arXiv subjects

Y. Takahashi

Publications and source records attributed to Y. Takahashi.

At least 19 recordsLinked to original sources

arXiv2024

Determination of the Néel vector in rutile altermagnets through x-ray magnetic circular dichroism: the case of MnF$_2$

We present a numerical simulation of the X-ray magnetic circular dichroism (XMCD) at the $L_{2,3}$ edge of Mn in altermagnetic MnF$_2$ using a combination of density functional + exact diagonlization of an atomic model. We explore how the dichroic spectra vary with the light propagation vector and the Néel vector. We show how XMCD in rutile structures can be employed to determine the orientation of the Néel vector. An exact relationship between the XMCD spectra for different orientation of the Néel vector valid in the absence on the valence spin-orbit coupling and core-valence multipole interaction is derived and its approximate validity demonstrated by numerical calculation for the full Hamiltonian.

cond-mat.str-el↗

arXiv2024

Gapped nodal planes drive a large topological Nernst effect in a chiral lattice antiferromagnet

The electronic structure of compensated antiferromagnets (CAF) has drawn attention for its ability to create large responses, reminiscent of ferromagnets and suitable for data storage and readout, despite (nearly) net-zero spontaneous magnetization. Many of the striking experimental signatures predicted for CAF, such as giant thermoelectric Nernst effects, are enhanced when two or more electronic bands are nearly degenerate in vicinity of the Fermi energy. Here, we use thermoelectric and electric transport experiments to study the electronic structure of the layered, chiral metal CoNb3S6 in its all-in-all-out CAF ground state and report near-degeneracies of electron bands at the upper and lower boundaries of the first Brillouin zone. Considering non-symmorphic spin-space group symmetries in the non-relativistic approximation for the ordered phase, these near-degeneracies are approximately protected by a lattice translation combined with spin rotation, and are vestiges of nodal planes enforced by a screw axis symmetry in the paramagnetic state. Hot spots of emergent, or fictitious, magnetic fields are formed at the slightly gapped nodal plane, generating the spontaneous Hall and Nernst effects in this CAF. Taking into account more than six hundred Wannier orbitals, our model quantitatively reproduces the observed spontaneous Nernst effect, emphasizes the role of proximate symmetries in the emergent responses of CAF, and demonstrates the promise of ab-initio search for functional responses in a wide class of materials with reconstructed unit cells due to spin or charge order.

cond-mat.mtrl-sci↗

arXiv2024

X-ray Magnetic Circular Dichroism in RuO$_2$

We present numerical simulation of the X-ray magnetic circular dichroism (XMCD) of the $L_{2,3}$ and $M_{2,3}$ edges of Ru in antiferromagnetic RuO$_2$ using a combination of density functional + dynamical mean-field theory and configuration interaction treatment of Anderson impurity model. We study the dependence of the dichroic spectra on the orientation of the Néel vector and discuss it in the context of altermagnetism. Approximate equivalence between the XMCD spectra for geometries with X-rays propagating parallel and perpendicular to the Néel vector is found and shown to be exact in absence of valence spin-orbit coupling and core-valence multipolar interaction.

cond-mat.str-el↗

arXiv2022

Terahertz lattice and charge dynamics in ferroelectric semiconductor Sn$_x$Pb$_{1-x}$Te

The symmetry breaking induced by the ferroelectric transition often triggers the emergence of topological electronic states such as Weyl fermions in ferroelectric-like metals/semimetals. Such strong coupling between the lattice deformation and electronic states is therefore essentially important for the control of novel topological phases. Here, we study the terahertz lattice and charge dynamics in ferroelectric semiconductor SnxPb1-xTe thin films hosting versatile topological phases by means of the terahertz time-domain spectroscopy. With lowering the temperature, the resonant frequency of transverse optical phonon shows the significant softening and upturn. This temperature anomaly of lattice dynamics directly indicates the displacive-type ferroelectric transition. The resulting phase diagram suggests the enhancement of ferroelectricity in the films due to compressive strain compared with the bulk crystals. The soft phonon induces the large DC and terahertz dielectric constant even in metallic state. Furthermore, we find that the Born effective charge of soft phonon mode is enhanced at around the compositions showing the band gap closing associated with the topological transition.

cond-mat.mtrl-sci↗

arXiv2021

Internal rubble properties of asteroid (101955) Bennu

Exploration of asteroid (101955) Bennu by the OSIRIS-REx mission has provided an in-depth look at this rubble-pile near-Earth asteroid. In particular, the measured gravity field and the detailed shape model of Bennu indicate significant heterogeneities in its interior structure, compatible with a lower density at its center. Here we combine gravity inversion methods with a statistical rubble-pile model to determine the density and size-frequency distribution (SFD) index of the rubble that constitutes Bennu. The best-fitting models indicate that the SFD of the interior is consistent with that observed on the surface, with a cumulative SFD index of approximately $-2.9$. The rubble bulk density is approximately $1.35$ g/cm$^3$, corresponding to a $12$% macro-porosity. We find the largest rubble particle to be approximately $145$ m, whereas the largest void is approximately $10$ m.

astro-ph.EP↗

arXiv2021

Experimental signature of parity anomaly in semi-magnetic topological insulator

A three-dimensional topological insulator features a two-dimensional surface state consisting of a single linearly-dispersive Dirac cone. Under broken time-reversal symmetry, the single Dirac cone is predicted to cause half-integer quantization of Hall conductance, which is a manifestation of the parity anomaly in quantum field theory. However, despite various observations of quantization phenomena, the half-integer quantization has been elusive because a pair of equivalent Dirac cones on two opposing surfaces are simultaneously measured in ordinary experiments. Here we demonstrate the half-integer quantization of Hall conductance in a synthetic heterostructure termed a 'semi-magnetic' topological insulator, where only one surface state is gapped by magnetic doping and the opposite one is non-magnetic and gapless. We observe half quantized Faraday/Kerr rotations with terahertz magneto-optical spectroscopy and half quantized Hall conductance in transport at zero magnetic field. Our results suggest a condensed-matter realization of the parity anomaly and open a way for studying unconventional physics enabled by a single Dirac fermion.

cond-mat.mes-hall↗

arXiv2020

Excitonic optical spectra and energy structures in a one-dimensional Mott insulator demonstrated by applying a many-body Wannier functions method to a charge model

We have applied a many-body Wannier functions method to theoretically calculate an excitonic optical conductivity spectrum and energy structure in a one-dimensional (1D) Mott insulator at absolute zero temperature with large system size. Focusing on full charge fluctuations associated with pairs of a holon and doublon, we employ a charge model, which is interpreted as a good effective model to investigate photoexcitations of a 1D extended Hubbard model at half-filling in the spin-charge separation picture. As a result, the theoretical spectra with appropriate broadenings qualitatively reproduce the recent experimental data of ET-F$_{2}$TCNQ at 294 K with and without a modulated electric field. Regarding the excitonic energy structure, we have found that the excitons, especially for even-parity, are weakly bound by many-body effects. This is also consistent with the fitting parameters reported in the recent experiment. Thus, our theoretical method presented in this paper is practically useful to understand physical roles of charge fluctuations in many-body excited states of a 1D Mott insulator.

cond-mat.str-el↗

arXiv2019

Neutron quadrupole transition strength in $^{10}$C deduced from the $^{10}$C$(α,α')$ measurement with the MAIKo active target

Elastic and inelastic alpha scatterings on $^{10}$C were measured using a 68-MeV/u radioactive $^{10}$C beam incident on the recently developed MAIKo active target system. The phenomenological effective $α$-$N$ interaction and the point-nucleon density distribution in the ground state were determined from the elastic scattering data. The cross sections of the inelastic alpha scattering were calculated using this interaction and density distribution and were compared with the experiment to determine the neutron quadrupole transition matrix element $M_{n}$ between the ground state and the $2_{1}^{+}$ state at $E_{x} = 3.35$ MeV in $^{10}$C. The deduced neutron transition matrix element is $M_{n} = 6.9\, \pm0.7\, \mathrm{(fit)}\, \pm1.2\, \mathrm{(sys)}$ fm$^{2}$. The ratio of the neutron transition strength to proton transition strength was determined as $M_{n}/M_{p} = 1.05\, \pm0.11\, \mathrm{(fit)}\, \pm0.17\, \mathrm{(sys)}$, which indicates that the quadrupole transition between the ground state and the $2_{1}^{+}$ state in $^{10}$C is less neutron dominant compared to that in $^{16}$C.

nucl-ex↗

arXiv2019

Biexciton in one-dimensional Mott insulators

Mott insulators sometimes show dramatic changes in their electronic states after photoirradiation, as indicated by photoinduced Mott-insulator-to-metal transition. In the photoexcited states of Mott insulators, electron wavefunctions are more delocalized than in the ground state, and long-range Coulomb interactions play important roles in charge dynamics. However, their effects are difficult to discriminate experimentally. Here, we show that in a one-dimensional Mott insulator, bis(ethylenedithio)tetrathiafulvalene-difluorotetracyanoquinodimethane (ET-F2TCNQ), long-range Coulomb interactions stabilize not only excitons, doublon-holon bound states, but also biexcitons. By measuring terahertz-electric-field-induced reflectivity changes, we demonstrate that odd- and even-parity excitons are split off from a doublon-holon continuum. Further, spectral changes of reflectivity induced by a resonant excitation of the odd-parity exciton reveals that an exciton-biexciton transition appears just below the exciton-transition peak. Theoretical simulations show that long-range Coulomb interactions over four sites are necessary to stabilize the biexciton. Such information is indispensable for understanding the non-equilibrium dynamics of photoexcited Mott insulators.

cond-mat.str-el↗

arXiv2019

Observation of a Be double-Lambda hypernucleus in the J-PARC E07 experiment

A double-$Λ$ hypernucleus, ${}_{ΛΛ}\mathrm{Be}$, was observed by the J-PARC E07 collaboration in nuclear emulsions tagged by the $(K^{-},K^{+})$ reaction. This event was interpreted as a production and decay of $ {}_{ΛΛ}^{\;10}\mathrm{Be}$, ${}_{ΛΛ}^{\;11}\mathrm{Be}$, or ${}_{ΛΛ}^{\;12}\mathrm{Be}^{*}$ via $Ξ^{-}$ capture in ${}^{16}\mathrm{O}$. By assuming the capture in the atomic 3D state, the binding energy of two $Λ$ hyperons$\,$($B_{ΛΛ}$) of these double-$Λ$ hypernuclei are obtained to be $15.05 \pm 0.11\,\mathrm{MeV}$, $19.07 \pm 0.11\,\mathrm{MeV}$, and $13.68 \pm 0.11\,\mathrm{MeV}$, respectively. Based on the kinematic fitting, ${}_{ΛΛ}^{\;11}\mathrm{Be}$ is the most likely explanation for the observed event.

nucl-ex↗

arXiv2018

Quantum paramagnet near spin-state transition

Spin-state transition, also known as spin crossover, plays a key role in diverse systems, including minerals and biological materials. In theory, the boundary range between the low- and high-spin states is expected to enrich the transition and give rise to unusual physical states. However, no compound that realizes a nearly degenerate critical range as the ground state without requiring special external conditions has yet been experimentally identified. This study reports that, by comprehensive measurements of macroscopic physical properties, X-ray diffractometry, and neutron spectroscopy, the Sc substitution in LaCoO$_3$ destabilizes its nonmagnetic low-spin state and generates an anomalous paramagnetic state accompanied by the enhancement of transport gap and magneto-lattice-expansion as well as the contraction of Co--O distance with the increase of electron site-transfer. These phenomena are not well described by the mixture of conventional low- and high-spin states, but by their quantum superposition occurring on the verge of a spin-state transition. The present study enables us to significantly accelerate the design of new advanced materials without requiring special equipment based on the concept of quantum spin-state criticality.

cond-mat.str-el↗

arXiv2018

Emergence of spin-orbit order in the spinel CuCr$_2$O$_4$

We determined the magnetic structure of CuCr$_2$O$_4$ using neutron diffraction and irreducible representation analysis. The measurements identified a new phase between 155 K and 125 K as nearly collinear magnetic ordering in the Cr pyrochlore lattice. Below 125 K, a Cu-Cr ferrimagnetic component develops the noncollinear order. Along with the simultaneously obtained O positions and the quantum effect of spin-orbit coupling, the magnetic structure is understood to involve spin-orbit ordering, accompanied by an appreciably deformed orbital of presumably spin-only Cu and Cr.

cond-mat.str-el↗

arXiv2018

Construction and Response of a Highly Granular Scintillator-based Electromagnetic Calorimeter

A highly granular electromagnetic calorimeter with scintillator strip readout is being developed for future lepton collider experiments. A prototype of 21.5 $X_0$ depth and $180 \times 180 $mm$^2$ transverse dimensions was constructed, consisting of 2160 individually read out $10 \times 45 \times 3$ mm$^3$ scintillator strips. This prototype was tested using electrons of 2--32 GeV at the Fermilab Test Beam Facility in 2009. Deviations from linear energy response were less than 1.1\%, and the intrinsic energy resolution was determined to be $(12.5 \pm 0.1 (\mathrm{stat.}) \pm0.4 (\mathrm{syst.}))\%/\sqrt{E[\mathrm{GeV}]}\oplus (1.2 \pm 0.1(\mathrm{stat.})^{+0.6}_{-0.7}(\mathrm{syst.}))\%$, where the uncertainties correspond to statistical and systematic sources, respectively.

physics.ins-det↗

arXiv2018

Anisotropic spin-density distribution and magnetic anisotropy of strained La$_{1-x}$Sr$_x$MnO$_3$ thin films: Angle-dependent x-ray magnetic circular dichroism

Magnetic anisotropies of ferromagnetic thin films are induced by epitaxial strain from the substrate via strain-induced anisotropy in the orbital magnetic moment and that in the spatial distribution of spin-polarized electrons. However, the preferential orbital occupation in ferromagnetic metallic La$_{1-x}$Sr$_x$MnO$_3$ (LSMO) thin films studied by x-ray linear dichroism (XLD) has always been found out-of-plane for both tensile and compressive epitaxial strain and hence irrespective of the magnetic anisotropy. In order to resolve this mystery, we directly probed the preferential orbital occupation of spin-polarized electrons in LSMO thin films under strain by angle-dependent x-ray magnetic circular dichroism (XMCD). Anisotropy of the spin-density distribution was found to be in-plane for the tensile strain and out-of-plane for the compressive strain, consistent with the observed magnetic anisotropy. The ubiquitous out-of-plane preferential orbital occupation seen by XLD is attributed to the occupation of both spin-up and spin-down out-of-plane orbitals in the surface magnetic dead layer.

cond-mat.mtrl-sci↗

arXiv2017

Search for Anisotropy in the Ultra High Energy Cosmic Ray Spectrum using the Telescope Array Surface Detector

The Telescope Array (TA) experiment is located in the western desert of Utah, USA, and observes ultra high energy cosmic rays (UHECRs) in the Northern hemisphere. At the highest energies, $E>10$~EeV, the shape of cosmic ray energy spectrum may carry an imprint of the source density distribution along the line of sight different in different directions of the sky. In this study, we search for such directional variations in the shape of the energy spectrum using events observed with the Telescope Array's surface detector. We divide the TA field of view into two nearly equal-exposure regions: the "on-source" region which we define as $\pm 30^\circ$ of the supergalactic plane containing mostly nearby structures, and the complementary "off-source" region where the sources are further away on average. We compare the UHECR spectra in these regions by fitting them to the broken power law and comparing the resulting parameters. We find that the off-source spectrum has an earlier break at highest energies. The chance probability to obtain such or larger difference in statistically equivalent distributions is estimated as $6.2\pm1.1\times10^{-4}$ ($3.2σ$) by a Monte-Carlo simulation. The observed difference in spectra is in a reasonable quantitative agreement with a simplified model that assumes that the UHECR sources trace the galaxy distribution from the 2MRS catalogue, primary particles are protons and the magnetic deflections can be neglected.

astro-ph.HE↗

arXiv2017

Magnetic anisotropy of $L1_0$-ordered FePt thin films studied by Fe and Pt $L_{2,3}$-edges x-ray magnetic circular dichroism

The strong perpendicular magnetic anisotropy of $L{\rm1_0}$-ordered FePt has been the subject of extensive studies for a long time. However, it is not known which element, Fe or Pt, mainly contributes to the magnetic anisotropy energy (MAE). We have investigated the anisotropy of the orbital magnetic moments of Fe 3$d$ and Pt 5$d$ electrons in $L{\rm1_0}$-ordered FePt thin films by Fe and Pt $L_{2,3}$-edge x-ray magnetic circular dichroism (XMCD) measurements for samples with various degrees of long-range chemical order $S$. Fe $L_{2,3}$-edge XMCD showed that the orbital magnetic moment was larger when the magnetic field was applied perpendicular to the film than parallel to it, and that the anisotropy of the orbital magnetic moment increased with $S$. Pt $L_{2,3}$-edge XMCD also showed that the orbital magnetic moment was smaller when the magnetic field was applied perpendicular to the film than parallel to it, opposite to the Fe $L_{2,3}$-edge XMCD results although the anisotropy of the orbital magnetic moment increases with $S$ like the Fe edge. These results are qualitatively consistent with the first-principles calculation by Solovyev ${\it et\ al.}$ [Phys. Rev. B $\bf{52}$, 13419 (1995).], which also predicts the dominant contributions of Pt 5$d$ to the magnetic anisotropy energy rather than Fe 3$d$ due to the strong spin-orbit coupling and the small spin splitting of the Pt 5$d$ bands in $L{\rm1_0}$-ordered FePt.

cond-mat.mtrl-sci↗

arXiv2017

Thickness-dependent electronic and magnetic properties of $γ'$-Fe$_{\mathrm 4}$N atomic layers on Cu(001)

Growth, electronic and magnetic properties of $γ'$-Fe$_{4}$N atomic layers on Cu(001) are studied by scanning tunneling microscopy/spectroscopy and x-ray absorption spectroscopy/magnetic circular dichroism. A continuous film of ordered trilayer $γ'$-Fe$_{4}$N is obtained by Fe deposition under N$_{2}$ atmosphere onto monolayer Fe$_{2}$N/Cu(001), while the repetition of a bombardment with 0.5 keV N$^{+}$ ions during growth cycles results in imperfect bilayer $γ'$-Fe$_{4}$N. The increase in the sample thickness causes the change of the surface electronic structure, as well as the enhancement in the spin magnetic moment of Fe atoms reaching $\sim$ 1.4 $μ_{\mathrm B}$/atom in the trilayer sample. The observed thickness-dependent properties of the system are well interpreted by layer-resolved density of states calculated using first principles, which demonstrates the strongly layer-dependent electronic states within each surface, subsurface, and interfacial plane of the $γ'$-Fe$_{4}$N atomic layers on Cu(001).

cond-mat.mtrl-sci↗

arXiv2017

Electromagnon resonance in a collinear spin state of a polar antiferromagnet Fe2Mo3O8

Magnetic excitations are investigated for a hexagonal polar magnet Fe2Mo3O8 by terahertz spectroscopy. We observed magnon modes including an electric-field active magnon, electromagnon, in the collinear antiferromagnetic phase with spins parallel to the c axis. We unravel the nature of these excitations by investigating the correlation between the evolution of the mode profile and the magnetic transition from antiferromagnetic to ferrimagnetic order induced by magnetic field or Zn-doping. We propose that the observed electromagnon mode involves the collective precession of the spins with oscillating in-plane electric polarization through the mechanism of the linear magnetoelectric effect.

cond-mat.str-el↗