arXiv subjects

Tadashi Togashi

Publications and source records attributed to Tadashi Togashi.

At least 19 recordsLinked to original sources

arXiv2025

All-optical magnetization reversal via x-ray magnetic circular dichroism

Light polarization is one of the most fundamental features, equivalent to energy and coherence. Magnetism changes light polarization, and vice versa. The irradiation of intense circularly polarized femtosecond pules to magnetic materials can alter the magnetic orders and elementary excitations, particularly in the visible to infrared spectral regions. Furthermore, the recent development of x-ray free-electron laser enables the element-specific trace of the ultrafast dynamics with high time and spatial resolution. However, the light helicity of x-ray photons has not yet been used to control order parameters in condensed matter materials, not limited to such magnetic phenomenon. Here, we demonstrate the deterministic magnetization reversal of a ferromagnetic Pt/Co/Pt multilayer solely by irradiating femtosecond pulses of circularly polarized hard x-rays. The observed all-optical magnetization switching depends on the helicity of incident x-ray pulses and is strongly resonant with the photon energy at the Pt $L_3$ edge. These results originate in the x-ray magnetic circular dichroism of Pt, involving helicity-dependent excitation from the 2$p_{3/2}$ core level to the exchange-split 5$d$ valence states owing to the magnetic proximity effect with Co. These findings mark a new frontier for examining interactions between light and matter in the x-ray region.

cond-mat.mtrl-sci↗

arXiv2025

Phase transitions of eutectic high entropy alloy AlCoCrFeNi2.1 under shock compression

High entropy alloys (HEAs) are a new class of metals that exhibit unique mechanical performance. Among HEAs, additively manufactured eutectic high entropy alloys (AM-EHEAs) have recently emerged as candidate materials for use in extreme conditions due to their simultaneous high strength and ductility. However, the deformation and structural evolution of AM-EHEAs under conditions of high pressure have not been well characterized, limiting their use in extreme applications. We present dynamic compression experiments and molecular dynamics simulations studying the structural evolution of AM-EHEA AlCoCrFeNi2.1 when compressed to pressures up to 400 GPa. Our in-situ X-ray diffraction measurements capture the appearance of fcc and bcc phases at different pressure conditions, with pure- and mixed-phase regions. Understanding the phase stability and structural evolution of the AM EHEA offers new insights to guide the development of high-performance complex materials for extreme conditions.

cond-mat.mtrl-sci↗

arXiv2025

Pushing the limits of flow strength in diamond

Extreme pressures and temperatures create conditions that allow even hard and brittle materials to deform plastically. Despite extensive research, the upper limits of flow strength, the resistance to plastic flow, remain uncertain, and the mechanisms driving deformation at the relevant stresses are a subject of debate. Using femtosecond in situ X-ray diffraction experiments and large-scale molecular dynamics simulations, we demonstrate that stacking fault-mediated strengthening enables shock-compressed nano-polycrystalline diamond to achieve a peak flow strength of 107+-5 GPa at a stress of 227+-8 GPa. Our findings show that extreme conditions can unlock unusual strength via mechanisms that can be used as design tools in targeted applications.

cond-mat.mtrl-sci↗

arXiv2025

Discovery of transient topological crystalline order in optically driven SnSe

Ultrafast optical excitation provides a powerful route for accessing emergent quantum phases far from equilibrium, enabling transient light-induced phenomena such as magnetism, ferroelectricity, and superconductivity. However, extending this approach to induce topological phases, especially in conventional semiconductors, remains challenging. Here, we report the observation of a thermally inaccessible, transient topological crystalline order in the layered semiconductor SnSe, a trivial insulator with a sizable (~ 0.8 eV) band gap, induced by femtosecond above-gap excitation. Time- and angle-resolved photoemission spectroscopy directly reveals the sub-picosecond emergence of Dirac-like linear dispersions within the band gap. Their features, including a high Fermi velocity (~ 2.5x10^5 m/s), multiple Dirac points away from high-symmetry momenta, and independence from probe photon energy, are consistent with mirror-symmetry-protected surface states of a topological crystalline insulator. The observed spectral dynamics, combined with density functional theory calculations, indicate that the femtosecond excitation transiently increases lattice symmetry, enabling topological crystalline order to emerge. Our discovery opens new avenues for ultrafast optical control of topological quantum phases in semiconductors, with potential applications in quantum and spintronic devices.

cond-mat.mtrl-sci↗

arXiv2024

A simple method to find temporal overlap between THz and X-ray pulses using X-ray-induced carrier dynamics in semiconductors

X-ray-induced carrier dynamics in silicon and gallium arsenide were investigated through intensity variations of transmitted terahertz (THz) pulses in the pico to microsecond time scale with X-ray free-electron laser and synchrotron radiation. We observed a steep reduction in THz transmission with a picosecond scale due to the X-ray-induced carrier generation, followed by a recovery on a nano to microsecond scale caused by the recombination of carriers. The rapid response in the former process is applicable to a direct determination of temporal overlap between THz and X-ray pulses for THz pump-X-ray probe experiments with an accuracy of a few picoseconds.

cond-mat.mtrl-sci↗

arXiv2024

Comment on "Comments regarding "Transonic dislocation propagation in diamond" by Katagiri, et al. (Science 382, 69-72, 2023)" by Hawreliak, et al. (arXiv:2401.04213)

In their comment (1), Hawreliak et al. claims that our observation of stacking fault formation and transonic dislocation propagation in diamond (2) is not valid as they interpret the observed features as cracks. In this response letter, we describe our rationale for interpreting the observed features as stacking faults. We also address other points raised in their comments, including the clarifications of how the results of Makarov et al. (3) are not in conflict with our study.

cond-mat.mtrl-sci↗

arXiv2024

(Sub-)picosecond surface correlations of femtosecond laser excited Al-coated multilayers observed by grazing-incidence x-ray scattering

Femtosecond high-intensity laser pulses at intensities surpassing $10^{14} \,\text{W}/\text{cm}^2$ can generate a diverse range of functional surface nanostructures. Achieving precise control over the production of these functional structures necessitates a thorough understanding of the surface morphology dynamics with nanometer-scale spatial resolution and picosecond-scale temporal resolution. In this study, we show that individual XFEL pulses can elucidate structural changes on surfaces induced by laser-generated plasmas, employing grazing-incidence small-angle x-ray scattering (GISAXS). Using aluminum-coated multilayer samples we can differentiate between ultrafast surface morphology dynamics and subsequent subsurface density dynamics, achieving nanometer-depth sensitivity and subpicosecond temporal resolution. The observed subsurface density dynamics serve to validate advanced simulation models depicting matter under extreme conditions. Our findings promise to unveil novel avenues for laser material nanoprocessing and high-energy-density science.

physics.plasm-ph↗

arXiv2023

Transonic Dislocation Propagation in Diamond

The motion of line defects (dislocations) has been studied for over 60 years but the maximum speed at which they can move is unresolved. Recent models and atomistic simulations predict the existence of a limiting velocity of dislocation motions between the transonic and subsonic ranges at which the self-energy of dislocation diverges, though they do not deny the possibility of the transonic dislocations. We use femtosecond x-ray radiography to track ultrafast dislocation motion in shock-compressed single-crystal diamond. By visualizing stacking faults extending faster than the slowest sound wave speed of diamond, we show the evidence of partial dislocations at their leading edge moving transonically. Understanding the upper limit of dislocation mobility in crystals is essential to accurately model, predict, and control the mechanical properties of materials under extreme conditions.

cond-mat.mtrl-sci↗

arXiv2023

Disentangling the Evolution of Electrons and Holes in photoexcited ZnO nanoparticles

The evolution of charge carriers in photoexcited room temperature ZnO nanoparticles in solution is investigated using ultrafast ultraviolet photoluminescence spectroscopy, ultrafast Zn K-edge absorption spectroscopy and ab-initio molecular dynamics (MD) simulations. The photoluminescence is excited at 4.66 eV, well above the band edge, and shows that electron cooling in the conduction band and exciton formation occur in <500 fs, in excellent agreement with theoretical predictions. The X-ray absorption measurements, obtained upon excitation close to the band edge at 3.49 eV, are sensitive to the migration and trapping of holes. They reveal that the 2 ps transient largely reproduces the previously reported transient obtained at 100 ps time delay in synchrotron studies. In addition, the X-ray absorption signal is found to rise in ~1.4 ps, which we attribute to the diffusion of holes through the lattice prior to their trapping at singly-charged oxygen vacancies. Indeed, the MD simulations show that impulsive trapping of holes induces an ultrafast expansion of the cage of Zn atoms in <200 fs, followed by an oscillatory response at a frequency of ~100 cm-1, which corresponds to a phonon mode of the system involving the Zn sub-lattice.

cond-mat.mtrl-sci↗

arXiv2023

Ultrafast Control of Crystal Structure in a Topological Charge-Density-Wave Material

Optical control of crystal structures is a promising route to change physical properties including topological nature of a targeting material. Time-resolved X-ray diffraction measurements using the X-ray free-electron laser are performed to study the ultrafast lattice dynamics of VTe$_2$, which shows a unique charge-density-wave (CDW) ordering coupled to the topological surface states as a first-order phase transition. A significant oscillation of the CDW amplitude mode is observed at a superlattice reflection as well as Bragg reflections. The frequency of the oscillation is independent of the fluence of the pumping laser, which is prominent to the CDW ordering of the first-order phase transition. Furthermore, the timescale of the photoinduced 1$T^{\prime\prime}$ to 1$T$ phase transition is independent of the period of the CDW amplitude mode.

cond-mat.str-el↗

arXiv2023

Picosecond volume expansion drives a later-time insulator-metal transition in a nano-textured Mott Insulator

Technology moves towards ever faster switching between different electronic and magnetic states of matter. Manipulating properties at terahertz rates requires accessing the intrinsic timescales of electrons (femtoseconds) and associated phonons (10s of femtoseconds to few picoseconds), which is possible with short-pulse photoexcitation. Yet, in many Mott insulators, the electronic transition is accompanied by the nucleation and growth of percolating domains of the changed lattice structure, leading to empirical time scales dominated by slow coarsening dynamics. Here, we use time-resolved X-ray diffraction and reflectivity measurements to investigate the photoinduced insulator-to-metal transition in an epitaxially strained thin film Mott insulator Ca2RuO4. The dynamical transition occurs without observable domain formation and coarsening effects, allowing the study of the intrinsic electronic and lattice dynamics. Above a fluence threshold, the initial electronic excitation drives a fast lattice rearrangement, followed by a slower electronic evolution into a metastable non-equilibrium state. Microscopic calculations based on time-dependent dynamical mean-field theory and semiclassical lattice dynamics within a recently published equilibrium energy landscape picture explain the threshold-behavior and elucidate the delayed onset of the electronic phase transition in terms of kinematic constraints on recombination. Analysis of satellite scattering peaks indicates the persistence of a strain-induced nano-texture in the photoexcited film. This work highlights the importance of combined electronic and structural studies to unravel the physics of dynamic transitions and elucidates the role of strain in tuning the timescales of photoinduced processes.

cond-mat.str-el↗

arXiv2022

Ultrafast x-ray scattering reveals composite amplitude collective mode in the Weyl charge density wave material (TaSe$_4$)$_2$I

We report ultrafast x-ray scattering experiments of the quasi-1D charge density wave (CDW) material (TaSe$_4$)$_2$I following photoexcitation with femtosecond infrared laser pulses. From the time-dependent diffraction signal at the CDW sidebands we identify an amplitude mode derived primarily from the transverse acoustic component of the CDW static distortion. The dynamics of this acoustic amplitude mode are described well by a model of a displacive excitation, which we interpret as mediated through a coupling to the optical phonon component associated with the tetramerization of the Ta chains.

cond-mat.mtrl-sci↗

arXiv2022

Melting of magnetic order in ${\mathrm{NaOsO}}_{3}$ by fs laser pulses

NaOsO$_3$ has recently attracted significant attention for the strong coupling between its electronic band structure and magnetic ordering. Here, we used time-resolved magnetic X-ray diffraction to determine the timescale of the photoinduced \afm dynamics in NaOsO$_3$. Our measurements are consistent with a sub-100~fs melting of the \afm long-range order, that occurs significantly faster than the lattice dynamics as monitored by the transient change in intensity of selected Bragg structural reflections, which instead show a decrease of intensity on a timescale of several ps.

cond-mat.str-el↗

arXiv2021

Nanoscale subsurface dynamics of solids upon high-intensity laser irradiation observed by femtosecond grazing-incidence x-ray scattering

Observing ultrafast laser-induced structural changes in nanoscale systems is essential for understanding the dynamics of intense light-matter interactions. For laser intensities on the order of $10^{14} \, \rm W/cm^2$, highly-collisional plasmas are generated at and below the surface. Subsequent transport processes such as heat conduction, electron-ion thermalization, surface ablation and resolidification occur at picosecond and nanosecond time scales. Imaging methods, e.g. using x-ray free-electron lasers (XFEL), were hitherto unable to measure the depth-resolved subsurface dynamics of laser-solid interactions with appropriate temporal and spatial resolution. Here we demonstrate picosecond grazing-incidence small-angle x-ray scattering (GISAXS) from laser-produced plasmas using XFEL pulses. Using multi-layer (ML) samples, both the surface ablation and subsurface density dynamics are measured with nanometer depth resolution. Our experimental data challenges the state-of-the-art modeling of matter under extreme conditions and opens new perspectives for laser material processing and high-energy-density science.

physics.plasm-ph↗

arXiv2021

Photoemission from the gas phase using soft x-ray fs pulses: An investigation of the space-charge effects

An experimental and computational investigation of the space-charge effects occurring in ultrafast photoelectron spectroscopy from the gas phase is presented. The target sample CF$_3$I is excited by ultrashort (100 fs) far-ultraviolet radiation pulses produced by a free-electron laser. The modification of the energy distribution of the photoelectrons, i.e. the shift and broadening of the spectral structures, is monitored as a function of the pulse intensity. The experimental results are compared with computational simulations which employ a Barnes-Hut algorithm to calculate the effect of individual Coulomb forces acting among the particles. In the presented model, a survey spectrum acquired at low radiation fluence is used to determine the initial energy distribution of the electrons after the photoemission event. The spectrum modified by the space-charge effects is then reproduced by $N$-body calculations that simulate the dynamics of the photoelectrons subject to the individual mutual Coulomb repulsion and to the attractive force of the positive ions. The employed numerical method accounts for the space-charge effects on the energy distribution and allows to reproduce the complete photoelectron spectrum and not just a specific photoemission structure. The simulations also provide information on the time evolution of the space-charge effects on the picosecond scale. Differences with the case of photoemission from solid samples are highlighted and discussed. The presented simulation procedure, although it omits the analysis of angular distribution, constitutes an effective simplified model that allows to predict and account for space-charge effects on the photoelectron energy spectrum in time-resolved photoemission experiments with high-intensity pulsed sources.

physics.ins-det↗

arXiv2020

Structurally assisted melting of excitonic correlations in 1T-TiSe2

The simultaneous condensation of electronic and structural degrees of freedom gives rise to new states of matter, including superconductivity and charge-density-wave formation. When exciting such a condensed system, it is commonly assumed that the ultrafast laser pulse disturbs primarily the electronic order, which in turn destabilizes the atomic structure. Contrary to this conception, we show here that structural destabilization of few atoms causes melting of the macroscopic ordered charge-density wave in 1T-TiSe2. Using ultrafast pump-probe non-resonant and resonant X-ray diffraction, we observe full suppression of the Se 4p orbital order and the atomic structure at excitation energies more than one order of magnitude below the suggested excitonic binding energy. Complete melting of the charge-density wave occurs 4-5 times faster than expected from a purely electronic charge-screening process, strongly suggesting a structurally assisted breakup of excitonic correlations. Our experimental data clarifies several questions on the intricate coupling between structural and electronic order in stabilizing the charge-density-wave in 1T-TiSe2. The results further show that electron-phonon-coupling can lead to different, energy dependent phase-transition pathways in condensed matter systems, opening new possibilities in the conception of non-equilibrium phenomena at the ultrafast scale.

cond-mat.str-el↗

arXiv2018

Ultrafast demagnetization of Pt magnetic moment in L1${}_0$-FePt probed by hard x-ray free electron laser

We demonstrate ultrafast magnetization dynamics in a 5d transition metal using circularly-polarized x-ray free electron laser in the hard x-ray region. A decay time of light-induced demagnetization of L1${}_0$-FePt was determined to be $τ_\textrm{Pt} = 0.6\ \textrm{ps}$ using time-resolved x-ray magnetic circular dichroism at the Pt L${}_3$ edge, whereas magneto-optical Kerr measurements indicated the decay time for total magnetization as $τ_\textrm{total} = 0.1\ \textrm{ps}$. A transient magnetic state with the photo-modulated magnetic coupling between the 3d and 5d elements is firstly demonstrated.

cond-mat.mtrl-sci↗

arXiv2018

Dynamics of the photoinduced insulator-to-metal transition in a nickelate film

The control of materials properties with light is a promising approach towards the realization of faster and smaller electronic devices. With phases that can be controlled via strain, pressure, chemical composition or dimensionality, nickelates are good candidates for the development of a new generation of high performance and low consumption devices. Here we analyze the photoinduced dynamics in a single crystalline NdNiO$_3$ film upon excitation across the electronic gap. Using time-resolved reflectivity and resonant x-ray diffraction, we show that the pump pulse induces an insulator-to-metal transition, accompanied by the melting of the charge order. Finally we compare our results to similar studies in manganites and show that the same model can be used to describe the dynamics in nickelates, hinting towards a unified description of these photoinduced phase transitions.

cond-mat.str-el↗