petahertz functions of superconductors and of strongly correlated systems., Here the authors show second harmonic generation (SHG) from a centrosymmetric organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br. They find unusual temperature dependence and CEP-sensitive nature of the SHG which are explained in terms of nonlinear current.

We show that a static electric field induces the transition from a ferromagnetic metal to an antiferromagnetic insulator owing to the Bloch oscillation of conduction electrons. In the steady state, the electric current is inversely proportional to the applied electric field, implying the nonperturbative insulating nature that is different from the Wannier-Stark localization. Possible experimental realization based on recent terahertz pulse sources is discussed., 2 pages, 3 figures

High-harmonic generation (HHG) in crystalline solids have been examined so far on the basis of one-body energy-band structures arising from electron itineracy in a periodic potential. Here, we show emergence of HHG signals which are attributed to dynamics of many-body states in a low-dimensional correlated electron system. An interacting fermion model and its effective pseudo-spin model on a one-dimensional dimer-type lattice are analyzed. Observed HHG signals in a spontaneously symmetry-broken state, where charge densities are polarized inside of dimer units, show threshold behavior with respect to light amplitude and are interpreted in terms of tunneling and recombination of kink-antikink excitations in an electric field., 8 pages, 9 figures (including supplemental material)

A charge excitation in a two-dimensional Mott insulator is strongly coupled with the surrounding spins, which is observed as magnetic-polaron formations of doped carriers and a magnon sideband in the Mott-gap transition spectrum. However, the dynamics related to the spin sector are difficult to measure. Here, we show that pump-probe reflection spectroscopy with seven-femtosecond laser pulses can detect the optically induced spin dynamics in Nd2CuO4, a typical cuprate Mott insulator. The bleaching signal at the Mott-gap transition is enhanced at ~18 fs. This time constant is attributable to the spin-relaxation time during magnetic-polaron formation, which is characterized by the exchange interaction. More importantly, ultrafast coherent oscillations appear in the time evolution of the reflectivity changes, and their frequencies (1400–2700 cm−1) are equal to the probe energy measured from the Mott-gap transition peak. These oscillations can be interpreted as the interference between charge excitations with two magnons originating from charge–spin coupling., Understanding the dynamics of cuprates following photoexcitation can provide insights into the complex coupling mechanisms that underlie their exotic equilibrium behaviour. Here the authors use pump-probe reflection spectroscopy to investigate the nonequilibrium spin dynamics of Mott-insulating Nd2CuO4.

Intense light-field application to solids produces enormous/ultrafast non-linear phenomena such as high-harmonic generations 1, 2 and attosecond charge dynamics 3, 4. They are distinct from conventional photonics. However, main targets have been limited to insulators and semiconductors, although theoretical approaches have been made also for correlated metals and superconductors 5. Here, in a layered organic superconductor, a non-linear charge oscillation driven by a nearly single-cycle strong electric field of >10 megavolts /cm is observed as a stimulated emission. The charge oscillation is different from a linear response and ascribed to a polar charge oscillation with a period of 6 fs. This non-linear polar charge oscillation is enhanced by critical fluctuations near a superconducting transition temperature and a critical end point of first order Mott transitions. Its observation on an ultrafast timescale of 10 fs clarifies that the Coulomb repulsion plays an essential role in superconductivity of organic superconductors., 5 Figs. for main text, 3 Figs for supplementary information

Understanding and controlling atomic motions in solid are essential in ionic materials, such as ion conductors, electrolyte and electrodes of batteries, and gas separators. Although photoinduced proton transport in solid materials has been demonstrated, photoinduced transport of heavier ions, such as the oxide ion, in solid materials remains entirely elusive. Herein, we apply ultrafast time-resolved electron diffraction to observe the large displacement of oxide ions in a strongly correlated material, EuBaCo2O5.39, under near-ultraviolet photoexcitation at room temperature. The mechanism of the photoinduced transport of oxide ions is revealed via the combination of ultrafast optical spectroscopy and density functional theory calculations. Our findings show that photoexcitation induces ionic motions in a ceramic crystal at room temperature through the charge transfer that strongly couples with large ionic displacement under the nonequilibrium condition.

Strongly enhanced quantum fluctuations often lead to a rich variety of quantum-disordered states. Developing approaches to enhance quantum fluctuations may open paths to realize even more fascinating quantum states. Here, we demonstrate that a coupling of localized spins with the zero-point motion of hydrogen atoms, that is, proton fluctuations in a hydrogen-bonded organic Mott insulator provides a different class of quantum spin liquids (QSLs). We find that divergent dielectric behavior associated with the approach to hydrogen-bond order is suppressed by the quantum proton fluctuations, resulting in a quantum paraelectric (QPE) state. Furthermore, our thermal-transport measurements reveal that a QSL state with gapless spin excitations rapidly emerges upon entering the QPE state. These findings indicate that the quantum proton fluctuations give rise to a QSL—a quantum-disordered state of magnetic and electric dipoles—through the coupling between the electron and proton degrees of freedom., The organic material κ-H3(Cat-EDT-TTF)2 has been suggested to exhibit a quantum spin liquid phase in which quantum fluctuations prevent the formation of magnetic order. Here, the authors show that this may be a result of fluctuations of hydrogen atoms, rather than more conventional geometric frustration.

Effects of the spin-orbit coupling (SOC) and magnetic field on excitonic insulating (EI) states are investigated. We introduce the two-orbital Hubbard model with the crystalline field splitting, which is a minimal model for discussing the exciton condensation in strongly correlated electron systems, and analyze its effective Hamiltonian in the strong correlation limit by using the mean-field theory. In the absence of the SOC and magnetic field, the ground state changes from the nonmagnetic band-insulating state to the EI state by increasing the Hund coupling. In an applied magnetic field, the magnetic moment appears in the EI state, which is continuously connected to the forced ferromagnetic state. On the other hand, in the presence of the SOC, they are separated by a phase boundary. We find that the magnetic susceptibility is strongly enhanced in the EI phase near the boundary with a small SOC. This peculiar behavior is attributed to the low-energy fluctuation of the spin nematicity inherent in the high-spin local state stabilized by the Hund coupling. The present study not only reveals the impact of the SOC for the EI state but also sheds light on the role of quantum fluctuations of the spin nematicity for the EI state., Comment: 9 pages, 8 figures

The electronic structure of ${\mathrm{SrRuO}}_{3}$ thin film with a thickness from 1 to 50 unit cell (u.c.) is investigated via the resonant inelastic x-ray scattering (RIXS) technique at the O $K$ edge to unravel the intriguing interplay of orbital and charge degrees of freedom. We found that the orbital-selective quantum confinement effect (QCE) induces the splitting of Ru $4d$ orbitals. At the same time, we observed a clear suppression of the electron-hole continuum across the metal-to-insulator transition occurring in the 4-u.c. sample. From these two clear observations we conclude that the QCE gives rise to a Mott insulating phase in ultrathin ${\mathrm{SrRuO}}_{3}$ films. Our interpretation of the RIXS spectra is supported by the configuration interaction calculations of ${\mathrm{RuO}}_{6}$ clusters.

Optical responses in an excitonic insulating (EI) system with strong electron correlation are studied. We adopt the two-orbital Hubbard model with a finite energy difference between the two orbitals where the spin state degree of freedom exists. This model is analyzed by the variational cluster approach. In order to include the local electron correlation effect, the vertex correction is taken into account in the formulation of the optical conductivity spectra. We calculate a finite-temperature phase diagram, in which an EI phase appears between a low-spin band insulating state and a high-spin Mott insulating state. Characteristic components of the optical conductivity spectra consisting of a sharp peak and continuum appear in the EI phase. Integrated intensity almost follows the order parameter of the EI state, suggesting that this component is available to identify the EI phases and transitions., Comment: 9 pages, 11 figures

We theoretically investigate photoresponses of a correlated electron system upon stimuli of a pulsed laser light. Real-time dynamics of an interacting spinless fermion model on a one-dimensional chain, as a model of charge order (CO), are numerically simulated using the time-dependent Hartree-Fock method. In particular, we discuss the differences between two situations as the initial state:the homogeneous order and the presence of a domain wall, i.e., a kink structure embedded in the CO bulk. Coherent dynamics are seen in the former case: When the frequency of the pump light $\omega_{p}$ is varied, along with single particle excitations across the CO gap ($\Delta_\textrm{CO}$), the resonantly-excited collective phase mode near $\omega_{p} \simeq \Delta_\textrm{CO}/2$ efficiently destabilizes CO. In clear contrast, in the latter case, when $\omega_{p}$ is tuned at such in-gap frequencies and the intensity of light is sufficiently large,inhomogeneity spreads out from the kink to the bulk region through kink creations. Moreover, even stronger intensity induces the inhomogeneous melting of CO where the CO gap is destroyed., Comment: 8 pages, 9 figures, published version

The charge disproportionation of Mn $3d$ and O $2p$ electronic states was investigated in two $({\mathrm{LaMnO}}_{3}{)}_{2}{({\mathrm{SrMnO}}_{3})}_{2}$ superlattices using the resonant x-ray scattering technique. The superlattices show significantly different conductivity even with the same composition. The large modulation of the Mn $3d$ electronic state was observed in the insulating sample, however the charge disproportionation and magnetic modulation of O $2p$ were shown in the semiconductive sample. It is suggested that the charge modulation appears in either Mn $3d$ or O $2p$, which is strongly related to strength of the $p\text{\ensuremath{-}}d$ hybridization, which is the main factor determining the electric conductivity in the superlattices.

Photoinduced nonequilibrium spin structure is examined in the double-exchange model, in which itinerant electrons couple with localized spins through the ferromagnetic Hund coupling. In particular, we focus on the transient spin structure from the initial ferromagnetic metallic state to the steady antiferromagnetic ordered state reported in [Phys. Rev. Lett. 119, 207202 (2017)]. By solving the Schr\"odinger equation combined with the Landau-Lifshitz-Gilbert equation, we find finite winding number and chirality, which implies emergence of topological chiral spin textures. These observations are reproduced by a calculation where spin dynamics after sudden quench of the chemical potential are examined in larger clusters. A possible mechanism of the topological spin texture in the transient dynamics is discussed., Comment: 5 pages, 4 figures

Magnetic, dielectric, and magnetoelectric properties in a spin-state transition system are examined, motivated by the recent discovery of a multiferroic behavior in a cobalt oxide. We construct an effective model Hamiltonian based on the two-orbital Hubbard model, in which the spin-state degrees of freedom in magnetic ions couple with ferroelectric-type lattice distortions. A phase transition occurs from the high-temperature low-spin phase to the low-temperature high-spin ferroelectric phase with accompanying an increase of the spin entropy. The calculated results are consistent with the experimental pressure-temperature phase diagram. We predict the magnetic-field induced electric polarization in the low-spin paraelectric phase near the ferroelectric phase boundary., 5 pages, 4 figures

Dynamical properties in interacting fermion systems on geometrically frustrated lattices are introduced. As a minimal model for the electronic charge order phenomena, we adopt the so-called spin-less fermion V – t model where the hoppings and interactions of spin-less fermions are taken into account in frustrated lattices. By analyzing this model, we examine (1) dielectric and optical properties in a double triangular lattice, (2) a collective excitation in a dimer-molecular system on a triangular lattice, and (3) a photo-excited state in a triangular lattice. We demonstrate that frustration effects influence directly dynamical properties in these charge systems, and explain successfully recent some experimental results.

Motivated by recent experimental studies of hydrogen-bonded molecular conductors $\kappa$-$X_3$(Cat-EDT-TTF)$_2$ [$X$=H, D], interplays of protons and correlated electrons, and their effects on magnetic, dielectric, and optical properties, are studied theoretically. We introduce a model Hamiltonian for $\kappa$-$X_3$(Cat-EDT-TTF)$_2$, in which molecular dimers are connected by hydrogen bonds. Ground-state phase diagram and optical conductivity spectra are examined by using the mean-field approximation and the exact diagonalization method in finite-size cluster. Three types of the competing electronic and protonic phases, charge density wave phase, polar charge-ordered phase, and antiferromagnetic dimer-Mott insulating phase are found. Observed softening of the inter-dimer excitation due to the electron-proton coupling implies reduction of the effective electron-electron repulsion, i.e. "Hubbard $U$", due to the quantum proton motion. Contrastingly, the intra-dimer charge excitation is harden due to the proton-electron coupling. Implications of the theoretical calculations to the recent experimental results in $\kappa$-$X_3$(Cat-EDT-TTF)$_2$ are discussed., Comment: 13 pages, 9 figures

Nonequilibrium susceptibility in photoinduced Floquet states is studied. We analyze an electron system coupled with a heat bath in a time-periodic oscillating electric field. Spin/charge susceptibility is formulated on the basis of the Floquet Green function method, and is calculated numerically in a wide range of amplitude and frequency of light. When the frequency is larger than the bandwidth, the susceptibility is enhanced due to the dynamical localization effect, and their peak positions in the momentum space are shifted by the Fermi surface deformation. In the case of the small frequency and amplitude, multiple-peak structure emerges in the susceptibility, originating from the multiple Floquet bands which cross the Fermi level. To confirm those numerical results and provide the interpretation, an approximated expression of the susceptibility is derived for small electric-field amplitude., Comment: 15 pages, 10 figures

We study the photoinduced magnetic transition in an itinerant magnet described by the double-exchange model, in which conduction electrons couple with localized spins through the ferromagnetic (FM) Hund coupling. It is shown that intense light applied to the FM ground state induces an antiferromagnetic (AFM) order, in contrast to the AFM-to-FM transition due to the photocarrier injection. In particular, we focus on the mechanism for instability of the FM structure by the light irradiation. The magnon spectrum in the Floquet state is formulated on the basis of the pertrubative expansion of the Floquet Green function. The magnon dispersion shows softening at momentum $(\pi,\pi)$ in the square lattice with increasing the light amplitude, implying photoinduced AFM instability. This result is mainly attributed to a nonequilibrium electron distribution, which promotes low-energy Stoner excitations. The transient optical conductivity spectra characterized by interband excitations and Floquet sidepeaks are available to identify the photoinduced AFM state., Comment: 13 pages, 9 figures

Spin-state transition, also known as spin crossover, plays a key role in diverse systems, including minerals and biological materials. In theory, the boundary range between the low- and high-spin states is expected to enrich the transition and give rise to unusual physical states. However, no compound that realizes a nearly degenerate critical range as the ground state without requiring special external conditions has yet been experimentally identified. This study reports that, by comprehensive measurements of macroscopic physical properties, X-ray diffractometry, and neutron spectroscopy, the Sc substitution in LaCoO$_3$ destabilizes its nonmagnetic low-spin state and generates an anomalous paramagnetic state accompanied by the enhancement of transport gap and magneto-lattice-expansion as well as the contraction of Co--O distance with the increase of electron site-transfer. These phenomena are not well described by the mixture of conventional low- and high-spin states, but by their quantum superposition occurring on the verge of a spin-state transition. The present study enables us to significantly accelerate the design of new advanced materials without requiring special equipment based on the concept of quantum spin-state criticality., Keywords: excitonic insulator, bi-exciton

Transient quantum dynamics in an interacting fermion-phonon system are investigated. In particular, a charge order (CO) melting after a short optical-pulse irradiation and roles of the quantum phonons on the transient dynamics are focused on. A spinless-fermion model in a one-dimensional chain coupled with local phonons is analyzed numerically. The infinite time-evolving block decimation algorithm is adopted as a reliable numerical method for one-dimensional quantum many-body systems. Numerical results for the photoinduced CO melting dynamics without phonons are well interpreted by the soliton picture for the CO domains. This interpretation is confirmed by the numerical simulation for an artificial local excitation and the classical soliton model. In the case of the large phonon frequency corresponding to the antiadiabatic condition, the CO melting is induced by propagations of the polaronic solitons with the renormalized soliton velocity. On the other hand, in the case of the small phonon frequency corresponding to the adiabatic condition, the first stage of the CO melting dynamics occurs due to the energy transfer from the fermionic to phononic systems, and the second stage is brought about by the soliton motions around the bottom of the soliton band. Present analyses provide a standard reference for the photoinduced CO melting dynamics in low-dimensional many-body quantum systems., 11pages, 13 figures

Photoinduced nonequilibrium phenomena in magnets with strong electron correlation are reviewed. Owing to the strong competition between electron itineracy and localization and the existence of mult...

Photo-induced electronic dynamics in strongly correlated electron system with spin-charge coupling are studied. Motivated from recent optical pump-probe experiments in perovskite manganites and cobaltites, the double-exchange system and the spin-state transition system are theoretically analyzed. First, we focus on the double-exchange interaction in photo-excited state. The time evolutions for the spin and charge structures are examined as functions of the pump photon amplitudes. In the weak-pumping case, the initial charge-ordered antiferromagnetic insulating state tends to become the ferromagnetic metallic state. On the other hand, in the strong-pumping case, the initial antiferromagnetic correlation and insulating character are enhanced after photoirradiation. This unexpected photo-excited state is detectable by the ultrafast optical measurement. Second, in the spin-state transition system, the two-band Hubbard model is analyzed by the time-dependent mean-field approximation. By introducing photons in the low-spin band insulator, high-spin states are generated through the electron-hole pair annihilation process which is governed by the electron band width. We also take into account the relativistic spin-orbit interaction, and compare the results with and without the spin-orbit interaction.

Motivated by recent experiments on Ba3CuSb2O9, we study the dynamical Jahn-Teller effect in a spin-orbital coupled system. We analyze a low-energy model Hamiltonian which includes the superexchange-type interaction and the Jahn-Teller interaction by using a combined method of the mean-field approximation and the quantum Monte-Carlo simulation. We find that magnetic orders do not appear in a wide parameter region and that the spin-dimer state with an orbital order is realized. The orbital order is strongly suppressed by the dynamical Jahn-Teller effect.

Spin and charge fluctuations at vicinity of metal-to-Mott insulator transitions are studied in an organic solid with molecular dimers. The extended Hubbard model taking account of the internal electronic degree of freedom in a molecular dimer is analyzed using the variational Monte Carlo method. Three kinds of the electronic phases, i.e. a metallic phase, an antiferromagnetic insulating phase and a polar charge ordered phase, compete with each other in the ground state. It is found that the polar-charge fluctuation is dominant in a wide range of the molecular dimerization and Coulomb interaction amplitudes, and is enhanced remarkably near the metal-insulator phase boundary, in which the spin fluctuation is almost unchanged. Implication for the $\kappa$-type BEDT-TTF salts is discussed., Comment: 5 pages, 5 figures, to be published in J. Phys. Soc. Jpn

The double-exchange (DE) interaction, that is, a ferromagnetic (FM) interaction due to a combination of electron motion and the Hund coupling, is a well known source of a wide class of FM orders. Here, we show that the DE interaction in highly photoexcited states is antiferromagnetic (AFM). Transient dynamics of quantum electrons coupled with classical spins are analyzed. An ac field applied to a metallic FM state results in an almost perfect N\'eel state. A time characterizing the FM-to-AFM conversion is scaled by light amplitude and frequency. This hidden AFM interaction is attributable to the electron-spin coupling under nonequilibrium electron distribution., Comment: 6 pages, 7 figures (including supplemental material)

The resonant X-ray scattering (RXS) and the resonant inelastic X-ray scattering (RIXS) in correlated electron systems with the orbital degree of freedom are reviewed from the theoretical view points. In the first part, the orbital order and RXS as an experimental tool to detect the orbital order are reviewed. We introduce characteristic interacting-orbital models and orbital orders, in which a unique orbital frustration effect emerges. As a case study, the theoretical and experimental studies in an impurity effect on the orbital order are presented. In the second part, the RIXS studies for observation of the orbital excitation are reviewed. In particular, we focus on the polarization dependence of RIXS which is crucial to identify the scattering from the orbital excitation. We introduce the recent progresses of the collective orbital excitations coupled with the lattice vibration. The non-resonant inelastic X-ray scattering as a tool to detect the orbital excitations is also reviewed.

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 $\mathit{\Omega}$ and a damping constant $\gamma$. In the case of low fluence, both $\mathit{\Omega}$ and $\gamma$ are scaled by the pulse amplitude in a wide range of the parameter values of the model. In the case of high fluence, $\mathit{\Omega}$ and $\gamma$ 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., Comment: 8 pages, 6 figures

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., 9 pages, 10 figures

The strong light-field effect of (TMTTF)2AsF6 was investigated utilizing 1.5-cycle, 7-fs infrared pulses. The ultarfast (20 fs) and large (40%) response of the plasma-like reflectivity edge (0.7 eV) was analyzed by the changes in omega_p=sqrt(ne2/(epsilon_0*epsilon(infty)*m)} (n: number of charges in the 1/4 filled-band, m: mass of charge, epsilon(infty): dielectric constants for high-frequency and vacuum, e: elementary charge). The 3% reduction in omega_p is attributed to the 6% increase in m. Furthermore, 20 fs oscillation of omega_p in the time domain indicates that the plasma-like edge is affected by the charge gap (0.2 eV) nature. Theoretical calculations suggest that the Coulomb repulsion plays an important role in the increase in m.

In strongly correlated electron systems, enhanced fluctuations in the proximity of the ordered states of electronic degrees of freedom often induce anomalous electronic properties such as unconventional superconductivity. While spin fluctuations in the energy-momentum space have been studied widely using inelastic neutron scattering, other degrees of freedom, i.e., charge and orbital, have hardly been explored thus far. Here, we use resonant inelastic x-ray scattering to observe charge fluctuations proximate to the charge-order phase in transition metal oxides. In the two-leg ladder of Sr14−xCaxCu24O41, charge fluctuations are enhanced at the propagation vector of the charge order ( qCO) when the order is melted by raising temperature or by doping holes. In contrast, charge fluctuations are observed not only at qCO but also at other momenta in a geometrically frustrated triangular bilayer lattice of LuFe2O4. The observed charge fluctuations have a high energy (~1 eV), suggesting that the Coulomb repulsion between electrons plays an important role in the formation of the charge order.

Photoinduced spin-charge dynamics in strongly correlated electron systems is studied based on an extended double-exchange model. Solving a time-dependent Schrodinger equation with the Lanczos method, we trace the process of photoinduced melting of an antiferromagnetic charge order and analyze the excitation-density dependence on that. In the case of low density photoexcitation, both charge and spin orders are melted by photocarrier doping. This is interpreted with a conventional double-exchange mechanism. In the case of high density photoexcitation, however, the charge order is melted. The antiferromagnetic spin order is transiently weakened but after turnoff of the photoirradiation it recovers. This phenomenon strikingly differs from the weak photoexcitation case.

Photo-induced spin-state change in correlated electron systems is examined theoretically, motivated by the recent pump-probe experiments in perovskite cobaltites. Effective Hamiltonian for the photo-excited state is derived from the two-orbital Hubbard model. High-spin state is induced by the photo-irradiation in the low-spin ground state. We demonstrate that this photo-induced spin-state change is detected by the pump-probe spectroscopy.

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., 15 pages, 9 figures

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 $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ is discussed., Comment: 9 pages, 7 figures

Dynamical localization that is, reduction of the intersite electronic transfer integral, was realized by strong electric filed (10 MV/cm) of 1.5-cycle (7 fs) near infrared light in layered organic conductor.

Suppression of electron motion under an alternating current (AC) electric field is examined in a one-dimensional Hubbard model. Utilizing three complementary calculation methods, it is found unambiguously that magnitudes of the kinetic-energy suppressions are influenced sensitively by the Coulomb interaction as well as the electron density. The phase and frequency in the AC field do not bring about major effects. The results are interpreted as a combined effect of the Coulomb interaction and the AC field, and provide a guiding principle for the photocontrol of correlated electron motion., Comment: 7 pages, 4 figures

Strong (> 10 MV/cm) light-field effect of (TMTTF)2AsF6 was investigated utilizing 7-fs infrared pulses. The ultrafast (~20 fs) and large (~40%) response of the plasma-like reflectivity edge (~0.7 eV) was discussed in terms of dynamical localization.

Dilution effects in orbital ordered systems with strong electron correlation are studied theoretically. Vacancy-concentration x dependence of the orbital-ordering temperature T OO calculated in the two-representative orbital models, i.e. the e g -orbital model in a three dimensional square lattice and the two-dimensional orbital compass model. Reduction of T OO as a function of x is stronger than that in dilute magnets and is not explained by the simple percolation theory. This phenomenon is attributed to the directional interaction between orbitals. It is shown that due to the quantum effects, the effective dimensionality in the system is increased and the orbital order becomes robust against dilution. The present calculation provides a new aspect of the dilute ordered state in strongly correlated electron systems.

Photo-control of material properties on femto- (10−15) and pico- (10−12) second timescales at room temperature has been a long-sought goal of materials science. Here we demonstrate a unique ultrafast conversion between the metallic and insulating state and the emergence of a hidden insulating state by tuning the carrier coherence in a wide temperature range in the two-leg ladder superconductor Sr14-xCaxCu24O41 through femtosecond time-resolved reflection spectroscopy. We also propose a theoretical scenario that can explain the experimental results. The calculations indicate that the holes injected by the ultrashort light reduce the coherence among the inherent hole pairs and result in suppression of conductivity, which is opposite to the conventional photocarrier-doping mechanism. By using trains of ultrashort laser pulses, we successively tune the carrier coherence to within 1 picosecond. Control of hole-pair coherence is shown to be a realistic strategy for tuning the electronic state on ultrafast timescales at room temperature., Controlling material properties on sub-picosecond scales using photons could allow for ultrafast optoelectronic devices. Here, the authors propose an ultrafast photoinduced metal-to-insulator transition in a two-leg ladder cuprate superconductor based on time-resolved reflection spectroscopy.

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., 22 pages, 5 figures

To elucidate the pressure evolution of the electronic structure in an antiferromagnetic dimer-Mott (DM) insulator ${\ensuremath{\beta}}^{\ensuremath{'}}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{ICl}}_{2}$, which exhibits superconductivity at 14.2 K under 8 GPa, we measured the polarized infrared (IR) optical spectra under high pressure. At ambient pressure, two characteristic bands due to intra- and interdimer charge transfers have been observed in the IR spectra, supporting that this salt is a typical half-filled DM insulator at ambient pressure. With increasing pressure, however, the intradimer charge transfer excitation shifts to much lower energies, indicating that the effective electronic state changes from half-filled to 3/4-filled as a result of weakening of dimerization. This implies that the system approaches a charge-ordered state under high pressure, in which charge degrees of freedom emerge as an important factor. The present results suggest that charge fluctuation inside of dimers plays an important role in the high-temperature superconductivity.