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K. Goto

Publications and source records attributed to K. Goto.

12 recordsLinked to original sources
arXiv2024

A new air shower array in the Southern Hemisphere looking for the origins of Cosmic rays: the ALPACA experiment

The Tibet AS$\gamma$ experiment successfully detected sub-PeV $\gamma$-rays from the Crab nebula using a Surface Array and underground muon detector. Considering this, we are building in Bolivia a new experiment to explore the Southern Hemisphere, looking for the origins of cosmic rays in our Galaxy. The name of this project is Andes Large area PArticle detector for Cosmic ray physics and Astronomy (ALPACA). A prototype array called ALPAQUITA, with $1/4$ the total area of the full ALPACA, started observations in September $2022$. In this paper we introduce the status of ALPAQUITA and the plans to extend the array. We also report the results of the observation of the moon shadow in cosmic rays.

astro-ph.HE↗
arXiv2023

Description and stability of a RPC-based calorimeter in electromagnetic and hadronic shower environments

The CALICE Semi-Digital Hadron Calorimeter technological prototype completed in 2011 is a sampling calorimeter using Glass Resistive Plate Chamber (GRPC) detectors as the active medium. This technology is one of the two options proposed for the hadron calorimeter of the International Large Detector for the International Linear Collider. The prototype was exposed in 2015 to beams of muons, electrons, and pions of different energies at the CERN Super Proton Synchrotron. The use of this technology for future experiments requires a reliable simulation of its response that can predict its performance. GEANT4 combined with a digitization algorithm was used to simulate the prototype. It describes the full path of the signal: showering, gas avalanches, charge induction, and hit triggering. The simulation was tuned using muon tracks and electromagnetic showers for accounting for detector inhomogeneity and tested on hadronic showers collected in the test beam. This publication describes developments of the digitization algorithm. It is used to predict the stability of the detector performance against various changes in the data-taking conditions, including temperature, pressure, magnetic field, GRPC width variations, and gas mixture variations. These predictions are confronted with test beam data and provide an attempt to explain the detector properties. The data-taking conditions such as temperature and potential detector inhomogeneities affect energy density measurements but have a small impact on detector efficiency.

physics.ins-det↗
arXiv2022

Light-induced magnetization driven by interorbital charge motion in a spin-orbit assisted Mott insulator alpha-RuCl3

In a honeycomb-lattice spin-orbit assisted Mott insulator α-RuCl3, an ultrafast magnetization is induced by circularly polarized excitation below the Mott gap. Photo-carriers play an important role, which are generated by turning down the synergy of the on-site Coulomb interaction and the spin-orbit interaction realizing the insulator state. An ultrafast 6- fs measurement of photo-carrier dynamics and a quantum mechanical analysis clarify the mechanism, according to which the magnetization emerges from a coherent charge motion between different t2g orbitals (dyz-dxz-dxy) of Ru3+ ions. This ultrafast magnetization is weakened in the antiferromagnetic (AF) phase, which is opposite to the general tendency that the inverse Faraday effect is larger in AF compounds than in paramagnetic ones. This temperature dependence indicates that the interorbital charge motion is affected by pseudo-spin rotational symmetry breaking in the AF phase.

cond-mat.str-el↗
arXiv2017

Band-to-band transitions, selection rules, effective mass and exciton binding energy parameters in monoclinic β-Ga2O3

We employ an eigen polarization model including the description of direction dependent excitonic effects for rendering critical point structures within the dielectric function tensor of monoclinic β-Ga2O3 yielding a comprehensive analysis of generalized ellipsometry data obtained from 0.75 eV--9 eV. The eigen polarization model permits complete description of the dielectric response, and we obtain single-electron and excitonic band-to-band transition anisotropic critical point structure model parameters including their polarization eigenvectors within the monoclinic lattice. We compare our experimental analysis with results from density functional theory calculations performed using a recently proposed Gaussian-attenuation-Perdue-Burke-Ernzerhof hybrid density functional, and we present and discuss the order of the fundamental direct band-to-band transitions and their polarization selection rules, the electron and hole effective mass parameters for the three lowest band-to-band transitions, and their exciton binding energy parameters, in excellent agreement with our experimental results. We find that the effective masses for holes are highly anisotropic and correlate with the selection rules for the fundamental band-to-band transitions, where the observed transitions are polarized closely in the direction of the lowest hole effective mass for the valence band participating in the transition.

cond-mat.mtrl-sci↗
arXiv2015

Anisotropy, phonon modes, and free charge carrier parameters in monoclinic $β$-gallium oxide single crystals

We derive a dielectric function tensor model approach to render the optical response of monoclinic and triclinic symmetry materials with multiple uncoupled infrared and farinfrared active modes. We apply our model approach to monoclinic $β$-Ga$_2$O$_3$ single crystal samples. Surfaces cut under different angles from a bulk crystal, (010) and ($\bar{2}$01), are investigated by generalized spectroscopic ellipsometry within infrared and farinfrared spectral regions. We determine the frequency dependence of 4 independent $β$-Ga$_2$O$_3$ Cartesian dielectric function tensor elements by matching large sets of experimental data using a point by point data inversion approach. From matching our monoclinic model to the obtained 4 dielectric function tensor components, we determine all infared and farinfrared active transverse optic phonon modes with $A_u$ and $B_u$ symmetry, and their eigenvectors within the monoclinic lattice. We find excellent agreement between our model results and results of density functional theory calculations. We derive and discuss the frequencies of longitudinal optical phonons in $β$-Ga$_2$O$_3$. We derive and report density and anisotropic mobility parameters of the free charge carriers within the tin doped crystals. We discuss the occurrence of longitudinal phonon plasmon coupled modes in $β$-Ga$_2$O$_3$ and provide their frequencies and eigenvectors. We also discuss and present monoclinic dielectric constants for static electric fields and frequencies above the reststrahlen range, and we provide a generalization of the Lyddane-Sachs-Teller relation for monoclinic lattices with infrared and farinfrared active modes. We find that the generalized Lyddane-Sachs-Teller relation is fulfilled excellently for $β$-Ga$_2$O$_3$.

cond-mat.mtrl-sci↗
arXiv2015

Long-range order and spin-liquid states of polycrystalline Tb$_{2+x}$Ti$_{2-x}$O$_{7+y}$

Low-temperature states of polycrystalline samples of a frustrated pyrochlore oxide Tb$_{2+x}$Ti$_{2-x}$O$_{7+y}$ have been investigated by specific heat, magnetic susceptibility, and neutron scattering experiments. We have found that this system can be tuned by a minute change of $x$ from a spin-liquid state ($x < x_{\text{c}}$) to a partly ordered state with a small antiferromagnetic ordering of the order of $0.1 μ_{\text{B}}$. Specific heat shows a sharp peak at a phase transition at $T_{\text{c}}= 0.5$ K for $x=0.005$. Magnetic excitation spectra for this sample change from a quasielastic to a gapped type through $T_{\text{c}}$. The possibility of a Jahn-Teller transition is discussed.

cond-mat.str-el↗
arXiv2007

Softening of Magnetic Excitations Leading to Pressure-Induced Quantum Phase Transition in Gapped Spin System KCuCl$_3$

KCuCl$_3$ is a three dimensionally coupled spin dimer system, which undergoes a pressure-induced quantum phase transition from a gapped ground state to an antiferromagnetic state at a critical pressure of $P_{\rm c} \simeq 8.2$ kbar. Magnetic excitations in KCuCl$_3$ at a hydrostatic pressure of 4.7 kbar have been investigated by conducting neutron inelastic scattering experiments using a newly designed cylindrical high-pressure clamp cell. A well-defined single excitation mode is observed. The softening of the excitation mode due to the applied pressure is clearly observed. From the analysis of the dispersion relations, it is found that an intradimer interaction decreases under hydrostatic pressure, while most interdimer interactions increase.

cond-mat.str-el↗
arXiv2006

Magentic Ordering under Hydrostatic Pressure in Doped Spin Gap Systems ACu$_{1-x}$Mg$_x$Cl$_3$: A$=$Tl and K

Magnetic phase transitions under hydrostatic pressures in spin gap systems TlCu$_{0.988}$Mg$_{0.012}$Cl$_3$ and KCu$_{0.973}$Mg$_{0.027}$Cl$_3$ were investigated by magnetization measurements. The present doped systems exhibit impurity-induced magnetic orderings. With increasing pressure, ordering temperature $T_{\rm N}$ increases. With a further increase in pressure, the present systems undergo phase transitions to uniform antiferromagnetic phases due to the closing of the triplet gap in the intact dimers. The crossover from the impurity-induced ordered phase to the uniform antiferromagnetic phase occurs at $P \simeq 1.3$ kbar for TlCu$_{0.988}$Mg$_{0.012}$Cl$_3$.

cond-mat.str-el↗
arXiv2006

Crossover from impurity-induced ordered phase to uniform antiferromagnetic phase under hydrostatic pressure in the doped spin-gap system TlCu$_{1-x}$Mg$_x$Cl$_3$

Magnetic phase transition under hydrostatic pressure in TlCu$_{0.988}$Mg$_{0.012}$Cl$_3$ was investigated by magnetization measurements. The parent compound TlCuCl$_3$ is a coupled spin dimer system, which undergoes a pressure-induced quantum phase transition from a gapped ground state to an antiferromagnetic state at $P_{\rm c} = 0.42$ kbar due to the shrinkage of the gap. At ambient pressure, the present doped system exhibits impurity-induced magnetic ordering at $T_{\rm N}=2.5$ K. With increasing pressure, $T_{\rm N}$ increases. This is because the effective exchange interaction $J_{\rm eff}$ between unpaired spins is enhanced by the shrinkage of the gap. With a further increase in pressure, the present system undergoes a phase transition to a uniform antiferromagnetic phase due to the closing of the triplet gap in the intact dimers. The crossover from the impurity-induced ordered phase to the uniform antiferromagnetic phase occurs at $P \simeq 1.3$ kbar.

cond-mat.str-el↗
arXiv2001

Relativistic Electron Facility for Education and Research at Hiroshima University

The Relativistic Electron Facility for Education and Research (REFER) at Hiroshima University accepts a 150 MeV electron beam from a microtron, the injector to the 700 MeV storage ring of the Hiroshima Synchrotron Research Center, and keep it circulating for 2.5ms without a RF acceleration. It acts as a beam stretcher and has been used for the following researches: 1) X-ray generation from an internal multiple foil crystalline target due to the parametric X-radiation (PXR) and the resonant transition radiation (RTR). 2) Laser backward Compton scattering. 3) Coherent pair creation from Si crystal by 150 MeV bremsstrahlung. 4) Study of the Ter-Mikaelian effect in the low energy part of the bremsstrahlung in an extracted electron line. 5) A novel scheme of beam stabilization by the induction coil. This paper reports the experimental results and the current status of the REFER electron ring.

physics.acc-ph↗