arXiv subjects

Naotaka Yoshikawa

Publications and source records attributed to Naotaka Yoshikawa.

10 recordsLinked to original sources

arXiv2025

Light-induced anomalous Hall conductivity in massive 3D Dirac semimetal Co$_3$Sn$_2$S$_2$

Weyl semimetals can emerge from Dirac semimetals when the time-reversal or spatial-inversion symmetries are broken. Recently, it has been proposed based on the Floquet theory that Dirac semimetals can be converted into Weyl semimetals even by shining circularly polarized light (CPL). Here we have investigated the possibility of such a Dirac-Weyl conversion by measuring the CPL-induced anomalous Hall conductivity (AHC) in a massive 3D Dirac semimetal Co$_3$Sn$_2$S$_2$ in the paramagnetic phase using ultrafast mid-infrared pump-terahertz Faraday rotation probe spectroscopy. We find that the field-strength and driving frequency dependence of the observed AHC is well accounted for by CPL-induced nonzero Berry curvature associated with the splitting of the Dirac bands as predicted by the Floquet theory. The estimated splitting of the Dirac bands reaches about 60 % of the mass gap and the calculated CPL-induced AHC quantitatively reproduces the experimental observation, demonstrating a promising route toward the realization of Floquet-Weyl states from massive Dirac semimetals.

cond-mat.mes-hall↗

arXiv2024

Emergence of high-mobility carriers in topological kagome bad metal Mn$_3$Sn by intense photoexcitation

Kagome-lattice materials offer novel playgrounds of exploring topologically nontrivial states of electrons under influence of many-body interactions. A noncollinear kagome antiferromagnet Mn$_3$Sn has attracted particular interest for application in spintronics owing to the large anomalous Hall effect related to the Weyl dispersion near the Fermi energy. In addition, strong electronic correlation suggesting the Kondo physics has also been implied. However, the effect of correlation on the band topology and their interplay remains elusive. Here, we investigate nonequilibrium Hall transport in a photoexcited Mn$_3$Sn using time-resolved terahertz Faraday rotation spectroscopy. In equilibrium, Mn$_3$Sn is a bad metal close to the Mott-Ioffe-Regal limit with low carrier mobility, and thus only the anomalous Hall effect is discerned. By contrast, intense photoexcitation beyond an approximate threshold gives rise to a clear cyclotron resonance, namely the normal Hall effect, indicating the emergence of unusual carriers with 50 times lighter effective mass and 40 times less scattering. The lifetime of high-mobility carriers as long as a few tens of picoseconds and a threshold-like behavior for the pump fluence are hardly explained by contribution of photoexcited hot carriers. Instead, the emergence of unusual carriers may be accounted for by dielectric screening of the on-site Coulomb interaction by high-density delocalized photocarriers. A possible role of electronic correlation in equilibrium transport in Mn$_3$Sn beyond the single-particle picture is discussed.

cond-mat.str-el↗

arXiv2023

Ultrafast Dynamics of Intrinsic Anomalous Hall Effect in the Topological Antiferromagnet Mn3Sn

We investigate ultrafast dynamics of the anomalous Hall effect (AHE) in the topological antiferromagnet Mn3Sn with sub-100 fs time resolution. Optical pulse excitations largely elevate the electron temperature up to 700 K, and terahertz probe pulses clearly resolves ultrafast suppression of the AHE before demagnetization. The result is well reproduced by microscopic calculation of the intrinsic Berry-curvature mechanism while the extrinsic contribution is clearly excluded. Our work opens a new avenue for the study of nonequilibrium AHE to identify the microscopic origin by drastic control of the electron temperature by light.

cond-mat.mtrl-sci↗

arXiv2023

Anomalous Hall effect of light-driven three-dimensional Dirac electrons in bismuth

Recent advancement in laser technology has opened the path toward the manipulation of functionalities in quantum materials by intense coherent light. Here, we study three-dimensional (3D) Dirac electrons driven by circularly polarized light (CPL), when the photon energy lies within the Dirac bands. As an experimental realization of this setup, we irradiate a thin film sample of elemental bismuth, which is a well-known semimetal hosting 3D Dirac electrons, with mid-infrared CPL. We successfully observe the emergence of the anomalous Hall effect (AHE) via terahertz Faraday rotation that is both pump-helicity-dependent and instantaneous. We compare our experimental findings with the results of Floquet theory, which is a powerful framework for analyzing the electronic band structure driven by coherent light. The contribution from the band structures near the one-photon resonant positions to the AHE shows a field-strength dependence consistent with our experimental results. The effective Hamiltonian on which we base our model calculations also implies that a pair of "double Weyl points" emerge due to the CPL-induced hybridization between the occupied and unoccupied 3D Dirac bands. Our findings shed light on ultrafast control of material properties in nonlinear topological optics.

cond-mat.mes-hall↗

arXiv2020

Phase-resolved Higgs response in superconducting cuprates

In high energy physics, the Higgs field couples to gauge bosons and fermions and gives mass to their elementary excitations. Experimentally, such couplings can be inferred from the decay product of the Higgs boson, i.e. the scalar (amplitude) excitation of the Higgs field. In superconductors, Cooper pairs bear a close analogy to the Higgs field. Interaction between the Cooper pairs and other degrees of freedom provides dissipation channel for the amplitude mode, which may reveal important information about the microscopic pairing mechanism. To this end, we investigate the Higgs (amplitude) mode of several cuprate thin films using phase-resolved terahertz third harmonic generation (THG). In addition to the heavily damped Higgs mode itself, we observe a universal jump in the phase of the driven Higgs oscillation as well as a non-vanishing THG above Tc. These findings indicate coupling of the Higgs mode to other collective modes and potentially a nonzero pairing amplitude above Tc.

cond-mat.supr-con↗

arXiv2020

Dynamical symmetry of strongly light-driven electronic system in crystalline solids

The Floquet state, which is a periodically and intensely light driven quantum state in solids, has been attracting attention as a novel state that is coherently controllable on an ultrafast time scale. An important issue has been to demonstrate experimentally novel electronic properties in the Floquet state. One technique to demonstrate them is the light scattering spectroscopy, which offers an important clue to clarifying the symmetries and energy structures of the states through symmetry analysis of the polarization selection rules. Here, we determine circular and linear polarization selection rules of light scattering in a mid-infrared-driven Floquet system in monolayer MoS2 and provide a comprehensive understanding in terms of the "dynamical symmetry" of the Floquet state.

physics.optics↗

arXiv2019

Inherent Limit to Coherent Phonon Generation under Non-Resonant Light Field Driving

Coherent manipulation of quasi-particles is a crucial method to realize ultrafast switching of the relating macroscopic order. In this letter, we studied coherent phonon generation under strong light field which allows to induce non-perturbative nonlinear optical phenomena in solids. The efficiency of coherent phonon generation starts to saturate and deviate from the pronounced linear power dependence when the light intensity goes into the non-perturbative regime. We propose a novel theoretical model based on Floquet picture and show that the saturation is due to a limitation of the driving force inherent in non-resonant driving of the electronic system in the non-perturbative regime.

physics.optics↗

arXiv2019

Interband resonant high-harmonic generation by valley polarized electron-hole pairs

We demonstrated nonperturbative high harmonics induced by intense mid-infrared light up to 18th order that well exceed the material bandgap in monolayer transition metal dichalcogenides. The intensities of the even-order high-harmonic radiation did not monotonically decrease as the harmonic order increased. By comparing the high harmonic spectra with the optical absorption spectra, we found that the enhancement in the even-order high harmonics could be attributed to the resonance to the band nesting energy. The symmetry analysis shows that the valley polarization and anisotropic band structure lead to polarization of the high-harmonic radiation under excitation with the polarization along the zigzag direction. We also examined the possible recombination pathways of electrons and holes by calculating their dynamics in real and momentum spaces based on three-step model in solids. It revealed that, by considering the electrons and holes generated at neighboring lattice sites, the electron-hole polarization driven to the band nesting region should contribute to the high harmonic radiation. Our findings open the way for attosecond science with monolayer materials having widely tunable electronic structures.

cond-mat.mes-hall↗

arXiv2019

Light-induced nonequilibrium response of the superconducting cuprate La$_{2-x}$Sr$_{x}$CuO$_4$

We report the dynamics of the cuprate superconductor La$_{2-x}$Sr$_{x}$CuO$_4$ ($x = 0.14$) after intense photoexcitation utilizing near-infrared (800 nm) optical pump-terahertz probe spectroscopy. In the superconducting state at 5 K, we observed a redshift of the Josephson plasma resonance that sustains for hundreds of picoseconds after the photoexcitation, indicating the destruction of the $c$-axis superconducting coherence. We show that the metastable spectral features can be described by the photoinduced surface heating of the sample. We also demonstrate that the conventional analysis used to extract the spectra of the photoexcited surface region can give rise to artifacts in the nonequilibrium response.

cond-mat.supr-con↗

arXiv2019

Charge carrier dynamics of FeSe thin film investigated by terahertz magneto-optical spectroscopy

We performed terahertz magneto-optical spectroscopy of FeSe thin film to elucidate the charge carrier dynamics. The measured diagonal (longitudinal) and off-diagonal (Hall) conductivity spectra are well reproduced by two-carrier Drude model, from which the carrier densities, scattering times and effective masses of electron and hole carriers are determined in a wide range of temperature. The hole density decreases below the structural transition temperature while electron density increases, which is attributed to the band structure modification in the electronic nematic phase. The scattering time of the hole carrier becomes substantially longer than that of the electron at lower temperature, which accounts for the increase of the positive dc Hall coefficient at low temperature.

cond-mat.supr-con↗