arXiv subjects

Nobuya Maeshima

Publications and source records attributed to Nobuya Maeshima.

At least 19 recordsLinked to original sources

arXiv2024

Multi-Triplon Excitations of Hubbard Ladders with Site-Dependent Potentials

We study low-lying spin-singlet excitations of two-leg Hubbard ladders with site-dependent potentials. Using general formulas of the charge disproportionation induced by the site-dependent potentials, we derive the contributions of spin degrees of freedom to the spectral functions such as the dynamical charge structure factor $N(\boldsymbol{k},ω)$ and the optical conductivity $σ_γ(ω)$ along the $γ(=x,y)$-direction of the two-leg ladders. Numerical results obtained by the Lanczos diagonalization method have clarified that the multi-triplon singlet states, including two- and three-triplon excitations, can be detected by observing these quantities. Furthermore, we have found that ladders with random potentials also have non-negligible contributions of these excitations to $σ_x(ω)$.

cond-mat.str-el↗

arXiv2023

Laser induced surface magnetization in Floquet-Weyl semimetals

We investigate optically induced magnetization in Floquet-Weyl semimetals generated by irradiation of a circularly-polarized continuous-wave laser from the group II-V narrow gap semiconductor Zn$_3$As$_2$ in a theoretical manner. Here, this trivial and nonmagnetic crystal is driven by the laser with a nearly resonant frequency with a band gap to generate two types of Floquet-Weyl semimetal phases composed of different spin states. These two phases host nontrivial two-dimensional surface states pinned to the respective pairs of the Weyl points. By numerically evaluating the laser-induced transient carrier-dynamics, it is found that both spins are distributed in an uneven manner on the corresponding surface states due to significantly different excitation probabilities caused by the circularly-polarized laser with the nearly resonant frequency. It is likely that such spin-polarized surface states produce surface magnetization, and furthermore the inverse Faraday effect also contributes almost as much as the spin magnetization. To be more specific, excited carries with high density of the order of $10^{21}\: {\rm cm}^{-3}$ are generated by the laser with electric field strength of a few MV/cm to result in the surface magnetization that becomes asymptotically constant with respect to time, around 1 mT. The magnitude and the direction of it depend sharply on both of the intensity and frequency of the driving laser, which would be detected by virtue of the magneto-optic Kerr effect.

cond-mat.mtrl-sci↗

arXiv2022

Floquet-Weyl semimetals generated by an optically resonant interband-transition

Floquet-Weyl semimetals (FWSMs) generated by irradiation of a continuous-wave laser with left-hand circular polarization (rotating in counterclockwise sense with time) on the group II-V narrow gap semiconductor Zn$_3$As$_2$ are theoretically investigated, where the frequency of the laser is set nearly {\it resonant} with a band gap of the crystal. It is found that the excitation of the crystal by such a laser induce two types of FWSM phases that differ absolutely in characters. To be specific, the associated two pairs of Weyl points are stably formed by band touching between Floquet sidebands ascribable to a valence band labeled as $J_z=\pm3/2$ and a conduction band labeled as $J_z=\pm 1/2$, where $J_z$ represents the $z$-component of total angular momentum quantum number of $Γ$-point and a double sign corresponds. Here, one FWSM state composed of the up-spin Floquet sidebands relevant to $J_z=3/2$ and $1/2$ shows almost quadratic band-touching in the vicinity of the associated pair of Weyl points, while the other FWSM state composed of the down-spin Floquet sidebands relevant to $J_z=-3/2$ and $-1/2$ shows linear band-touching. Further, it is revealed that both up-spin and down-spin sidebands host nontrivial two-dimensional surface states that are pinned to the respective pairs of the Weyl points. Both surface states also show different energy dispersions and physical properties. More detailed discussion is made in the text on the origin of the above findings, chirality of the FWSM phases, alteration of topological order, laser-induced magnetic properties, and so on.

cond-mat.mtrl-sci↗

arXiv2020

Edge states of Floquet-Dirac semimetal in a laser-driven semiconductor quantum-well

Band crossings observed in a wide range of condensed matter systems are recognized as a key to understand low-energy fermionic excitations that behave as massless Dirac particles. Despite rapid progress in this field, the exploration of non-equilibrium topological states remains scarce and it has potential ability of providing a new platform to create unexpected massless Dirac states. Here we show that in a semiconductor quantum-well driven by a cw-laser with linear polarization, the optical Stark effect conducts bulk-band crossing, and the resulting Floquet-Dirac semimetallic phase supports an unconventional edge state in the projected one-dimensional Brillouin zone under a boundary condition that an electron is confined in the direction perpendicular to that of the laser polarization. Further, we reveal that this edge state mediates a transition between topological and non-topological edge states that is caused by tuning the laser intensity. We also show that the properties of the edge states are strikingly changed under a different boundary condition. It is found that such difference originates from that nearly fourfold-degenerate points exist in a certain intermediate region of the bulk Brillouin zone between high-symmetry points.

cond-mat.mes-hall↗

arXiv2019

Dynamical Charge Structure Factor of a One-Dimensional Ionic Hubbard Model in the Low-Energy Region

We present a numerical study of the charge dynamical structure factor N(k,omega) of a one-dimensional (1D) ionic Hubbard model in the Mott insulator phase. We show that the low-energy spectrum of N(k,omega) is expressed in terms of the spin operators for the spin degrees of freedom. Numerical results of N(k,omega) for the spin degrees of freedom, obtained by the Lanczos diagonalization method, well reproduce the low-energy spectrum of N(k,omega) of the 1D ionic Hubbard model. In addition, we show that these spectral peaks probe the dispersion of the spin-singlet excitations of the system and are observed in the wide parameter region of the MI phase.

cond-mat.str-el↗

arXiv2018

Ultrafast asymmetric Rosen-Zener-like coherent phonon responses observed in silicon

We investigate the spectral profiles of time signals attributed to coherent phonon generation in an undoped Si crystal. Here, the retarded longitudinal-optical (LO) phonon Green function relevant to the temporal variance of induced charge density of ionic cores is calculated by employing the polaronic quasiparticle model developed by the authors [Y. Watanabe et al., Phys. Rev. B 95, 014301 (2017); ibid., 96, 125204 (2017)]. The spectral asymmetry is revealed in the frequency domain of the signals under the condition that an LO phonon mode stays almost energetically resonant with a plasmon mode in the early time region; this lasts for approximately 100 fs immediately after the irradiation of an ultrashort pump-laser pulse. It is understood that based on the adiabatic picture in time, this asymmetry is caused by the Rosen-Zener coupling between both modes. The associated experimental results are obtained by measuring time-dependent electro-optic reflectivity signals, and it is proved that these are in harmony with the calculated ones. The spectra become more symmetric, as the photoexcited carrier density further changes from that meeting the above condition to higher and lower sides of carrier densities. Moreover, the effect of optical nutation of carrier density on the CP signals is addressed, and the present results are compared with the asymmetry caused by transient Fano resonance, and the spectral profiles observed in a GaAs crystal in the text.

cond-mat.mtrl-sci↗

arXiv2017

Irregular Oscillatory-Patterns in the Early-Time Region of Coherent Phonon Generation in Silicon

Coherent phonon (CP) generation in an undoped Si crystal is theoretically investigated to shed light on unexplored quantum-mechanical effects in the early-time region immediately after the irradiation of ultrashort laser pulse. One examines time signals attributed to an induced charge density of an ionic core, placing the focus on the effects of the Rabi frequency $Ω_{0cv}$ on the signals; this frequency corresponds to the peak electric-field of the pulse. It is found that at specific $Ω_{0cv}$'s where the energy of plasmon caused by photoexcited carriers coincides with the longitudinal-optical phonon energy, the energetically {\it resonant } interaction between these two modes leads to striking anticrossings, revealing irregular oscillations with anomalously enhanced amplitudes in the observed time signals. Also, the oscillatory pattern is subject to the Rabi flopping of the excited carrier density that is controlled by $Ω_{0cv}$. These findings show that the early-time region is enriched with quantum-mechanical effects inherent in the CP generation, though experimental signals are more or less masked by the so-called coherent artifact due to nonlinear optical effects.

cond-mat.mtrl-sci↗

arXiv2016

Polaronic-Quasiparticle Picture for Generation Dynamics of Coherent Phonons in Semiconductors: Transient and Non-Linear Fano Resonance

We examine generation dynamics of coherent phonons (CPs) in both of polar and non-polar semiconductors -- such as GaAs and Si -- based on a polaronic-quasiparticle (PQ) model. In the model concerned, the PQ operator is composed of two kinds of operators. One is a quasiboson operator -- defined as a linear combination of a set of pairs of electron operators -- and the other is a longitudinal optical (LO) phonon operator. The problem of transient and non-linear Fano resonance (FR) is tackled in particular; the vestige of this quantum interference effect was observed exclusively in lightly $n$-doped Si immediately after carriers were excited by an ultrashort pulse-laser [M. Hase et. al., Nature 426, 51 (2003)], though not observed yet in GaAs. It is shown that the phonon energy state is embedded in a continuum state formed by a set of adiabatic eigenstates of the quasiboson. This result implies the possibility of manifestation of the transient FR in the present optically-non-linear system.

cond-mat.mtrl-sci↗

arXiv2015

Manifestation of anomalous Floquet states with longevity in dynamic fractional Stark ladder with high AC electric fields

We examine a resonance structure of Floquet state in dynamic fractional Stark ladder (DFSL) realized in biased semiconductor superlattices driven by a terahertz cw laser on the basis of the R-matrix Floquet theory. To do this, we calculate an excess density of state $ρ^{(ex)}(E)$ corresponding to lifetime of the Floquet state with a fractional matching ratio $η$, where $η$ is the ratio of a Bloch frequency $Ω_B$ to a laser frequency $ω$, namely, $η=Ω_B/ω$. The results for $η=3/2$ demonstrate the appearance of discernibly large peaks associated with Floquet states with longevity in a region of relatively high laser-intensity. The underlying physics is discussed in terms of an analytical expression of $ρ^{(ex)}(E)$ and the associated Green function in which ponderomotive couplings are included in a non-perturbative way.

cond-mat.mes-hall↗

arXiv2011

Photoexcitation-Energy-Dependent Transition Pathways from a Dimer Mott Insulator to a Metal

We theoretically study pump-photon-energy-dependent pathways of a photoinduced dimer-Mott-insulator-to-metal transition, on the basis of numerical solutions to the time-dependent Schrödinger equation for the exact many-body wave function of a two-dimensional three-quarter-filled extended Peierls-Hubbard model. When molecular degrees of freedom inside a dimer are utilized, photoexcitation can weaken the effective interaction or increase the density of photocarriers. In the organic dimer Mott insulator, $ κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br, the intradimer and the interdimer charge-transfer excitations have broad bands that overlap with each other. Even in this disadvantageous situation, the photoinduced conductivity change depends largely on the pump photon energy, confirming the two pathways recently observed experimentally. The characteristic of each pathway is clarified by calculating the modulation of the effective interaction and the number of carriers involved in low-energy optical excitations. The pump-photon-energy-dependent pathways are confirmed to be realized from the finding that, although the effective interaction is always and slowly weakened, the introduction of carriers is sensitive to the pump-photon energy and proceeds much faster.

cond-mat.str-el↗

arXiv2010

Photoinduced coherent oscillations in the one-dimensional two-orbital Hubbard model

We study photoinduced ultrafast coherent oscillations originating from orbital degrees of freedom in the one-dimensional two-orbital Hubbard model. By solving the time-dependent Schrödinger equation for the numerically exact many-electron wave function, we obtain time-dependent optical response functions. The calculated spectra show characteristic coherent oscillations that vary with the frequency of probe light. A simple analysis for the dominant oscillating components clarifies that these photoinduced oscillations are caused by the quantum interference between photogenerated states. The oscillation attributed to the Raman-active orbital excitations (orbitons) clearly appears around the charge-transfer peak.

cond-mat.str-el↗

arXiv2009

Coupling-dependent rates of energy transfers from photoexcited Mott insulators to lattice vibrations

Photoexcited states are relaxed by transferring energy to the environments. In order to study which coupling allows fast energy transfer to lattice vibrations in correlated electron systems, we calculate the time evolutions of the kinetic energies of different types and frequencies of lattice vibrations. The one-dimensional half-filled Hubbard model is augmented with electron-lattice couplings that modulate transfer integrals, site energies, and Coulomb repulsion strengths. The time-dependent Schrödinger equation is solved for exact many-electron wave functions, and the classical equation of motion for the lattice displacements. In order to transfer energy to classical lattice vibrations that modulate transfer integrals or site energies, the translational invariance must be broken to give optical activity to an electronic excitation with wave number $ π$ and to these lattice vibrations. On the other hand, a certain amount of energy is always transferred to lattice vibrations that modulate Coulomb repulsion strengths, irrespective of the symmetry of the ground state, as long as the corresponding electron-lattice couplings are present. In strongly correlated electron systems, these couplings can be strong, although they are usually insignificant because their effects on the equilibrium properties can be absorbed into redefinition of Coulomb repulsion strengths. We will discuss competition or collaboration between energy transfer pathways through different types of electron-lattice couplings.

cond-mat.str-el↗

arXiv2008

Polaronic states with Spin-Charge-Coupled Excitation in a 1D Mott Insulator K-TCNQ

We discuss photogenerated midgap states of a one-dimensional (1D) dimerized Mott insulator, potassium-tetracyanoquinodimethane (K-TCNQ). Two types of phonon modes are taken into account: intermolecular and intramolecular vibrations. We treat these phonon modes adiabatically and analyze a theoretical model by using the density-matrix renormalization group (DMRG). Our numerical results demonstrate that the intermolecular lattice distortion is necessary to reproduce the photoinduced midgap absorption in K-TCNQ. We find two types of midgap states. One is a usual polaronic state characterized by a localized elementary excitation. The other is superposition of two types of excitations, a doped-carrier state and a triplet-dimer state, which can be generally observed in 1D dimerized Mott insulators, not limited to K-TCNQ.

cond-mat.str-el↗

arXiv2007

Magnetic Field versus Temperature Phase Diagram of the Spin-1/2 Alternating Chain Compound F5PNN

We have measured the specific heat of the S = 1/2 alternating Heisenberg antiferromagnetic chain compound pentafluorophenyl nitronyl nitroxide in magnetic fields using a single crystal and powder. A sharp peak due to field-induced magnetic ordering (FIMO) is observed in both samples. The H-T phase boundary of the FIMO of the single crystal is symmetric with respect to the central field of the gapless field region HC1 < H < HC2, whereas it is distorted for the powder whose ordering temperatures are lower. An analysis employing calculations based on the finite temperature density matrix renormalization group indicates the possibility of novel incommensurate ordering due to frustration in the powder around the central field.

cond-mat.str-el↗

arXiv2007

Suppression of rectification at metal-Mott-insulator interfaces

Charge transport through metal-Mott-insulator interfaces is studied and compared with that through metal-band-insulator interfaces. For band insulators, rectification has been known to occur owing to a Schottky barrier, which is produced by the work-function difference. For Mott insulators, however, qualitatively different current-voltage characteristics are obtained. Theoretically, we use the one-dimensional Hubbard model for a Mott insulator and attach to it the tight-binding model for metallic electrodes. A Schottky barrier is introduced by a solution to the Poisson equation with a simplified density-potential relation. The current density is calculated by solving the time-dependent Schrödinger equation. We mainly use the time-dependent Hartree-Fock approximation, and also use exact many-electron wave functions on small systems for comparison. Rectification is found to be strongly suppressed even for large work-function differences. We show its close relationship with the fact that field-effect injections into one-dimensional Mott insulators are ambipolar. Experimentally, we fabricated asymmetric contacts on top of single crystals of quasi-one-dimensional organic Mott and band insulators. Rectification is strongly suppressed at an interface between metallic magnesium and Mott-insulating (BEDT-TTF)(F$_2$TCNQ) [BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene, F$_2$TCNQ=2,5-difluorotetracyanoquinodimethane].

cond-mat.str-el↗

arXiv2007

Photoinduced melting of charge order in a quarter-filled electron system coupled with different types of phonons

Photoinduced melting of charge order is calculated by using the exact many-electron wave function coupled with classically treated phonons in the one-dimensional quarter-filled Hubbard model with Peierls and Holstein types of electron-phonon couplings. The model parameters are taken from recent experiments on (EDO-TTF)_2PF_6 (EDO-TTF=ethylenedioxy-tetrathiafulvalene) with (0110) charge order, where transfer integrals are modulated by molecular displacements (bond-coupled phonons) and site energies by molecular deformations (charge-coupled phonons). The charge-transfer photoexcitation from (0110) to (0200) configurations and that from (0110) to (1010) configurations have different energies. The corresponding excited states have different shapes of adiabatic potentials as a function of these two phonon amplitudes. The adiabatic potentials are shown to be useful in understanding differences in the photoinduced charge dynamics and the efficiency of melting, which depend not only on the excitation energy but also on the relative phonon frequency of the bond- and charge-coupled phonons.

cond-mat.str-el↗

arXiv2007

Charge-Transfer Excitations in One-Dimensional Dimerized Mott Insulators

We investigate the optical properties of one-dimensional (1D) dimerized Mott insulators using the 1D dimerized extended Hubbard model. Numerical calculations and a perturbative analysis from the decoupled-dimer limit clarify that there are three relevant classes of charge-transfer (CT) states generated by photoexcitation: interdimer CT unbound states, interdimer CT exciton states, and intradimer CT exciton states. This classification is applied to understanding the optical properties of an organic molecular material, 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA), which is known for its photoinduced transition from the dimerized spin-singlet phase to the regular paramagnetic phase. We conclude that the lowest photoexcited state of TTTA is the interdimer CT exciton state and the second lowest state is the intradimer CT exciton state.

cond-mat.str-el↗

arXiv2006

Dynamics of photoexcited states in one-dimensional dimerized Mott insulators

Dynamical properties of photoexcited states are theoretically studied in a one-dimensional Mott insulator dimerized by the spin-Peierls instability. Numerical calculations combined with a perturbative analysis have revealed that the lowest photoexcited state without nearest-neighbor interaction corresponds to an interdimer charge transfer excitation that belongs to dispersive excitations. This excited state destabilizes the dimerized phase, leading to a photoinduced inverse spin-Peierls transition. We discuss the purely electronic origin of midgap states that are observed in a latest photoexcitation experiment of an organic spin-Peierls compound, K-TCNQ (potassium-tetracyanoquinodimethane).

cond-mat.str-el↗