arXiv subjects

Yasuhiro Tanaka

Publications and source records attributed to Yasuhiro Tanaka.

At least 19 recordsLinked to original sources

arXiv2026

A Rotation-Compensated Smartphone Accelerometer Application for Undergraduate Mechanics Experiments

Smartphones equipped with sensors such as accelerometers, gyroscopes, and magnetometers offer valuable opportunities for physics education, allowing students to measure motion using their own devices. However, commonly used applications provide acceleration only in the device-fixed coordinate system, which makes it difficult to analyze two- or three-dimensional motion when the device rotates. To address this limitation, we developed a web-based accelerometer application that can provide acceleration in a stationary global coordinate system. This is achieved by simultaneously recording acceleration in the device-fixed coordinate system and Euler angles, and converting them to rotation-compensated acceleration in real time. We also built a companion web application for numerical integration, noise reduction, and visualization of the measured data. Both applications are installation-free and can be accessed directly through a smartphone browser. We demonstrate the capabilities of the newly developed system through several representative types of motion, including sliding motion, projectile motion, and circular motion, by showing that rotation-compensated acceleration enables accurate reconstruction of velocity, displacement, and trajectories even when the smartphone changes its orientation. The applications were implemented in undergraduate mechanics classes, where students used them in group-based experiments. Classroom observations suggested that the use of these tools facilitated a deeper understanding of the relationships among acceleration, velocity, and position. These results suggest that rotation-compensated smartphone measurements provide a practical and effective tool for physics education.

physics.ed-ph↗

arXiv2025

Photoinduced melting dynamics and collective mode in a correlated charge-ordered system

We theoretically investigate the transient spectral function during the photoinduced melting of charge order in a correlated electron system, to unravel the dynamical processes triggered by different initial excitations. We employ a one-dimensional interacting spinless fermion model introducing a pulsed laser light, and perform a comparative study by the Hartree-Fock approximation and by the exact diagonalization method to numerically solve the time-dependent Schr\"odinger equation. We find characteristic behavior in the transient spectral function, whose features strongly depend on the pump light frequency $\omega_p$. When $\omega_p$ is resonant with the collective phase mode of frequency $\Omega_c\simeq \Delta_{\rm CO}/2$, where $\Delta_{\rm CO}$ is the charge gap, the transient spectral function exhibits a photoinduced in-gap weight which triggers large responses. With increasing the laser intensity, the development of in-gap weight directly turns into the collapse of the gap. This charge-order destabilization process is in sharp contrast to the case of $\omega_p>\Delta_{\rm CO}$, where the photoirradiation induces interband electron-hole excitations giving rise to a shrinkage of the gap. The impact of quantum fluctuations and spatial inhomogeneity on the photoinduced dynamics is also discussed.

cond-mat.str-el↗

arXiv2024

Developing vocal system impaired patient-aimed voice quality assessment approach using ASR representation-included multiple features

The potential of deep learning in clinical speech processing is immense, yet the hurdles of limited and imbalanced clinical data samples loom large. This article addresses these challenges by showcasing the utilization of automatic speech recognition and self-supervised learning representations, pre-trained on extensive datasets of normal speech. This innovative approach aims to estimate voice quality of patients with impaired vocal systems. Experiments involve checks on PVQD dataset, covering various causes of vocal system damage in English, and a Japanese dataset focusing on patients with Parkinson's disease before and after undergoing subthalamic nucleus deep brain stimulation (STN-DBS) surgery. The results on PVQD reveal a notable correlation (>0.8 on PCC) and an extraordinary accuracy (<0.5 on MSE) in predicting Grade, Breathy, and Asthenic indicators. Meanwhile, progress has been achieved in predicting the voice quality of patients in the context of STN-DBS.

cs.SD↗

arXiv2023

Photoinduced inhomogeneous melting of charge order by ultrashort pulsed light excitation

We numerically investigate photo-responses of the charge ordered state upon stimuli of pulsed laser light, especially paying attention to the differences in the pulse width, whose shortness has been a key to experimentally realize large photo-induced effects. As a model for charge ordering, we consider an interacting spinless fermion model on a one-dimensional chain coupled to classical phonons and numerically simulate its real-time dynamics. First, we demonstrate the case where spatially uniform dynamics take place, and show the delayed response of phonons after the photo-irradiation causing destabilization of charge order. When a local impurity potential is introduced in the initial state, an inhomogeneous charge order melting occurs and the delay in the time scale also happens in the real space melting process. Finally, by investigating wider parameter ranges using different pulse widths, we find that the use of ultrashort pulses considerably expands the frequency window for charge order melting; it is indispensable in realizing the photo-induced phase transition with off-resonant conditions.

cond-mat.str-el↗

arXiv2022

Dynamical phase transitions in the photodriven charge-ordered Dirac-electron system

Photoinduced phase transitions and charge dynamics in the interacting Dirac-electron system with a charge-ordered ground state are theoretically studied by taking an organic salt $α$-(BEDT-TTF)$_2$I$_3$. By analysing the extended Hubbard model for this compound using a combined method of numerical simulations based on the time-dependent Schrödinger equation and the Floquet theory, we observe successive dynamical phase transitions from the charge-ordered insulator to a gapless Dirac semimetal and, eventually, to a Chern insulator phase under irradiation with circularly polarized light. These phase transitions occur as a consequence of two major effects of circularly polarized light, i.e., closing of the charge gap through melting the charge order and opening of the topological gap by breaking the time reversal symmetry at the Dirac points. We demonstrate that these photoinduced phenomena are governed by charge dynamics of driven correlated Dirac electrons.

cond-mat.str-el↗

arXiv2022

Predicted novel type of photoinduced topological phase transition accompanied by collision and collapse of Dirac-cone pair in organic salt $α$-(BEDT-TTF)$_2$I$_3$

Photoinduced topological phase transitions in the Dirac-electron systems have attracted intensive research interest since its theoretical prediction in graphene, where the application of circularly polarized light opens a gap at the Dirac points and renders the system a topologically nontrivial Chern insulator phase through breaking the time-reversal symmetry. However, most of the previously studied phenomena in two-dimensional Dirac systems are basically based on the same physicical mechanism, i.e., the gap opening in the Dirac electron bands with circularly polarized light, and it is lacking in variety of the physics. In this paper, we theoretically predict a novel type of photoinduced topological phase transition accompanied by collision and collapse of gapped Dirac points in the organic salt $α$-(BEDT-TTF)$_2$I$_3$. By constructing the Floquet theory for this compound, we demonstrate that the irradiation of elliptically polarized light causes collision of the Dirac points through the photoinduced band deformation and their collapse, which eventually results in the topological phase transition from a topological semimetal with gapped Dirac cones to a normal insulator when the elliptical axis is oriented at a specfic angle with respect to the crystallographic axes. We argue that this novel photoinduced phase transition can be experimentally detected by the measurement of Hall conductivity. The present work enriches the fundamental physics of photoinduced topological phase transitions and thus contribute to development of this rapidly growing research field.

cond-mat.str-el↗

arXiv2021

Predicted photoinduced pair annihilation of emergent magnetic charges in the organic salt $α$-(BEDT-TTF)$_2$I$_3$ irradiated by linearly polarized light

Prolonged experimental attempts to find magnetic monopoles (i.e., elementary particles with an isolated magnetic charge in three dimensions) have not yet been successful despite intensive efforts made since Dirac's proposal in 1931. Particle physicists have predicted the possible collision and pair annihilation of two magnetic charges with opposite signs. However, if such annihilation exists, its experimental observation would be difficult because its energy scale is predicted to be tremendously high ($\sim$10$^{16}$ GeV). In the present work, we theoretically predict using the Floquet theory that a pair of slightly gapped Dirac-cone bands in a weakly-charge-ordered organic conductor $α$-(BEDT-TTF)$_2$I$_3$, which behave as magnetic charges with opposite signs in the momentum space, exhibit pair annihilation under irradiation with linearly polarized light. This photoinduced pair annihilation is accompanied by a non-topological phase transition to the Floquet normal insulator phase in contrast to the well-known circularly-polarized-light-induced topological phase transition to the Floquet Chern insulator phase. We discuss that $α$-(BEDT-TTF)$_2$I$_3$ has a peculiar band structure capable of realizing a suitable experimental condition (i.e., off-resonant condition) and a charge ordered state providing a required staggered site potential and thereby provides a rare example of materials that can be used to observe the predicted pair annihilation phenomenon. The feasibility of experimental observation is also discussed.

cond-mat.str-el↗

arXiv2021

Machine-learning detection of the Berezinskii-Kosterlitz-Thouless transitions in the q-state clock models

We demonstrate that a machine learning technique with a simple feedforward neural network can sensitively detect two successive phase transitions associated with the Berezinskii-Kosterlitz-Thouless (BKT) phase in q-state clock models simultaneously by analyzing the weight matrix components connecting the hidden and output layers. We find that the method requires only a data set of the raw spatial spin configurations for the learning procedure. This data set is generated by Monte-Carlo thermalizations at selected temperatures. Neither prior knowledge of, for example, the transition temperatures, number of phases, and order parameters nor processed data sets of, for example, the vortex configurations, histograms of spin orientations, and correlation functions produced from the original spin-configuration data are needed, in contrast with most of previously proposed machine learning methods based on supervised learning. Our neural network evaluates the transition temperatures as T_2/J=0.921 and T_1/J=0.410 for the paramagnetic-to-BKT transition and BKT-to-ferromagnetic transition in the eight-state clock model on a square lattice. Both critical temperatures agree well with those evaluated in the previous numerical studies.

cond-mat.stat-mech↗

arXiv2021

Floquet theory of photoinduced topological phase transitions in the organic salt $α$-(BEDT-TTF)$_2$I$_3$ irradiated with elliptically polarized light

We theoretically investigate possible photoinduced topological phase transitions in the organic salt $α$-(BEDT-TTF)$_2$I$_3$, which possesses a pair of inclined massless Dirac-cone bands between the conduction and valence bands under uniaxial pressure. The Floquet analyses of a driven tight-binding model for this material reveal rich photoinduced variations of band structures, Chern numbers, and Hall conductivities under irradiation with elliptically polarized light. The obtained phase diagrams contain a variety of nonequilibrium steady phases, e.g., the Floquet Chern insulator, Floquet semimetal, and Floquet normal insulator phases. This work widens a scope of target materials for research on photoinduced topological phase transitions and contributes to development of research on the optical manipulations of electronic states in matters.

cond-mat.str-el↗

arXiv2021

Real-time dynamics of the photoinduced topological state in organic conductor $α$-(BEDT-TTF)$_2$I$_3$ under continuous-wave and pulse excitations

We theoretically study the real-time dynamics of the photoinduced topological phase transition to a nonequilibrium Floquet Chern insulator in an organic conductor $α$-(BEDT-TTF)$_2$I$_3$, which was recently predicted using the Floquet theory. By using a tight-binding model of $α$-(BEDT-TTF)$_2$I$_3$ that hosts a pair of tilted Dirac-cone bands at the Fermi level, we solve the time-dependent Schrödinger equation and obtained time evolutions of physical quantities for continuous-wave and pulse excitations with circularly polarized light. We demonstrate that, for the continuous-wave excitations, time profiles of the Chern number and the Hall conductivity show indications of the Floquet topological insulator. We argue that the Hall conductivity exhibits a slow oscillation with its frequency corresponding to a photoinduced direct gap determined by the Floquet band structure. With pulse excitations, transient excitation spectra are obtained, from which we infer the formation of Floquet bands and the gap opening at the Dirac point during the pulse irradiation. This dynamical gap formation is also manifested by the slow oscillation component of the Hall conductivity; that is, its frequency increases with time toward the pulse peak at which it nearly coincides with the photoinduced direct gap. The relevance of the results to experiments is also discussed.

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

Theory of the Inverse Faraday Effect due to the Rashba Spin-Oribt Interactions: Roles of Band Dispersions and Fermi Surfaces

We theoretically study the inverse Faraday effect, i.e., the optical induction of spin polarization with circularly polarized light, by particularly focusing on effects of band dispersions and Fermi surfaces in crystal systems with the spin-orbit interaction (SOI). By numerically solving the time-dependent Schrödinger equation of a tight-binding model with the Rashba-type SOI, we reproduce the light-induced spin polarization proportional to $E_0^2/ω^3$ where $E_0$ and $ω$ are the electric-field amplitude and the angular frequency of light, respectively. This optical spin induction is attributed to dynamical magnetoelectric coupling between the light electric field and the electron spins mediated by the SOI. We elucidate that the magnitude and sign of the induced spin polarization sensitively depend on the electron filling. To understand these results, we construct an analytical theory based on the Floquet theorem. The theory successfully explains the dependencies on $E_0$ and $ω$ and ascribes the electron-filling dependence to a momentum-dependent effective magnetic field governed by the Fermi-surface geometry. Several candidate materials and experimental conditions relevant to our theory and model parameters are also discussed. Our findings will enable us to engineer the magneto-optical responses of matters via tuning the material parameters.

cond-mat.str-el↗

arXiv2019

Photoinduced collective mode, inhomogeneity, and melting in a charge order system

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 $ω_{p}$ is varied, along with single particle excitations across the CO gap ($Δ_\textrm{CO}$), the resonantly-excited collective phase mode near $ω_{p} \simeq Δ_\textrm{CO}/2$ efficiently destabilizes CO. In clear contrast, in the latter case, when $ω_{p}$ 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.

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↗

arXiv2016

Correlation Effects on Charge Order and Zero-Gap State in the Organic Conductor α-(BEDT-TTF)2I3

The effects of electron correlation in the quasi-two-dimensional organic conductor α-(BEDT-TTF)2I3 are investigated theoretically by using an extended Hubbard model with on-site and nearest-neighbor Coulomb interactions. A variational Monte Carlo method is applied to study its ground-state properties. We show that there appears a nonmagnetic horizontal-stripe charge order in which nearest-neighbor correlation functions indicate a tendency toward a spin-singlet formation on the bonds with large transfer integrals along the charge-rich stripe. Under uniaxial pressure, a first-order transition from the nonmagnetic charge order to a zero-gap state occurs. Our results on a spin correlation length in the charge-ordered state suggest that a spin gap is almost unaffected by the uniaxial pressure in spite of the suppression of the charge disproportionation. The relevance of these contrasting behaviors in spin and charge degrees of freedom to recent experimental observations is discussed.

cond-mat.str-el↗

arXiv2015

Phase Separation Induced by Symmetric Monocycle Optical Pulse in Extended Hubbard Models

Many-electron dynamics induced by a symmetric monocycle electric-field pulse of large amplitude is theoretically investigated in one- and two-dimensional half-filled extended Hubbard models on regular lattices (i.e., without dimerization) using the exact diagonalization method for small systems and the Hartree-Fock approximation for large systems. The formation of a negative-temperature state and the change from repulsive interactions to effective attractive interactions are shown to be realized for a wide region of the field amplitude and the excitation energy. For a nonnegligible intersite repulsive interaction, the numerical results are consistent with the fact that the phase separation between charge-rich and charge-poor regions is caused by the corresponding effective attraction.

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

Roles of Potential Gradient and Electrode Bandwidth on Negative Differential Resistance in One-Dimensional Band Insulator

A negative differential resistance (NDR) in a one-dimensional band insulator attached to electrodes is investigated. We systematically examine the effects of an electrode bandwidth and a potential distribution inside the insulator on current-voltage characteristics. We show that, in uncorrelated systems, the NDR is generally caused by a linear potential gradient as well as by a finite electrode bandwidth. In particular, the former reduces the effective bandwidth of the insulator for elastic tunneling by tilting its energy band, so that it brings about the NDR even in the limit of large electrode bandwidth.

cond-mat.str-el↗