arXiv subjects

Ryusuke Matsunaga

Publications and source records attributed to Ryusuke Matsunaga.

33 records · Page 2Linked to original sources

arXiv2023

Semiconducting Electronic Structure of the Ferromagnetic Spinel $\mathbf{Hg}\mathbf{Cr}_2\mathbf{Se}_4$ Revealed by Soft-X-Ray Angle-Resolved Photoemission Spectroscopy

We study the electronic structure of the ferromagnetic spinel $\mathrm{Hg}\mathrm{Cr}_2\mathrm{Se}_4$ by soft-x-ray angle-resolved photoemission spectroscopy (SX-ARPES) and first-principles calculations. While a theoretical study has predicted that this material is a magnetic Weyl semimetal, SX-ARPES measurements give direct evidence for a semiconducting state in the ferromagnetic phase. Band calculations based on the density functional theory with hybrid functionals reproduce the experimentally determined band gap value, and the calculated band dispersion matches well with ARPES experiments. We conclude that the theoretical prediction of a Weyl semimetal state in $\mathrm{Hg}\mathrm{Cr}_2\mathrm{Se}_4$ underestimates the band gap, and this material is a ferromagnetic semiconductor.

cond-mat.mtrl-sci↗

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

Jitter correction for asynchronous optical sampling terahertz spectroscopy using free-running pulsed lasers

We demonstrate a jitter correction method for asynchronous optical sampling (ASOPS) terahertz (THz) time-domain spectroscopy using two free-running oscillators. This method simultaneously records the THz waveform and a harmonic of the laser repetition rate difference, to monitor the jitter information for software jitter correction. By suppressing the residual jitter below 0.1 ps, the accumulation of the THz waveform is achieved without losing the measurement bandwidth. Our measurement of water vapor successfully resolves the absorption linewidths below 1 GHz, demonstrating a robust ASOPS with a flexible, simple, and compact setup without any feedback control or additional continuous-wave THz source.

physics.optics↗

arXiv2022

Stimulated Rayleigh Scattering Enhanced by a Longitudinal Plasma Mode in a Periodically Driven Dirac Semimetal Cd$_3$As$_2$

Using broadband (12-45 THz) multi-terahertz spectroscopy, we show that stimulated Rayleigh scattering dominates the transient optical conductivity of cadmium arsenide, a Dirac semimetal, under an optical driving field at 30 THz. The characteristic dispersive lineshape with net optical gain is accounted for by optical transitions between light-induced Floquet subbands, strikingly enhanced by the longitudinal plasma mode. Stimulated Rayleigh scattering with an unprecedentedly large refractive index change may pave the way for slow light generation in conductive solids at room temperature.

cond-mat.mtrl-sci↗

arXiv2022

Tracking ultrafast change of multiterahertz broadband response functions in a photoexcited Dirac semimetal Cd$_3$As$_2$ thin film

The electromagnetic response of Dirac semimetals in the infrared and terahertz frequency ranges is attracting growing interest for potential applications in optoelectronics and nonlinear optics. The interplay between the free-carrier response and interband transitions in the gapless, linear dispersion relation plays a key role in enabling novel functionalities. Here we investigate ultrafast dynamics in thin films of a photoexcited Dirac semimetal Cd$_3$As$_2$ by probing the broadband response functions as complex quantities in the multiterahertz region (10-45 THz, 40-180 meV, or 7-30 $μ$m), which covers the crossover between the inter and intraband response. We resolve dynamics of the photoexcited nonthermal electrons which merge with originally existing carriers to form a single thermalized electron gas and how it is facilitated by high-density excitation. We also demonstrate that a large reduction of the refractive index by 80% dominates the nonequilibrium infrared response, which can be utilized for designing ultrafast switches in active optoelectronics.

cond-mat.mtrl-sci↗

arXiv2021

Optical parametric amplification of phase-stable terahertz-to-midinfrared pulses studied in the time domain

We report optical parametric amplification (OPA) of low-frequency infrared pulses in the intermediate region between terahertz (THz) frequency and mid-infrared (MIR), i.e., from 16.9 to 44.8 THz. The 255-fs laser output of the Yb:KGW regenerative amplifier is compressed to 11-fs pulses using a multi-plate broadening scheme, which generates THz-to-MIR pulses with a spectrum extending to approximately 50 THz by intra-p1ulse differential frequency generation (DFG) in GaSe. The THz-to-MIR pulses are further amplified using a two-stage OPA in GaSe. The temporal dynamics and photocarrier effects during OPA are characterized in the time domain. Owing to the intra-pulse DFG, the long-term phase drift of the THz-to-MIR pulses after two-stage OPA is as small as 16 mrad during a 6-h operation without any active feedback. Our scheme using the intra-pulse DFG and post-amplification proposes a new route to intense THz-to-MIR light sources with extreme phase stability.

physics.optics↗

arXiv2020

Efficient Terahertz Harmonic Generation with Coherent Acceleration of Electrons in the Dirac Semimetal Cd3As2

We report strong terahertz (~10^12 Hz) high harmonic generation in thin films of Cd3As2, a three-dimensional Dirac semimetal at room temperature. The third harmonics is detectable with tabletop light source and can be as strong as 100 V/cm by applying the fundamental field of 6.5 kV/cm inside the film, showing an unprecedented efficiency for terahertz frequency conversion. Our time-resolved terahertz spectroscopy and calculations also clarify the microscopic mechanism of the nonlinearity originating in the coherent acceleration of Dirac electrons in momentum space. Our results provide clear insights for nonlinear current of Dirac electrons driven by terahertz field under an influence of scattering, paving the way toward novel devices for high-speed electronics and photonics based on topological semimetals.

cond-mat.mtrl-sci↗

arXiv2020

Room-Temperature Terahertz Anomalous Hall Effect in Weyl Antiferromagnet Mn$_3$Sn Thin Films

Antiferromagnetic spin motion at terahertz (THz) frequencies attracts growing interests for fast spintronics, however their smaller responses to external field inhibit device application. Recently the noncollinear antiferromagnet Mn$_3$Sn, a Weyl semimetal candidate, was reported to show large anomalous Hall effect (AHE) at room temperature comparable to ferromagnets. Dynamical aspect of such large responses is an important issue to be clarified for future THz data processing. Here the THz anomalous Hall conductivity in Mn$_3$Sn thin films is investigated by polarization-resolved spectroscopy. Large anomalous Hall conductivity Re $σ_{xy} (ω) \sim$ 20 $\rm{Ω^{-1} cm^{-1}}$ at THz frequencies is clearly observed as polarization rotation. In contrast, Im $σ_{xy} (ω)$ is small up to a few THz, showing that the AHE remains dissipationless over a large frequency range. A peculiar temperature dependence corresponding to the breaking/recovery of symmetry in the spin texture is also discussed. Observation of the THz AHE at room temperature demonstrates the ultrafast readout for the antiferromagnetic spintronics using Mn$_3$Sn and will also open new avenue for studying nonequilibrium dynamics in Weyl antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2019

Infrared activation of the Higgs mode by supercurrent injection in superconducting NbN

Higgs mode in superconductors, i.e. the collective amplitude mode of the order parameter does not associate with charge nor spin fluctuations, therefore it does not couple to the electromagnetic field in the linear response regime. On the contrary to this common understanding, here, we demonstrate that, if the dc supercurrent is introduced into the superconductor, the Higgs mode becomes infrared active and is directly observed in the linear optical conductivity measurement. We observed a sharp resonant peak at $ω=2Δ$ in the optical conductivity spectrum of a thin-film NbN in the presence of dc supercurrent, showing a reasonable agreement with the recent theoretical prediction. The method as proven by this work opens a new pathway to study the Higgs mode in a wide variety of superconductors.

cond-mat.supr-con↗

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↗

arXiv2018

Light-induced switching to a Metastable Phase in a Layered Superconductor La$_{2-x}$Sr$_x$CuO$_4$ (x=0.15)

We investigated transient optical responses in an optimally-doped high-Tc superconductor La2-xSrxCuO4 (x=0.15) by using 800-nm optical pump and terahertz probe spectroscopy. With increasing the photoexcitation intensities, the Josephson plasma resonance shows a gradual redshift, indicating the suppression of superconductivity by the photoexcitation. With further increasing the photoexcitation intensities, a new longitudinal mode in the loss function spectrum appears and grows from the high energy side, accompanied by a new transverse mode as manifested in the conductivity spectrum. The observed spectra are described by the multilayer model with alternating interlayer Josephson couplings. The new longitudinal and transverse modes sustain much longer than several hundred picoseconds after the photoexcitation, indicating that the new metastable phase with possessing alternating interlayer Josephson couplings is induced by the strong photoexcitation.

cond-mat.supr-con↗

arXiv2017

Higgs mode in the d-wave superconductor Bi2Sr2CaCu2O8+x driven by an intense terahertz pulse

We investigated the terahertz (THz)-pulse driven nonlinear response in the d-wave cuprate superconductor Bi2Sr2CaCu2O8+x (Bi2212) using a THz pump near-infrared probe scheme in the time domain. We have observed an oscillatory behavior of the optical reflectivity that follows the THz electric field squared and is strongly enhanced below Tc. The corresponding third-order nonlinear effect exhibits both A1g and B1g symmetry components, which are decomposed from polarization-resolved measurements. Comparison with a BCS calculation of the nonlinear susceptibility indicates that the A1g component is associated with the Higgs mode of the d-wave order parameter.

cond-mat.supr-con↗

arXiv2017

Polarization-resolved terahertz third-harmonic generation in a superconductor NbN: dominance of Higgs mode beyond the BCS approximation

Recent advances in time-domain terahertz (THz) spectroscopy have unveiled that resonantly-enhanced strong THz third-harmonic generation (THG) mediated by the collective Higgs amplitude mode occurs in s-wave superconductors, where charge-density fluctuations (CDF) have also been shown to contribute to the nonlinear third-order susceptibility. It has been theoretically proposed that the nonlinear responses of Higgs and CDF exhibit essentially different polarization dependences. Here we experimentally discriminate the two contributions by polarization-resolved intense THz transmission spectroscopy for a single-crystal NbN film. The result shows that the resonant THG in the transmitted light always appears in the polarization parallel to that of the incident light with no appreciable crystal axis dependence. When we compare this with the theoretical calculation here with the BCS approximation and the dynamical mean-field theory for a model of NbN constructed from first principles, the experimental result strongly indicates that the Higgs mode rather than the CDF dominates the THG resonance in NbN. A possible mechanism for this is discussed such as the retardation effect in the phonon-mediated pairing interaction beyond BCS.

cond-mat.supr-con↗

arXiv2013

Higgs Amplitude Mode in BCS Superconductors Nb$_{1-x}$Ti$_{x}$N induced by Terahertz Pulse Excitation

Ultrafast responses of BCS superconductor Nb1-xTixN films in a nonadiabatic excitation regime were investigated by using terahertz (THz) pump-THz probe spectroscopy. After an instantaneous excitation with the monocycle THz pump pulse, a transient oscillation emerges in the electromagnetic response in the BCS gap energy region. The oscillation frequency coincides with the asymptotic value of the BCS gap energy, indicating the appearance of the theoretically-anticipated collective amplitude mode of the order parameter, namely the Higgs amplitude mode. Our result opens a new pathway to the ultrafast manipulation of the superconducting order parameter by optical means.

cond-mat.supr-con↗

arXiv2011

Observation of charged excitons in hole-doped carbon nanotubes using photoluminescence and absorption spectroscopy

We report the first observation of trions (charged excitons), three-particle bound states consisting of one electron and two holes, in hole-doped carbon nanotubes at room temperature. When p-type dopants are added to carbon nanotube solutions, the photoluminescence and absorption peaks of the trions appear far below the E11 bright exciton peak, regardless of the dopant species. The unexpectedly large energy separation between the bright excitons and the trions is attributed to the strong electron-hole exchange interaction in carbon nanotubes.

cond-mat.mes-hall↗