arXiv subjects

Kenji Yonemitsu

Publications and source records attributed to Kenji Yonemitsu.

At least 19 recordsLinked to original sources

arXiv2026

Ultrafast decoupling of quasiparticles and spin fluctuations in superconducting cuprates

Understanding how quasiparticles are generated following a rapid quench of superconductivity in high-Tc cuprates is a key unresolved problem in nonequilibrium superconductivity. Here we resolve these processes in optimally doped YBCO [YBa2Cu3Oy(y=6.94, Tc=92 K)] using broadband (0.16 -4.1 eV, ca. 100 fs) and nearly single-cycle (6 fs) transient reflectivity spectroscopy. We show that within a few femtosecond, enhanced electron-electron Umklapp scattering dominates, signaling a transient modulation of long-range Coulomb interactions on the eV scale. This regime is followed by a rapid suppression of the scattering rate of the mid-infrared absorption associated with carriers dressed by spin fluctuations. We attribute this observation to an ultrafast decoupling of quasiparticles from the spin-fluctuation background, occurring on a 90 fs timescale set by the inverse optical gap. These findings reveal the correlated many-body dynamics underlying quasiparticle generation in cuprates and provide further clues for unconventional pairing mechanism.

cond-mat.str-el↗

arXiv2025

Onsager Reciprocal Relations for Charge and Spin Transport in Periodically Driven Systems

A time-periodic driving field can be used to generate and control transport phenomena. Any transport coefficients in the linear-response regime are restricted by the Onsager reciprocal relations, but these relations in periodically driven systems have been poorly understood. In particular, the Onsager reciprocal relation in spin transport of these systems is lacking despite its vital role. Here we establish the Onsager reciprocal relations for charge and spin transport in periodically driven systems. We consider the time-averaged charge and spin off-diagonal dc conductivities $\sigma_{yx}^{\textrm{C}}$ and $\sigma_{yx}^{\textrm{S}}$ in the nonequilibrium steady state with the pump field of light. First, we argue the Onsager reciprocal relations for these conductivities with the pump field of circularly, linearly, or bicircularly polarized light. We show that $\sigma_{yx}^{\textrm{C}}$ and $\sigma_{yx}^{\textrm{S}}$ satisfy the Onsager reciprocal relations in all the cases considered, but their main terms depend on the polarization of light. Our numerical calculations validate our general arguments. Therefore, the spin current generated in periodically driven systems is detectable by the inverse spin Hall effect. Our numerical calculations also show that $\sigma_{yx}^{\textrm{C}}$ cannot necessarily be regarded as the anomalous Hall conductivity even with broken time-reversal symmetry, whereas $\sigma_{yx}^{\textrm{S}}$ can be regarded as the spin Hall conductivity in all the cases considered. Our results suggest that it is highly required to check the dominant terms of the charge and spin off-diagonal conductivities in discussing the anomalous Hall and spin Hall effects, respectively. This study will become a cornerstone of theoretical and experimental studies of transport phenomena in periodically driven systems.

cond-mat.mes-hall↗

arXiv2025

Optically tunable spin Hall effect in periodically driven monolayer transition metal dichalcogenides

We show that the driving field of circularly polarized light (CPL) can be used to enhance and reverse the spin current generated in the spin Hall effect for some transition-metal dichalcogenides. This is demonstrated by analyzing the time-averaged spin Hall conductivities in the nonequilibrium steady states of monolayers WS$_{2}$, MoS$_{2}$, MoTe$_{2}$, and WTe$_{2}$ driven by CPL with the Floquet linear-response theory. We argue that the enhancement and reversals of the spin current come from a combination of the non-Rashba effect of broken inversion symmetry and the nonperturbative effect of CPL beyond the dynamical localization. This work allows optical control of the magnitude and direction of the pure spin current.

cond-mat.mes-hall↗

arXiv2024

Light-Induced Mirror Symmetry Breaking and Charge Transport

We propose that light can break mirror symmetries and combining symmetries with a uniform time translation, and their breaking is characterized by an off-diagonal charge conductivity. Taking periodically driven graphene as an example, we show that mirror symmetries about the $xz$ and $yz$ planes and the combining symmetries, the symmetries of combinations of the mirror operations about these planes and a uniform time translation, can be broken by linearly or circularly polarized light. We also show that this symmetry breaking induces the time-averaged off-diagonal symmetric or antisymmetric charge conductivity in a nonequilibrium steady state with linearly or circularly, respectively, polarized light. Our results are experimentally testable in pump-probe measurements. This work will pave the way for controlling mirror symmetries via light and utilizing the light-induced mirror symmetry breaking.

cond-mat.mes-hall↗

arXiv2024

Light-induced large and tunable valley-selective Hall effect in a centrosymmetric system

We propose that a large and tunable valley-selective Hall effect can be realized in a centrosymmetric system via light-induced breaking of inversion and time-reversal symmetries. This is demonstrated in graphene driven by bicircularly polarized light, which consists of a linear combination of left- and right-handed circularly polarized light with different frequencies. We also show that our Hall conductivity is two orders of magnitude larger than the maximum value obtained in noncentrosymmetric systems, and that the main valley can be switched by tuning a phase difference between the left- and right-handed circularly polarized light. Our results will enable us to generate and control the valley-selective Hall effect in centrosymmetric systems.

cond-mat.mes-hall↗

arXiv2023

Symmetry-protected difference between spin Hall and anomalous Hall effects of a periodically driven multiorbital metal

Nonequilibrium quantum states can be controlled via the driving field in periodically driven systems. Such control, which is called Floquet engineering, has opened various phenomena, such as the light-induced anomalous Hall effect. There are expected to be some essential differences between the anomalous Hall and spin Hall effects of periodically driven systems because of the difference in time-reversal symmetry. However, these differences remain unclear due to the lack of Floquet engineering of the spin Hall effect. Here we show that when the helicity of circularly polarized light is changed in a periodically driven $t_{2g}$-orbital metal, the spin current generated by the spin Hall effect remains unchanged, whereas the charge current generated by the anomalous Hall effect is reversed. This difference is protected by the symmetry of a time reversal operation. Our results offer a way to distinguish the spin current and charge current via light and could be experimentally observed in pump-probe measurements of periodically driven Sr$_{2}$RuO$_{4}$.

cond-mat.mes-hall↗

arXiv2022

Photoinduced Pseudospin Polarization in a Three-Orbital Hubbard Model

In a Hubbard model for the Kitaev spin-liquid candidate material $\alpha$-RuCl$_3$ with three $t_{2g}$ orbitals per Ru site, we calculate photoinduced dynamics based on the exact diagonalization method and interpret them with the help of a high-frequency expansion in quantum Floquet theory. The high-frequency expansion shows two types of effective magnetic fields during the application of a circularly polarized light field. One of them originates from spin-orbit coupling and is within the honeycomb lattice. The other is of purely kinetic origin and perpendicular to the lattice. The former fields are antiparallel at the two sites within a unit cell and rotate in accordance with the momentum distribution of holes that follow the light field. When the light field is weak, pseudospin dynamics are governed by the former fields; thus, the average of the pseudospins almost vanishes. The latter fields are parallel at the two sites within a unit cell and produce nonzero perpendicular components of the pseudospins when the light field is strong. Numerically obtained perpendicular components are consistent with the latter fields when the frequency of the light field is well below the Mott gap. The relevance to the inverse Faraday effect recently observed in $\alpha$-RuCl$_3$ is discussed.

cond-mat.str-el↗

arXiv2021

Polarization-dependent magnetic properties of periodically driven $α$-RuCl$_{3}$

We study magnetic properties of a periodically driven Mott insulator with strong spin-orbit coupling and show some properties characteristic of linearly polarized light. We consider a $t_{2g}$-orbital Hubbard model driven by circularly or linearly polarized light with strong spin-orbit coupling and derive its effective Hamiltonian in the strong-interaction limit for a high-frequency case. We show that linearly polarized light can change not only the magnitudes and signs of the exchange interactions, but also their bond anisotropy even without the bond-anisotropic hopping integrals. Because of this property, the honeycomb-network spin system could be transformed into weakly coupled zigzag or step spin chains for the light field polarized along the $b$- or $a$-axis, respectively. Then, analyzing how the light fields affect several magnetic states in a mean-field approximation, we show that linearly polarized light can change the relative stability of the competing magnetic states, whereas such a change is absent for circularly polarized light. We also analyze the effects of both the bond anisotropy of nearest-neighbor hopping integrals and a third-neighbor hopping integral on the magnetic states and show that the results obtained in a simple model, in which the bond-averaged nearest-neighbor hopping integrals are considered, remain qualitatively unchanged except for the stability of zigzag states in the non-driven case and the degeneracy lifting of the zigzag or stripy states.

cond-mat.str-el↗

arXiv2021

Floquet engineering of Mott insulators with strong spin-orbit coupling

We propose a method for controlling the exchange interactions of Mott insulators with strong spin-orbit coupling. We consider a multiorbital system with strong spin-orbit coupling and a circularly polarized light field and derive its effective Hamiltonian in the strong-interaction limit. Applying this theory to a minimal model of $α$-RuCl$_{3}$, we show that the magnitudes and signs of three exchange interactions, $J$, $K$, and $Γ$, can be changed simultaneously. Then, considering another case in which one of the hopping integrals has a different value and the other parameters are the same as those for $α$-RuCl$_{3}$, we show that the Heisenberg interaction $J$ can be made much smaller than the anisotropic exchange interactions $K$ and $Γ$.

cond-mat.str-el↗

arXiv2021

Mechanism for Synchronization of Charge Oscillations in Dimer Lattices

We discuss the mechanism and the conditions for the appearance of synchronized charge oscillations which have been observed experimentally and theoretically after strong photoexcitation of dimerized systems. In the Hubbard model with on-site repulsion, the Bloch equations for a wave-number-dependent pseudospin -- whose components describe the charge-density difference, current density, and bond density between the two sublattices -- involve an alternatingly tilted pseudomagnetic field, which assists the synchronization of pseudospins with different wave numbers, irrespective of the initial condition. This fact is numerically confirmed by the dynamics in finite lattices based on the exact diagonalization method. In the presence of nearest-neighbor repulsion, however, the synchronization can be hindered by excitons. Therefore, the excitation of a sufficiently large density of free electron-hole pairs, but low density of excitons, is needed to achieve synchronization.

cond-mat.str-el↗

arXiv2020

Photoinduced dynamics of excitonic order and Rabi oscillation in the two-orbital Hubbard model

We investigate the condition for the photoinduced enhancement of an excitonic order in a two-orbital Hubbard model, which has been theoretically proposed in our previous work [Phys. Rev. B 97, 115105 (2018)], and analyze it from the viewpoint of the Rabi oscillation. Within the mean-field approximation, we simulate real-time dynamics of an excitonic insulator with a direct gap, where the pair condensation in the initial state is of BEC nature and the photoexcitation is introduced by electric dipole transitions. We first discuss that in the atomic limit our model is reduced to a two-level system that undergoes the Rabi oscillation, so that for single cycle pulses physical quantities after the photoirradiation are essentially determined by the ratio of the Rabi frequency to the pump-light frequency. Then, it is shown that this picture holds even in the case of nonzero transfer integrals where each one-particle state exhibits the Rabi oscillation leading to the enhancement of the excitonic order. We demonstrate that effects of electron-phonon interactions do not alter the results qualitatively. We also examine many-body dynamics by the exact diagonalization method on small clusters, which strongly suggests that our mechanism for the enhancement of the exctionic order survives even when quantum fluctuations are taken into account.

cond-mat.str-el↗

arXiv2020

Competition between Interactions and Randomness in Photoinduced Synchronization of Charge Oscillations on a Dimer Lattice

The synchronization of charge oscillations after photoexcitation that has been realized through the emergence of an electronic breathing mode on dimer lattices is studied here from the viewpoint of the competition between interactions and randomness. We employ an extended Hubbard model at three-quarter filling on a simple dimer lattice and add random numbers to all transfer integrals between nearest-neighbor sites. Photoinduced dynamics are calculated using the time-dependent Schrödinger equation by the exact diagonalization method. Although the randomness tends to unsynchronize charge oscillations on different bonds during and after photoexcitation, sufficiently strong on-site repulsion $U$ overcomes this effect and synchronizes these charge oscillations some time after strong photoexcitation. The degree of synchronization is evaluated using an order parameter that is derived from the time profiles of the current densities on all bonds. As to the nearest-neighbor interaction $V$, if $V$ is weakly attractive, it increases the order parameter by facilitating the charge oscillations. The relevance of these findings to previously reported experimental and theoretical results for the organic conductor $κ$-(bis[ethylenedithio]tetrathiafulvalene)$_2$Cu[N(CN)$_2$]Br is discussed.

cond-mat.str-el↗

arXiv2018

Photoinduced Dynamics of Commensurate Charge Density Wave in 1T-TaS$_{2}$ Based on Three-Orbital Hubbard Model

We study the coupled charge-lattice dynamics in the commensurate charge density wave (CDW) phase of the layered compound 1T-TaS$_{2}$ driven by an ultrashort laser pulse. For describing its electronic structure, we employ a tight-binding model of previous studies including the effects of lattice distortion associated with the CDW order. We further add on-site Coulomb interactions and reproduce an energy gap at the Fermi level within a mean-field analysis. On the basis of coupled equations of motion for electrons and the lattice distortion, we numerically study their dynamics driven by an ultrashort laser pulse. We find that the CDW order decreases and even disappears during the laser irradiation while the lattice distortion is almost frozen. We also find that the lattice motion sets in on a longer time scale and causes a further decrease in the CDW order even after the laser irradiation.

cond-mat.str-el↗

arXiv2018

Charge Oscillations Emerging after Application of an Intense Light Field to Superconductors on a Dimer Lattice

Motivated by recent experimental findings for an organic superconductor, charge oscillations that emerge after a strong pulse of an oscillating electric field is applied are studied in electron systems in a superconducting phase on a lattice with a dimerized structure. They are analyzed using Fourier spectra of charge densities whose time profiles are obtained by numerically solving the time-dependent Schrödinger equation within a mean-field approximation. Depending on the strengths of attractive interactions, different charge-oscillation modes appear. For weak attractions, the charge-oscillation mode is an electronic breathing mode, which was previously found for repulsive interactions. For strong attractions, it is a pair analog of the electronic breathing mode (a `pair' breathing mode). For intermediate attractions, it is another mode whose transient current distributions are considerably different from those of the breathing modes. We investigate how their frequencies and amplitudes depend on interactions and transfer integrals.

cond-mat.supr-con↗

arXiv2018

Strong light-field effects driven by nearly single-cycle 7-fs light field in correlated organic conductors

We have demonstrated transient charge localization effects with a driving high-frequency field of 7-fs, 1.5-cycle near infrared light in correlated organic conductors. In a layered organic conductor alpha-(BEDT-TTF)2I3 (BEDT-TTF: bis[ethylenedithio]-tetrathiafulvalene), a transient short-range charge order (CO) state is induced in a metallic phase. In contrast to such drastic change in the electronic state from the metal to the transient CO in alpha-(BEDT-TTF)2I3, dynamics of a field-induced reduction of a transfer integral are captured as a red shift of the plasma-like reflectivity edge in a quasi-one-dimensional organic conductor (TMTTF)2AsF6 (TMTTF: tetramethyltetrathiafulvalene). These studies on the field-induced charge localization have been motivated by the theory of dynamical localization on the basis of tight-binding models with no electron correlation under a strong continuous field. However, the results of pump-probe transient reflectivity measurements using nearly single-cycle 7-fs, 11 MV/cm pulses and the theoretical studies which are presented in this review indicate that the pulsed field contributes to the similar phenomenon with the help of a characteristic lattice structure and Coulomb repulsion.

cond-mat.str-el↗

arXiv2018

Photoinduced High-Frequency Charge Oscillations in Dimerized Systems

Photoinduced charge dynamics in dimerized systems is studied on the basis of the exact diagonalization method and the time-dependent Schrödinger equation for a one-dimensional spinless-fermion model at half filling and a two-dimensional model for $κ$-(bis[ethylenedithio]tetrathiafulvalene)$_2$X [$κ$-(BEDT-TTF)$_2$X] at three-quarter filling. After the application of a one-cycle pulse of a specifically polarized electric field, the charge densities at half of the sites of the system oscillate in the same phase and those at the other half oscillate in the opposite phase. For weak fields, the Fourier transform of the time profile of the charge density at any site after photoexcitation has peaks for finite-sized systems that correspond to those of the steady-state optical conductivity spectrum. For strong fields, these peaks are suppressed and a new peak appears on the high-energy side, that is, the charge densities mainly oscillate with a single frequency, although the oscillation is eventually damped. In the two-dimensional case without intersite repulsion and in the one-dimensional case, this frequency corresponds to charge-transfer processes by which all the bonds connecting the two classes of sites are exploited. Thus, this oscillation behaves as an electronic breathing mode. The relevance of the new peak to a recently found reflectivity peak in $κ$-(BEDT-TTF)$_2$X after photoexcitation is discussed.

cond-mat.str-el↗

arXiv2018

Photoinduced enhancement of excitonic order in the two-orbital Hubbard model

Photoinduced dynamics in an excitonic insulator is studied theoretically by using a two-orbital Hubbard model on the square lattice where the excitonic phase in the ground state is characterized by the BCS-BEC crossover as a function of the interorbital Coulomb interaction. We consider the case where the order has a wave vector $Q=(0,0)$ and photoexcitation is introduced by a dipole transition. Within the mean-field approximation, we show that the excitonic order can be enhanced by the photoexcitation when the system is initially in the BEC regime of the excitonic phase, whereas it is reduced if the system is initially in the BCS regime. The origin of this difference is discussed from behaviors of momentum distribution functions and momentum-dependent excitonic pair condensation. In particular, we show that the phases of the excitonic pair condensation have an important role in determining whether the excitonic order is enhanced or not.

cond-mat.str-el↗

arXiv2017

High-Frequency Analysis of Effective Interactions and Bandwidth for Transient States after Monocycle Pulse Excitation of Extended Hubbard Model

Using a high-frequency expansion in periodically driven extended Hubbard models, where the strengths and ranges of density-density interactions are arbitrary, we obtain the effective interactions and bandwidth, which depend sensitively on the polarization of the driving field. Then, we numerically calculate modulations of correlation functions in a quarter-filled extended Hubbard model with nearest-neighbor interactions on a triangular lattice with trimers after monocycle pulse excitation. We discuss how the resultant modulations are compatible with the effective interactions and bandwidth derived above on the basis of their dependence on the polarization of photoexcitation, which is easily accessible by experiments. Some correlation functions after monocycle pulse excitation are consistent with the effective interactions, which are weaker or stronger than the original ones. However, the photoinduced enhancement of anisotropic charge correlations previously discussed for the three-quarter-filled organic conductor $α$-(bis[ethylenedithio]-tetrathiafulvalene)$_2$I$_3$ [$α$-(BEDT-TTF)$_2$I$_3$] in the metallic phase is not fully explained by the effective interactions or bandwidth, which are derived independently of the filling.

cond-mat.str-el↗