arXiv subjects

Y. Harada

Publications and source records attributed to Y. Harada.

At least 19 recordsLinked to original sources

arXiv2024

Micro-scale Electrostatic Structures formed on the Rough Surfaces of the Moon

It is widely accepted that the surface potential of the lunar dayside is "on average" several to 10 V positive due to photoelectron emission in addition to the solar wind plasma precipitation. Recent studies, however, have shown that an insulating and rough regolith layer tends to make positive and negative charges separated and irregularly distributed on sub-Debye-length scales. The local charge separation then gives rise to an intense and structured electrostatic field. Such micro-scale electrostatic structures lie in the innermost part of the photoelectron sheath and may contribute to the mobilization of the charged dust particles. Since the electrostatic structures can take different states depending on the topography of the lunar surface, it is necessary to update the research approach. We have launched a research group to develop an integrated assessment framework that includes theoretical and numerical modeling, on-orbit observations, ground-based testing, and the development of charging measurement instruments, with the ultimate goal of comprehensively understanding the surface charging processes on the Moon.

physics.space-ph↗

arXiv2018

Half-metallicity of Mn2VAl ferrimagnet revealed by resonant inelastic soft x-ray scattering under magnetic field

Detailed information on the electronic states of both V and Mn 3d electrons in the ferrimagnet Mn2VAl is obtained by the bulk sensitive resonant inelastic soft x-ray scattering (SX-RIXS) excited with the circularly polarized light under an external magnetic field for the first time. The results under the V L-edge excitation have revealed the negligible partial density of states (PDOS) of the V 3d states around the Fermi energy as well as their rather localized character. Under the Mn L-edge excitation, on the other hand, the spectra are dominated by fluorescence with clear magnetic circular dichroism with noticeable excitation photon energy dependence. Compared with the theoretical prediction of the RIXS spectra based on the density-functional-theory band structure calculation, an itinerant, spin-dependent character of the Mn 3d states and decays of the Mn 2p core states are confirmed in consistence with the half-metallicity of the Mn 3d states.

cond-mat.mtrl-sci↗

arXiv2017

Tensile-Strain Dependent Spin States in Epitaxial LaCoO$_3$ Thin Films

The spin states of Co$^{3+}$ ions in perovskite-type LaCoO$_3$, governed by complex interplay between the electron-lattice interactions and the strong electron correlations, still remain controversial due to the lack of experimental techniques which can detect directly. In this letter, we revealed the tensile-strain dependence of spin states, $i. e.$ the ratio of the high- and low-spin states, in epitaxial thin films and a bulk crystal of LaCoO$_3$ via resonant inelastic soft x-ray scattering. The tensile-strain as small as 1.0% was found to realize different spin states from that in the bulk.

cond-mat.str-el↗

arXiv2017

Raman and fluorescence contributions to resonant inelastic soft x-ray scattering on LaAlO$_3$/SrTiO$_3$ heterostructures

We present a detailed study of the Ti 3$d$ carriers at the interface of LaAlO$_3$/SrTiO$_3$ heterostructures by high-resolution resonant inelastic soft x-ray scattering (RIXS), with special focus on the roles of overlayer thickness and oxygen vacancies. Our measurements show the existence of interfacial Ti 3$d$ electrons already below the critical thickness for conductivity and an increase of the total interface charge up to a LaAlO$_3$ overlayer thickness of 6 unit cells before it levels out. By comparing stoichiometric and oxygen deficient samples we observe strong Ti 3$d$ charge carrier doping by oxygen vacancies. The RIXS data combined with photoelectron spectroscopy and transport measurements indicate the simultaneous presence of localized and itinerant charge carriers. However, it is demonstrated that the relative amount of localized and itinerant Ti $3d$ electrons in the ground state cannot be deduced from the relative intensities of the Raman and fluorescence peaks in excitation energy dependent RIXS measurements, in contrast to previous interpretations. Rather, we attribute the observation of either the Raman or the fluorescence signal to the spatial extension of the intermediate state reached in the RIXS excitation process.

cond-mat.mtrl-sci↗

arXiv2017

Electronic Structure and Magnetic Properties of Half-metallic Ferrimagnet Mn$_{2}$VAl Probed by Soft X-ray Spectroscopies

We have studied the electronic structure of ferrimagnetic Mn2VAl single crystal by means of soft X-ray absorption spectroscopy (XAS), X-ray absorption magnetic circular dichroism (XMCD) and resonant soft X-ray inelastic scattering (RIXS). We have successfully observed the XMCD signals for all constitute elements, supporting the spin polarized states at the Fermi level. The Mn $L_{2,3}$ XAS and XMCD spectra are reproduced by the spectral simulation based on density-functional theory (DFT), indicating itinerant character of the Mn 3d states. On the other hand, V $3d$ electrons are rather localized since the ionic model can qualitatively explain the V $L_{2,3}$ XAS and XMCD spectra as well as the local dd excitation revealed by V $L_3$ RIXS.

cond-mat.str-el↗

arXiv2017

Observation of a dispersive charge mode in hole-doped cuprates using resonant inelastic x-ray scattering at the oxygen K edge

We investigate electronic excitations in La2-x(Br,Sr)xCuO4 using resonant inelastic x-ray scattering (RIXS) at the oxygen K edge. RIXS spectra of the hole-doped cuprates show clear momentum dependence below 1 eV. The spectral weight exhibits positive dispersion and shifts to higher energy with increasing hole concentration. Theoretical calculation of the dynamical charge structure factor on oxygen orbitals in a three-band Hubbard model is consistent with the experimental observation of the momentum and doping dependence, and therefore the dispersive mode is ascribed to intraband charge excitations which have been observed in electron-doped cuprates.

cond-mat.str-el↗

arXiv2016

Resonant inelastic x-ray scattering study of entangled spin-orbital excitations in superconducting PrFeAsO0.7

Low-energy electron excitation spectra were measured on a single crystal of a typical iron-based superconductor PrFeAsO$_{0.7}$ using resonant inelastic X-ray scattering (RIXS) at the Fe-$L_3$ edge. Characteristic RIXS features are clearly observed around 0.5, 1-1.5 and 2-3 eV energy losses. These excitations are analyzed microscopically with theoretical calculations using a 22-orbital model derived from first-principles electronic structure calculation. Based on the agreement with the experiment, the RIXS features are assigned to Fe-$d$ orbital excitations which, at low energies, are accompanied by spin flipping and dominated by Fe $d_{yz}$ and $d_{xz}$ orbital characters. Furthermore, our calculations suggest dispersive momentum dependence of the RIXS excitations below 0.5 eV, and predict remarkable splitting and merging of the lower-energy excitations in momentum space. Those excitations, which were not observed in the present experiment, highlight the potential of RIXS with an improved energy resolution to unravel new details of the electronic structure of the iron-based superconductors.

cond-mat.str-el↗

arXiv2016

Spatially resolved electronic structure of an isovalent nitrogen center in GaAs

Small numbers of nitrogen dopants dramatically modify the electronic properties of GaAs, generating very large shifts in the conduction-band energies with nonlinear concentration dependence, and impurity-associated spatially-localized resonant states within the conduction band. Cross-sectional scanning tunneling microscopy provides the local electronic structure of single nitrogen dopants at the (110) GaAs surface, yielding highly anisotropic spatial shapes when the empty states are imaged. Measurements of the resonant states relative to the GaAs surface states and their spatial extent allow an unambiguous assignment of specific features to nitrogen atoms at different depths below the cleaved (110) surface. Multiband tight binding calculations around the resonance energy of nitrogen in the conduction band match the imaged features. The spatial anisotropy is attributed to the tetrahedral symmetry of the bulk lattice. Additionally, the voltage dependence of the electronic contrast for two features in the filled state imaging suggest these features could be related to a locally modified surface state.

cond-mat.mes-hall↗

arXiv2016

Interface electronic structure at the topological insulator - ferrimagnetic insulator junction

An interface electron state at the junction between a three-dimensional topological insulator (TI) film of Bi2Se3 and a ferrimagnetic insulator film of Y3Fe5O12 (YIG) was investigated by measurements of angle-resolved photoelectron spectroscopy and X-ray absorption magnetic circular dichroism (XMCD). The surface state of the Bi2Se3 film was directly observed and localized 3d spin states of the Fe3+ state in the YIG film were confirmed. The proximity effect is likely described in terms of the exchange interaction between the localized Fe 3d electrons in the YIG film and delocalized electrons of the surface and bulk states in the Bi2Se3 film. The Curie temperature (TC) may be increased by reducing the amount of the interface Fe2+ ions with opposite spin direction observable as a pre-edge in the XMCD spectra.

cond-mat.mes-hall↗

arXiv2014

Spin and orbital magnetic moments of Fe in the $n$-type ferromagnetic semiconductor (In,Fe)As

The electronic and magnetic properties of Fe atoms in the ferromagnetic semiconductor (In,Fe)As codoped with Be have been studied by x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at the Fe $L_{2,3}$ edge. The XAS and XMCD spectra showed simple spectral line shapes similar to Fe metal, but the ratio of the orbital and spin magnetic moments ($M_\mathrm{orb}$/$M_\mathrm{spin}$) estimated using the XMCD sum rules was significantly larger than that of Fe metal, indicating a significant orbital moment of Fe $3d$ electrons in (In,Fe)As:Be. The positive value of $M_\mathrm{orb}$/$M_\mathrm{spin}$ implies that the Fe $3d$ shell is more than half-filled, which arises from the hybridization of the Fe$^{3+}$ ($d^5$) state with the charge-transfer $d^6\underline{L}$ states, where $\underline{L}$ is a ligand hole in the host valence band. The XMCD intensity as a function of magnetic field indicated hysteretic behavior of the superparamagnetic-like component due to discrete ferromagnetic domains.

cond-mat.mtrl-sci↗

arXiv2013

Electronic excitations of magnetic impurity state in diluted magnetic semiconductor (Ga,Mn)As

The electronic structure of doped Mn in (Ga,Mn)As is studied by resonant inelastic X-ray scattering (RIXS). From configuration-interaction cluster-model calculations, the line shapes of the Mn $L_3$ RIXS spectra can be explained by $d$-$d$ excitations from the Mn$^{3+}$ ground state, dominated by charge-transferred states, rather than a Mn$^{2+}$ ground state. Unlike archetypical $d$-$d$ excitation, the peak widths are broader than the experimental energy resolution. We attribute the broadening to a finite lifetime of the $d$-$d$ excitations, which decay rapidly to electron-hole pairs in the host valence and conduction bands through hybridization of the Mn $3d$ orbital with the ligand band.

cond-mat.mtrl-sci↗

arXiv2008

Systematic characterization of upper critical fields for MgB$_2$ thin films using the two-band superconducting theory

We present experimental results of the upper critical fields $H_{\rm c2}$ of various MgB$_2$ thin films prepared by the molecular beam epitaxy, multiple-targets sputtering, and co-evaporation deposition apparatus. Experimental data of the $H_{\rm c2}(T)$ are successfully analyzed by applying the Gurevich theory of dirty two-band superconductivity in the case of $D_π/D_σ>1$, where $D_π$ and $D_σ$ are the intraband electron diffusivities for $π$ and $σ$ bands, respectively. We find that the parameters obtained from the analysis are strongly correlated to the superconducting transition temperature $T_{\rm c}$ of the films. We also discuss the anormalous narrowing of the transition width at intermediate temperatures confirmed by the magnetoresistance measurements.

cond-mat.supr-con↗

arXiv2007

Atomic-like behaviors and orbital-related Luttinger liquid in carbon nano-peapod quantum dots

We report influence of encapsulated C60 molecules on electron transport in carbon-nanotube peapod quantum dots. We find atomic-like behaviors with doubly degenerate electronic levels, which exist only around ground states, by single electron spectroscopy measured at low back-gate voltages (Vbg). Correlation with presence of nearly free electrons (NFEs) unique to the peapods is discussed. Moreover, we find anomalously high values of power a observed in power laws in conductance versus energy relationships, which are strongly associated with the doubly degenerate levels. It is revealed that the powers originate from Tomonaga-Luttinger liquid via the occupied doubly degenerate levels. Our observations clarify that the encapsulated C60 molecules form doubly degenerate levels only at ground state in peapod quantum dots and do not eliminate a ballistic charge transport.

cond-mat.mtrl-sci↗

arXiv2005

Direct Observation of Site-specific Valence Electronic Structure at Interface: SiO2/Si Interface

Atom specific valence electronic structures at interface are elucidated successfully using soft x-ray absorption and emission spectroscopy. In order to demonstrate the versatility of this method, we investigated SiO2/Si interface as a prototype and directly observed valence electronic states projected at the particular atoms of the SiO2/Si interface; local electronic structure strongly depends on the chemical states of each atom. In addition we compared the experimental results with first-principle calculations, which quantitatively revealed the interfacial properties in atomic-scale.

cond-mat.mtrl-sci↗

arXiv2004

Doping Dependent Changes in Nitrogen 2$p$ States in the Diluted Magnetic Semiconductor Ga$_{1-x}$Cr$_{x}$N

We study the electronic structure of the recently discovered diluted magnetic semiconductor Ga$_{1-x}$Cr$_{x}$N ($x$ = 0.01-0.10). A systematic study of the changes in the $occupied$ and $unoccupied$ ligand (N) partial density of states (DOS) of the host lattice is carried out using N 1$s$ soft x-ray emission and absorption spectroscopy, respectively. X-ray absorption measurements confirm the wurtzite N 2$p$ DOS and substitutional doping of Cr into Ga-sites. Coupled changes in the $occupied$ and $unoccupied$ N 2$p$ character DOS of Ga$_{1-x}$Cr$_{x}$N identify states responsible for ferromagnetism consistent with band structure calculations.

cond-mat.mtrl-sci↗

arXiv2004

Layer dependent band dispersion and correlations using tunable Soft X-ray ARPES

Soft X-ray Angle-Resolved Photoemission Spectroscopy is applied to study in-plane band dispersions of Nickel as a function of probing depth. Photon energies between 190 and 780 eV were used to effectively probe up to 3-7 layers. The results show layer dependent band dispersion of the Delta_2 minority-spin band which crosses the Fermi level in 3 or more layers, in contrast to known top 1-2 layers dispersion obtained using ultra-violet rays. The layer dependence corresponds to an increased value of exchange splitting and suggests reduced correlation effects in the bulk compared to the surface.

cond-mat.str-el↗

arXiv2004

Anisotropy of superconductivity of as-grown MgB$_2$ thin films by molecular beam epitaxy

Superconducting thin films of magnesium diboride (MgB$_2$) were prepared on MgO (001) substrate by a molecular beam epitaxy (MBE) method with the co-evaporation conditions of low deposition rate in ultra-high vacuum. The structural and physical properties of the films were studied by RHEED, XRD, XPS, resistivity and magnetization measurements.All films demonstrated superconductivity without use of any post-annealing process.The highest {\it T}$_{c,onset}$ determined by resistivity measurement was about 33K in the present samples.Anisotropic superconducting properties were evaluated by the resistivity and magnetic measurements.We will discuss the anisotropy of superconductivity for as-grown MgB$_2$ thin films.

cond-mat.supr-con↗

arXiv2004

Synthesis of as-grown superconducting MgB_2 thin films by molecular beam epitaxy in UHV conditions

As-grown superconducting MgB_2 thin films have been grown on SrTiO_3(001), MgO(001), and Al_2O_3(0001) substrates by a molecular beam epitaxy (MBE) method with novel co-evaporation conditions of low deposition rate in ultra-high vacuum. The structural and physical properties of the films were studied by RHEED, XRD, electrical resistivity measurements, and SQUID magnetometer. The RHEED patterns indicate three-dimensional growth for MgB_2. The highest T_c determined by resistivity measurement was about 36K in these samples. And a clear Meissner effect below T_c was observed using magnetic susceptibility measurement. We will discuss the influence of B buffer layer on the structural and physical properties.

cond-mat.supr-con↗