arXiv subjects

H. Nakamura

Publications and source records attributed to H. Nakamura.

At least 37 records · Page 2Linked to original sources

arXiv2023

A kinematic study of the disc-outflow system around a high-mass protostar G59.783+0.065 probed by methanol and water masers

Class II CH3OH masers are used as a convenient tracer of disc-like structures in high-mass star formation. However, more than half of them show a complex distribution in Very Long Baseline Interferometry (VLBI) maps. The origin of such a complex distribution is still unknown. We conducted VLBI monitoring observations to unveil the origin of a complex class II CH3OH maser in the high-mass star-forming region G59.783+0.065. We observed the CH3OH maser at 6.7 GHz and the H2O maser at 22 GHz to probe detailed circumstellar kinematics and structures by the Japanese VLBI network and the VLBI Exploration of Radio Astrometry. We found similar bipolar distributions in both masers, specifically two clusters located 2000 au apart along the East-West direction. We detected a linear distribution of CH3OH masers in the Western cluster. A position-velocity diagram shows that the Western CH3OH masers trace a rotating disc-wind or infalling component inside an edge-on disc-like structure. In contrast to the simple bipolar expanding motions of the H2O masers, the CH3OH masers exhibited complex motions despite their spatial coincidence. Some of the Eastern CH3OH masers showed bipolar expansions similar to the H2O masers, while others displayed random or even inward motions. Such complex kinematics and their close association with the H2O maser could occur at the boundary between outflow and inflow. We suggest that the complex distribution of class II CH3OH masers, like G59.783+0.065 arises from several distinct circumstellar structures that simultaneously achieve maser excitation.

astro-ph.SR↗

arXiv2023

Charge order above room-temperature in a prototypical kagome superconductor La(Ru$_{1-x}$Fe$_{x}$)$_{3}$Si$_{2}$

The kagome lattice is an intriguing and rich platform for discovering, tuning and understanding the diverse phases of quantum matter, which is a necessary premise for utilizing quantum materials in all areas of modern and future electronics in a controlled and optimal way. The system LaRu$_{3}$Si$_{2}$ was shown to exhibit typical kagome band structure features near the Fermi energy formed by the Ru-$dz^{2}$ orbitals and the highest superconducting transition temperature $T_{\rm c}$ ${\simeq}$ 7K among the kagome-lattice materials. However, the effect of electronic correlations on the normal state properties remains elusive. Here, we report the discovery of charge order in La(Ru$_{1-x}$Fe$_{x}$)$_{3}$Si$_{2}$ ($x$ = 0, 0.01, 0.05) beyond room-temperature. Namely, single crystal X-ray diffraction reveals charge order with a propagation vector of ($\frac{1}{4}$,0,0) below $T_{\rm CO-I}$ ${\simeq}$ 400K in all three compounds. At lower temperatures, we see the appearance of a second set of charge order peaks with a propagation vector of ($\frac{1}{6}$,0,0). The introduction of Fe, which is known to quickly suppress superconductivity, does not drastically alter the onset temperature for charge order. Instead, it broadens the scattered intensity such that diffuse scattering appears at the same onset temperature, however does not coalesce into sharp Bragg diffraction peaks until much lower in temperature. Our results present the first example of a charge ordered state at or above room temperature in the correlated kagome lattice with bulk superconductivity.

cond-mat.supr-con↗

arXiv2023

Nodal line resonance generating the giant anomalous Hall effect of Co$_3$Sn$_2$S$_2$

Giant anomalous Hall effect (AHE) and magneto-optical activity can emerge in magnets with topologically non-trivial degeneracies. However, identifying the specific band structure features like Weyl points, nodal lines or planes which generate the anomalous response is a challenging issue. Since the low-energy interband transitions can govern the static AHE, we addressed this question in the prototypical magnetic Weyl semimetal Co$_3$Sn$_2$S$_2$ also hosting nodal lines by broadband polarized reflectivity and magneto-optical Kerr effect spectroscopy with a focus on the far-infrared range. In the linear dichroism spectrum we observe a strong resonance at 40\,meV, which also shows up in the optical Hall conductivity spectrum and primarily determines the static AHE, thus, confirms its intrinsic origin. Our material-specific theory reproduces the experimental data remarkably well and shows that strongly tilted nodal line segments around the Fermi energy generate the resonance. While the Weyl points only give vanishing contributions, these segments of the nodal lines gapped by the spin-orbit coupling dominate the low-energy optical response.

cond-mat.str-el↗

arXiv2022

Probing the interlayer coupling in 2$H$-NbS$_2$ via soft x-ray angle-resolved photoemission spectroscopy

In the large family of two-dimensional (2D) layered materials including graphene, its honeycomb analogs, and transition-metal dichalcogenides, the interlayer coupling plays a rather intriguing role. On the one hand, the weak van der Waals interaction that holds the layers together endows these compounds with quasi-2D properties, which might imply small interlayer effects on the electronically active bands. On the other hand, the oft-witnessed differences in electronic, optical, and magnetic behaviors of monolayers, bilayers, and multilayers of the same compound must have as their microscopic origin the detailed interlayer hopping parameters. Given the few experimental reports that have attempted to explicitly extract these parameters, we employ soft-x-ray angle-resolved photoemission spectroscopy (SX-ARPES) to probe the interlayer coupling in superconducting 2$H$-NbS$_2$. We visualize the S 3$p_z$ bands that disperse with respect to the out-of-plane momentum and introduce a simple tight-binding model to extract the interlayer hopping parameters. From first-principles calculations, we clarify how atomic distances and the proper accounting for screening via hybrid functionals influence these bands. The knowledge of interlayer hopping parameters is particularly pertinent in NbS$_2$, where recent experiments have uncovered fingerprints of finite-momentum superconductivity in the bulk material and heterostructures.

cond-mat.supr-con↗

arXiv2022

Perspective: Molecular beam epitaxy of antiperovskite oxides

Antiperovskites, or inverse perovskites, have recently emerged as a material class with a plethora of promising electronic properties. This perspective describes the molecular beam epitaxy (MBE) growth of oxide antiperovskites Sr$_3$PbO and Sr$_3$SnO. We show that MBE offers great potential not only in growing antiperovskites with high structural quality, but also in providing a means to seamlessly connect with advanced characterization tools, including x-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), reflection high-energy electron diffraction (RHEED), and scanning tunneling microscopy (STM), to facilitate the analyses of their intrinsic properties. The initial results point toward the feasibility of atomically controlled antiperovskite growth, which could open doors to study topological and correlated electronic states in an electronic environment quite distinct from what is available in conventional complex oxides.

cond-mat.mtrl-sci↗

arXiv2021

Code O-SUKI-N 3D: Upgraded Direct-Drive Fuel Target 3D Implosion Code in Heavy Ion Inertial Fusion

The Code O-SUKI-N 3D is an upgraded version of the 2D Code O-SUKI (Comput. Phys. Commun. 240, 83 (2019)). Code O-SUKI-N 3D is an integrated 3-dimensional (3D) simulation program system for fuel implosion, ignition and burning of a direct-drive nuclear-fusion pellet in heavy ion beam (HIB) inertial confinement fusion (HIF).The Code O-SUKI-N 3D consists of the three programs of Lagrangian fluid implosion program, data conversion program, and Euler fluid implosion, ignition and burning program. The Code O-SUKI-N 3D can also couple with the HIB illumination and energy deposition program of OK3 (Comput. Phys. Commun. 181, 1332 (2010)). The spherical target implosion 3D behavior is computed by the 3D Lagrangian fluid code until the time just before the void closure of the fuel implosion. After that, all the data by the Lagrangian implosion code are converted to the data for the 3D Eulerian code. In the 3D Euler code, the DT fuel compression at the stagnation, ignition and burning are computed. The Code O-SUKI-N 3D simulation system provides a capability to compute and to study the HIF target implosion dynamics.

physics.plasm-ph↗

arXiv2021

Planar Hall effect with sixfold oscillations in a Dirac antiperovskite

The planar Hall effect (PHE), wherein a rotating magnetic field in the plane of a sample induces oscillating transverse voltage, has recently garnered attention in a wide range of topological metals and insulators. The observed twofold oscillations in $ρ_{yx}$ as the magnetic field completes one rotation are the result of chiral, orbital and/or spin effects. The antiperovskites $A_3B$O ($A$ = Ca, Sr, Ba; $B$ = Sn, Pb) are topological crystalline insulators whose low-energy excitations are described by a generalized Dirac equation for fermions with total angular momentum $J = 3/2$. We report unusual sixfold oscillations in the PHE of Sr$_3$SnO, which persisted nearly up to room temperature. Multiple harmonics (twofold, fourfold and sixfold), which exhibited distinct field and temperature dependencies, were detected in $ρ_{xx}$ and $ρ_{yx}$. These observations are more diverse than those in other Dirac and Weyl semimetals and point to a richer interplay of microscopic processes underlying the PHE in the antiperovskites.

cond-mat.mes-hall↗

arXiv2020

Strong coupling nature of kagome superconductivity in LaRu$_3$Si$_2$

We report muon spin rotation ($μ$SR) experiments together with first-principles calculations on microscopic properties of superconductivity in the kagome superconductor LaRu$_3$Si$_2$ with $T_{\rm c}$ ${\simeq}$ 7K. We find that the calculated normal state band structure features a kagome flat band and Dirac as well as van Hove points formed by the Ru-$dz^2$ orbitals near the Fermi level. Below $T_{\rm c}$, $μ$SR reveals isotropic type-II superconductivity, which is robust against hydrostatic pressure up to 2 GPa. Intriguingly, the ratio 2$Δ/k_{\rm B}T_{\rm c}$ ${\simeq}$ 4.3 (where $Δ$ is the superconducting energy gap) is in the strong coupling limit, and $T_{\rm c}$/$λ_{eff}^{-2}$ (where $λ$ is the penetration depth) is comparable to that of high-temperature unconventional superconductors. We also find that electron-phonon coupling alone can only reproduce small fraction of $T_{\rm c}$ from calculations, which suggests other factors in enhancing $T_{\rm c}$ such as the correlation effect from the kagome flat band, the van Hove point on the kagome lattice, and high density of states from narrow kagome bands. Our experiments and calculations taken together point to strong coupling and the unconventional nature of kagome superconductivity in LaRu$_3$Si$_2$.

cond-mat.supr-con↗

arXiv2020

Structure and soft magnetic properties of single-domain Mg$_{1-x}$Ni$_x$Fe$_2$O$_4$ ($0 \le x \le 1.0$) nanocrystals

The effects of Ni$^{2+}$ substitution on the structure (lattice parameters, cations distribution, average cations radii, cation-cation/anion bond lengths and angles) and the soft magnetic properties of Mg$_{1-x}$Ni$_x$Fe$_2$O$_4$ ($0 \le x \le 1.0$) nanocrystals have been studied by x-ray diffraction with Rietveld refinement, transmittance electron microscopy, Mossbauer spectroscopy and magnetization measurements. The mostly inverse spinel structure was found in MgFe$_2$O$_4$ nanoparticles and the inversion factor is further increased by Ni-substitution up to a complete inversion in NiFe$_2$O$_4$. The synthesized ferrites with a small particle size (20 - 30 nm) exhibit soft magnetic properties of a single-domain behavior with critical domain size about 30 - 40 nm. An observed monotonic increase in magnetization and drastic decrease in coercivity at 5 K by Ni-substitution result in a lowest anisotropy constant Ku (2.8 kJ/m3) for x = 0.5. We discuss the evolution of magnetic properties by Ni-substitution in a correlation with a subsequent cations-redistribution between sites of the two sublattices and corresponding structural changes in the inter-ionic distances.

cond-mat.mtrl-sci↗

arXiv2020

Bright X-ray radiation from plasma bubbles in an evolving laser wakefield accelerator

We show that the properties of the electron beam and bright x-rays produced by a laser wakefield accelerator can be predicted if the distance over which the laser self-focuses and compresses prior to self-injection is taken into account. A model based on oscillations of the beam inside a plasma bubble shows that performance is optimised when the plasma length is matched to the laser depletion length. With a 200~TW laser pulse this results in an x-ray beam with median photon energy of \unit[20]{keV}, $> 6\times 10^{8}$ photons above \unit[1]{keV} per shot and a peak brightness of $\unit[3 \times 10^{22}]{photons~s^{-1}mrad^{-2}mm^{-2} (0.1\% BW)^{-1}}$.

physics.plasm-ph↗

arXiv2020

Lattice dynamics and electronic excitations in a large family of lacunar spinels with a breathing pyrochlore lattice structure

Reproducing the electronic structure of AM$_4$X$_8$ lacunar spinels with a breathing pyrochlore lattice is a great theoretical challenge due to the interplay of various factors. The character of the M$_4$X$_4$ cluster orbitals is critically influenced by the Jahn-Teller instability, the spin-orbit interaction, and also by the magnetic state of the clusters. Consequently, to reproduce the narrow-gap semiconducting nature of these moderately correlated materials requires advanced approaches, since the strength of the inter-cluster hopping is strongly affected by the character of the cluster orbitals. In order to provide a solid experimental basis for theoretical studies, we performed broadband optical spectroscopy on a large set of lacunar spinels, with systematically changing ions at the A and M sites as well as the ligand (A=Ga, Ge, Al; M=V, Mo, Nb, Ta; X=S, Se). Our study covers the range of phonon excitations and also electronic transitions near the gap edge. In the phonon excitation spectrum a limited subset of the symmetry allowed modes is observed in the cubic state, with a few additional modes emerging upon the symmetry-lowering structural transition. All the infrared active modes are assigned to vibrations of the ligands and ions at the A sites, with no obvious contribution from the M-site ions. Concerning the electronic states, we found that all compounds are narrow-gap semiconductors ($E_\mathrm{g} = 130 - 350\,$meV) already in their room-temperature cubic state and their structural transitions induce weak, if any, changes in the band gap. The gap value is decreased when substituting S with Se and also when replacing $3d$ ions by $4d$ or $5d$ ions at the M sites.

cond-mat.str-el↗

arXiv2019

Unusual Valence State in the Antiperovskites Sr$_3$SnO and Sr$_3$PbO Revealed by X-ray Photoelectron Spectroscopy

The class of antiperovskite compounds $A_3B$O ($A$ = Ca, Sr, Ba; $B$ = Sn, Pb) has attracted interest as a candidate 3D Dirac system with topological surface states protected by crystal symmetry. A key factor underlying the rich electronic structure of $A_3B$O is the unusual valence state of $B$, i.e., a formal oxidation state of $-4$. Practically, it is not obvious whether anionic $B$ can be stabilized in thin films, due to its unusual chemistry, as well as the polar surface of $A_3B$O, which may render the growth-front surface unstable. We report X-ray photoelectron spectroscopy (XPS) measurements of single-crystalline films of Sr$_3$SnO and Sr$_3$PbO grown by molecular beam epitaxy (MBE). We observe shifts in the core-level binding energies that originate from anionic Sn and Pb, consistent with density functional theory (DFT) calculations. Near the surface, we observe additional signatures of neutral or cationic Sn and Pb, which may point to an electronic or atomic reconstruction with possible impact on putative topological surface states.

cond-mat.mes-hall↗

arXiv2019

Robust weak antilocalization due to spin-orbital entanglement in Dirac material Sr$_3$SnO

The presence of both inversion ($P$) and time-reversal ($T$) symmetries in solids leads to a well-known double degeneracy of the electronic bands (Kramers degeneracy). When the degeneracy is lifted, spin textures can be directly observed in momentum space, as in topological insulators or in strong Rashba materials. The existence of spin textures with Kramers degeneracy, however, is very difficult to observe directly. Here, we use quantum interference measurements combined with first-principle band structure calculations to provide evidence for the existence of hidden entanglement between spin and momentum in the antiperovskite-type 3D Dirac material Sr$_3$SnO. We find robust weak antilocalization (WAL) independent of the position of $E_\mathrm{F}$. The observed WAL signal at low doping is fitted using a single interference channel, which implies that the different Dirac valleys are mixed by disorder. Notably, this mixing does not suppress WAL, suggesting contrasting interference physics compared to graphene. We identify scattering among axially spin-momentum locked states as a key process that leads to a spin-orbital entanglement, giving rise to robust WAL. Our work sheds light on the subtle role of spin and pseudospin, when both could contribute to the same quantum effect.

cond-mat.mes-hall↗

arXiv2019

Spin splitting and strain in epitaxial monolayer WSe$_2$ on graphene

We present the electronic and structural properties of monolayer WSe$_{2}$ grown by pulsed-laser deposition on monolayer graphene (MLG) on SiC. The spin splitting in the WSe$_{2}$ valence band at $\overline{\mathrm{K}}$ was $Δ_\mathrm{SO}=0.469\pm0.008$ eV by angle-resolved photoemission spectroscopy (ARPES). Synchrotron-based grazing-incidence in-plane X-ray diffraction (XRD) revealed the in-plane lattice constant of monolayer WSe$_{2}$ to be $a_\mathrm{WSe_2}=3.2757\pm0.0008 \mathrm{Å}$. This indicates a lattice compression of -0.19 % from bulk WSe$_{2}$. By using experimentally determined graphene lattice constant ($a_\mathrm{MLG}=2.4575\pm0.0007 \mathrm{Å}$), we found that a 3$\times$3 unit cell of the slightly compressed WSe$_{2}$ is perfectly commensurate with a 4$\times$4 graphene lattice with a mismatch below 0.03 %, which could explain why the monolayer WSe$_{2}$ is compressed on MLG. From XRD and first-principles calculations, however, we conclude that the observed size of strain is negligibly small to account for a discrepancy in $Δ_\mathrm{SO}$ found between exfoliated and epitaxial monolayers in earlier ARPES. In addition, angle-resolved, ultraviolet and X-ray photoelectron spectroscopy shed light on the band alignment between WSe$_{2}$ and MLG/SiC and indicate electron transfer from graphene to the WSe$_{2}$ monolayer. As further revealed by atomic force microscopy, the WSe$_{2}$ island size depends on the number of carbon layers on top of the SiC substrate. This suggests that the epitaxy of WSe$_{2}$ favors the weak van der Waals interactions with graphene while it is perturbed by the influence of the SiC substrate and its carbon buffer layer.

cond-mat.mtrl-sci↗

arXiv2019

Disordered quantum spin state in the stripe lattice system consisting of triangular and square tilings

Quantum fluctuations originating phase competition or geometrical frustration of spins lead to novel states such as a quantum critical point and a quantum spin liquid where the strong quantum fluctuations suppress any ordered states even at 0 K. Utilizing site-selective NMR for a quasi-two dimensional organic conductor $λ$-(STF)$_2$GaCl$_4$, we investigate the non-magnetic insulating phase of the stripe lattice system consisting of triangular and square tilings. We found development of AF spin fluctuations with decreasing temperature. Regardless of large enhancement of spin-lattice relaxation rate $1/T_1$ owing to critical slowing down below 10 K, no long-range magnetic ordering was observed down to 1.63 K two orders of magnitude less than the exchange interaction $J/k_{\rm B} \simeq$ 194 K. Moreover, $1/T_1$ saturated below 3.5 K. These results are in stark contrast to observed behaviors so far in other non-magnetic ground states discussed in terms of spin liquids, demonstrating realization of an exotic quantum state accompanying quantum criticality.

cond-mat.str-el↗

arXiv2019

Coexisting spin resonance and long-range magnetic order of Eu in EuRbFe$_4$As$_4$

Magnetic excitations and magnetic structure of EuRbFe$_4$As$_4$ were investigated by inelastic neutron scattering (INS), neutron diffraction, and random phase approximation (RPA) calculations. Below the superconducting transition temperature $T_\text{c}=36.5$~K, the INS spectra exhibit the neutron spin resonances at $Q_\text{res}=1.27(2)$~$\textÅ^{-1}$ and $1.79(3)$~$\textÅ^{-1}$. They correspond to the $\mathbf{Q}=(0.5,0.5,1)$ and $(0.5,0.5,3)$ nesting wave vectors, showing three dimensional nature of the band structure. The characteristic energy of the neutron spin resonance is $E_\text{res}=17.7(3)$~meV corresponding to $5.7(1)k_\text{B}T_\text{c}$. Observation of the neutron spin resonance mode and our RPA calculations in conjunction with the recent optical conductivity measurements are indicative of the $s_\pm$ superconducting pairing symmetry in EuRbFe$_4$As$_4$. In addition to the neutron spin resonance mode, upon decreasing temperature below the magnetic transition temperature $T_\text{N}=15$~K, the spin wave excitation originating in the long-range magnetic order of the Eu sublattice was observed in the low-energy inelastic channel. Single-crystal neutron diffraction measurements demonstrate that the magnetic propagation vector of the Eu sublattice is $\mathbf{k}=(0, 0, 0.25)$, representing the three-dimensional antiferromagnetic order. Linear spin wave calculations assuming the obtained magnetic structure with the intra- and inter-plane nearest neighbor exchange couplings of $J_1/k_\text{B}=-1.31$~K and $J_c/k_\text{B}=0.08$~K can reproduce quantitatively the observed spin wave excitation. Our results show that superconductivity and long-range magnetic order of Eu coexist in EuRbFe$_4$As$_4$ whereas the coupling between them is rather weak.

cond-mat.supr-con↗

arXiv2018

Orbital-Order Driven Ferroelectricity and Dipolar Relaxation Dynamics in Multiferroic GaMo$_4$S$_8$

We present the results of broadband dielectric spectroscopy of GaMo$_4$S$_8$, a lacunar spinel system that recently was shown to exhibit non-canonical, orbitally-driven ferroelectricity. Our study reveals complex relaxation dynamics of this multiferroic material, both above and below its Jahn-Teller transition at T$_{\textrm{JT}}=47$ K. Above T$_{\textrm{JT}}$, two types of coupled dipolar-orbital dynamics seem to compete: relaxations within cluster-like regions with short-range polar order like in relaxor ferroelectrics and critical fluctuations of only weakly interacting dipoles, the latter resembling the typical dynamics of order-disorder type ferroelectrics. Below the Jahn-Teller transition, the onset of orbital order drives the system into long-range ferroelectric order and dipolar dynamics within the ferroelectric domains is observed. The coupled dipolar and orbital relaxation behavior of GaMo$_4$S$_8$ above the Jahn-Teller transition markedly differs from that of the skyrmion host GaV$_4$S$_8$, which seems to be linked to differences in the structural distortions of the two systems on the unit-cell level.

cond-mat.str-el↗

arXiv2018

Observation of Laser Power Amplification in a Self-Injecting Laser Wakefield Accelerator

We report on the depletion and power amplification of the driving laser pulse in a strongly-driven laser wakefield accelerator. Simultaneous measurement of the transmitted pulse energy and temporal shape indicate an increase in peak power from $187 \pm 11$ TW to a maximum of $318 \pm 12$ TW after 13 mm of propagation in plasma density of $0.9 \times 10^{18}$ cm$^{-3}$. The power amplification is correlated with the injection and acceleration of electrons in the nonlinear wakefield. This process is modeled by including localized redshift and subsequent group delay dispersion at the laser pulse front.

physics.plasm-ph↗