arXiv subjects

M. Yoshida

Publications and source records attributed to M. Yoshida.

At least 19 recordsLinked to original sources

arXiv2024

Acceleration of positive muons by a radio-frequency cavity

Acceleration of positive muons from thermal energy to $100~$keV has been demonstrated. Thermal muons were generated by resonant multi-photon ionization of muonium atoms emitted from a sheet of laser-ablated aerogel. The thermal muons were first electrostatically accelerated to $5.7~$keV, followed by further acceleration to 100 keV using a radio-frequency quadrupole. The transverse normalized emittance of the accelerated muons in the horizontal and vertical planes were $0.85 \pm 0.25 ~\rm{(stat.)}~^{+0.22}_{-0.13} ~\rm{(syst.)}~\pi~$mm$\cdot$mrad and $0.32\pm 0.03~\rm{(stat.)} ^{+0.05}_{-0.02} ~\rm{(syst.)}~\pi~$mm$\cdot$mrad, respectively. The measured emittance values demonstrated phase space reduction by a factor of $2.0\times 10^2$ (horizontal) and $4.1\times 10^2$ (vertical) allowing good acceleration efficiency. These results pave the way to realize the first-ever muon accelerator for a variety of applications in particle physics, material science, and other fields.

physics.acc-ph↗

arXiv2023

Behavior under magnetic field of resonance at the edge of the upper Hubbard band in 1T-TaS2

Recent theoretical investigations of quantum spin liquids have described phenomenology amenable to experimental observation using scanning tunneling microscopy. This includes characteristic resonances found at the edge of the upper Hubbard band of the host Mott insulator, that under certain conditions shift into the Mott gap under external magnetic field [W.-Y. He and P. A. Lee, arXiv:2212.08767]. In light of this we report scanning tunneling microscopy observations, in samples of the quantum spin liquid candidate 1T-TaS2, of a conductance peak at the upper Hubbard band edge and its magnetic field dependent behavior. These observations potentially represent evidence for the existence of a quantum spin liquid in 1T TaS2. We also observe samples in which such field dependence is absent, but with no observed correlate for the presence or absence of field dependence. This suggests one or more material properties controlling electronic behavior that are yet to be understood, and should help to motivate renewed investigation of the microscopic degrees of freedom in play in 1T-TaS2, as well as the possible realization of a quantum spin liquid phase.

cond-mat.str-el↗

arXiv2022

Quiet Sun Center to Limb Variation of the Linear Polarization Observed by CLASP2 Across the Mg II h & k Lines

The CLASP2 (Chromospheric LAyer SpectroPolarimeter 2) sounding rocket mission was launched on 2019 April 11. CLASP2 measured the four Stokes parameters of the Mg II h & k spectral region around 2800 Angstroms along a 200 arcsecond slit at three locations on the solar disk, achieving the first spatially and spectrally resolved observations of the solar polarization in this near ultraviolet region. The focus of the work presented here is the center-to-limb variation of the linear polarization across these resonance lines, which is produced by the scattering of anisotropic radiation in the solar atmosphere. The linear polarization signals of the Mg II h & k lines are sensitive to the magnetic field from the low to the upper chromosphere through the Hanle and magneto-optical effects. We compare the observations to theoretical predictions from radiative transfer calculations in unmagnetized semi-empirical models, arguing that magnetic fields and horizontal inhomogeneities are needed to explain the observed polarization signals and spatial variations. This comparison is an important step in both validating and refining our understanding of the physical origin of these polarization signatures, and also in paving the way toward future space telescopes for probing the magnetic fields of the solar upper atmosphere via ultraviolet spectropolarimetry.

astro-ph.SR↗

arXiv2022

Estimation of Gr\"uneisen parameter of high-entropy-alloy-type functional materials

In functional materials like thermoelectric materials and superconductors, the interplay between functionality, electronic structure, and phonon characteristics is one of the key factors to improve functionality and to understand the mechanisms. In the first part of this article, we briefly review investigations on lattice anharmonicity in functional materials by Gr\"uneisen parameter ({\gamma}G). One can find that the {\gamma}G can be a good scale for large lattice anharmonicity and for detecting a change in anharmonicity amplitude in functional materials. Then, we show original results on estimation of {\gamma}G for recently-developed high-entropy-alloy-type (HEA-type) functional materials with a layered structure and a NaCl-type structure. As a common trend between those two systems with two- and three-dimensional structures, we find that {\gamma}G increases by a slight increase in configurational entropy of mixing ({\Delta}Smix), and then {\gamma}G decreases with increasing {\Delta}Smix in high-entropy region.

cond-mat.mtrl-sci↗

arXiv2022

Development of microwave cavities for measurement of muonium hyperfine structure at J-PARC

The MuSEUM collaboration is planning measurements of the ground-state hyperfine structure (HFS) of muonium at the Japan Proton Accelerator Research Complex (J-PARC), Materials and Life Science Experimental Facility. The high-intensity beam that will soon be available at H-line allows for more precise measurements by one order of magnitude. We plan to conduct two staged measurements. First, we will measure the Mu-HFS in a near-zero magnetic field, and thereafter we will measure it in a strong magnetic field. We have developed two microwave cavities for this purpose. Furthermore, we evaluated systematic uncertainties from such a fluctuation of microwave fields and confirm the requirement of the microwave system, we use a microwave field distribution calculated from the finite element method.

physics.ins-det↗

arXiv2021

Search for Solar Flare Neutrinos with the KamLAND detector

We report the result of a search for neutrinos in coincidence with solar flares from the GOES flare database. The search was performed on a 10.8 kton-year exposure of KamLAND collected from 2002 to 2019. This large exposure allows us to explore previously unconstrained parameter space for solar flare neutrinos. We found no statistical excess of neutrinos and established 90% confidence level upper limits of $8.4 \times 10^7$ cm$^{-2}$ ($3.0 \times 10^{9}$ cm$^{-2}$) on electron anti-neutrino (electron neutrino) fluence at 20 MeV normalized to the X12 flare, assuming that the neutrino fluence is proportional to the X-ray intensity.

astro-ph.SR↗

arXiv2021

Limits on astrophysical antineutrinos with the KamLAND experiment

We report on a search for electron antineutrinos ($\barν_e$) from astrophysical sources in the neutrino energy range 8.3 to 30.8 MeV with the KamLAND detector. In an exposure of 6.72 kton-year of the liquid scintillator, we observe 18 candidate events via the inverse beta decay reaction. Although there is a large background uncertainty from neutral current atmospheric neutrino interactions, we find no significant excess over background model predictions. Assuming several supernova relic neutrino spectra, we give upper flux limits of 60--110 cm$^{-2}$ s$^{-1}$ (90% CL) in the analysis range and present a model-independent flux. We also set limits on the annihilation rates for light dark matter pairs to neutrino pairs. These data improves on the upper probability limit of $^{8}$B solar neutrinos converting into $\barν_e$'s, $P_{ν_e \rightarrow \barν_e} < 3.5\times10^{-5}$ (90% CL) assuming an undistorted $\barν_e$ shape. This corresponds to a solar $\barν_e$ flux of 60 cm$^{-2}$ s$^{-1}$ (90% CL) in the analysis energy range.

astro-ph.HE↗

arXiv2021

Doublon-like excitations and their phononic coupling in a Mott charge-density-wave system

Electron-phonon-driven charge density waves can in some circumstances allow electronic correlations to become predominant, driving a system into a Mott insulating state. New insights into both the Mott state and preceding charge density wave may result from observations of the coupled dynamics of their underlying degrees of freedom. Here, tunneling injection of single electrons into the upper Hubbard band of the Mott charge-density-wave material 1T-TaS2 reveals extraordinarily narrow electronic excitations which couple to amplitude mode phonons associated with the charge density wave's periodic lattice distortion. This gives a vivid microscopic view of the interplay between excitations of the Mott state and the lattice dynamics of its charge density wave precursor.

cond-mat.str-el↗

arXiv2021

A Search for Charged Excitation of Dark Matter with the KamLAND-Zen Detector

There are many theories where a dark matter particle is part of a multiplet with an electrically charged state. If WIMP dark matter ($χ^{0}$) is accompanied by a charged excited state ($χ^{-}$) separated by a small mass difference, it can form a stable bound state with a nucleus. In supersymmetric models, the $χ^{0}$ and the $χ^{-}$ could be the neutralino and a charged slepton, such as the neutralino-stau degenerate model. The formation binding process is expected to result in an energy deposition of {\it O}(1--10 MeV), making it suitable for detection in large liquid scintillator detectors. We describe new constraints on the bound state formation with a xenon nucleus using the KamLAND-Zen 400 Phase-II dataset. In order to enlarge the searchable parameter space, all xenon isotopes in the detector were used. For a benchmark parameter set of $m_{χ^{0}} = 100$ GeV and $Δm = 10$ MeV, this study sets the most stringent upper limits on the recombination cross section $\langleσv\rangle$ and the decay-width of $χ^{-}$ of $2.0 \times 10^{-31}$ ${\rm cm^3/s}$ and $1.1 \times 10^{-18}$ GeV, respectively (90\% confidence level).

hep-ex↗

arXiv2020

Search for Low-energy Electron Antineutrinos in KamLAND Associated with Gravitational Wave Events

We present the results of a search for MeV-scale electron antineutrino events in KamLAND in coincident with the 60 gravitational wave events/candidates reported by the LIGO/Virgo collaboration during their second and third observing runs. We find no significant coincident signals within a $\pm$ 500 s timing window from each gravitational wave and present 90% C.L. upper limits on the electron antineutrino fluence between $10^{8}$-$10^{13}\,{\mathrm cm^2}$ for neutrino energies in the energy range of 1.8-111 MeV.

astro-ph.HE↗

arXiv2020

Non-resonant n = 1 helical core induced by m/n = 2/1 tearing mode in JT-60U

In JT-60U, simultaneous excitation of n = 1 helical cores (HCs) and m/n = 2/1 Tearing Modes (TMs) was observed [T. Bando et al., Plasma Phys. Control. Fusion 61 115014 (2019)]. In this paper, we have investigated the excitation mechanism of n = 1 HCs with m/n = 2/1 TMs based on the experimental observations and a simple quasi-linear MHD model. In the previous study, it was reported that a "coupling" on the phase of the MHD mode is observed between n = 1 HCs and m/n = 2/1 TMs. In this study, it is found that the coupling is observed with the mode frequency from several Hz to 6 kHz. This indicates that the resistive wall and the plasma control system do not induce the coupling because the both time scales are different from the mode frequency. In addition, n = 1 HCs appear to be the non-resonant mode from the two observations: n = 1 HCs do not rotate with the plasma around the q = 1 surface in the core and the coupling is also observed even when qmin > 1. It is also observed that the electron fluctuation due to an n = 1 HC in the core region disappears with the stabilization of an m/n = 2/1 neoclassical tearing mode by electron cyclotron current drive, implying that n = 1 HCs are driven by m/n = 2/1 TMs. This perspective, n = 1 HCs are driven by m/n = 2/1 TMs, is supported by the observation that the saturated amplitude of the m/n = 1/1 component of the radial displacement in the core is smaller than that of the m/n = 2/1 component. Finally, we revisit a quasi-linear MHD model where the m/n = 1/1 HC is induced directly by the sideband of the current for the m/n = 2/1 TM, which allows to excite the non-resonant m/n = 1/1 mode. The model also describes the characteristic of the coupling, fm/n=1/1(HC) = 2fm/n=2/1(TM).

physics.plasm-ph↗

arXiv2020

Impact of plasma shaping on tokamak microstability

We have used the local-$δ{f}$ gyrokinetic code GS2 to perform studies of the effect of flux-surface shaping on two highly-shaped, low- and high-$β$ JT-60SA-relevant equilibria, including a successful benchmark with the GKV code. We find a novel destabilization of electrostatic fluctuations with increased elongation for plasma with a strongly peaked pressure profile. We explain the results as a competition between the local magnetic shear and finite-Larmor-radius (FLR) stabilization. Electromagnetic studies indicate that kinetic ballooning modes are stabilized by increased shaping due to an increased sensitivity to FLR effects, relative to the ion-temperature-gradient instability. Nevertheless, at high enough $β$, increased elongation degrades the local magnetic shear stabilization that enables access to the region of ballooning second-stability.

physics.plasm-ph↗

arXiv2020

Design and performance of a high-pressure xenon gas TPC as a prototype for a large-scale neutrinoless double-beta decay search

A high-pressure xenon gas time projection chamber, with a unique cellular readout structure based on electroluminescence, has been developed for a large-scale neutrinoless double-beta decay search. In order to evaluate the detector performance and validate its design, a 180~L size prototype is being constructed and its commissioning with partial detector has been performed. The obtained energy resolution at 4.0~bar is 1.73 $\pm$ 0.07% (FWHM) at 511 keV. The energy resolution at the $^{136}$Xe neutrinoless double-beta decay Q-value is estimated to be between 0.79 and 1.52% (FWHM) by extrapolation. Reconstructed event topologies show patterns peculiar to track end-point which can be used to distinguish $0νββ$ signals from gamma-ray backgrounds.

physics.ins-det↗

arXiv2020

Effect of quenched disorder on a quantum spin liquid state of triangular-lattice antiferromagnet 1T-TaS$_2$

A quantum spin liquid (QSL) is an exotic state of matter characterized by quantum entanglement and the absence of any broken symmetry. A long-standing open problem, which is a key for fundamental understanding the mysterious QSL states, is how the quantum fluctuations respond to randomness due to quenched disorder. Transition metal dichalcogenide 1T-TaS$_2$ is a candidate material that hosts a QSL ground state with spin-1/2 on the two-dimensional perfect triangular lattice. Here, we performed systematic studies of low-temperature heat capacity and thermal conductivity on pure, Se-substituted and electron irradiated crystals of 1T-TaS$_2$. In pure 1T-TaS$_2$, the linear temperature term of the heat capacity $γT$ and the finite residual linear term of the thermal conductivity in the zero-temperature limit $κ_{0}/T\equivκ/T(T\rightarrow0)$ are clearly resolved, consistent with the presence of gapless spinons with a Fermi surface. Moreover, while the strong magnetic field slightly enhances $κ_0/T$, it strongly suppresses $γ$. These unusual contrasting responses to magnetic field imply the coexistence of two types of gapless excitations with itinerant and localized characters. Introduction of additional weak random exchange disorder in 1T-Ta(S$_{1-x}$Se$_x$)$_2$ leads to vanishing of $κ_0/T$, indicating that the itinerant gapless excitations are sensitive to the disorder. On the other hand, in both pure and Se-substituted systems, the magnetic contribution of the heat capacity obeys a universal scaling relation, which is consistent with a theory that assumes the presence of localized orphan spins forming random singlets. Electron irradiation in pure 1T-TaS$_2$ largely enhances $γ$ and changes the scaling function dramatically, suggesting a possible new state of spin liquid.

cond-mat.str-el↗

arXiv2020

Front-end-Electronics for the SiPM-readout gaseous TPC for neutrinoless double beta decay search

We have developed a dedicated front-end-electronics board for a high-pressure xenon gas time projection chamber for a neutrinoless double-beta decay search. The ionization signal is readout by detecting electroluminescence photons with SiPM's. The board readout the signal from 56~SiPM's through the DC-coupling and record the waveforms at 5 MS/s with a wide dynamic range up to 7,000 photons/200 ns. The SiPM bias voltages are provided by the board and can be adjusted for each SiPM. In order to calibrate and monitor the SiPM gain, additional auxiliary ADC measures 1 photon-equivalent dark current. The obtained performance satisfies the requirement for a neutrinoless double-beta decay search.

physics.ins-det↗

arXiv2019

AXEL: High-pressure Xe gas TPC for BG-free $0\nu2β$ decay search

AXEL is a high-pressure xenon gas time projection chamber for neutrinoless double-beta decay (0n2b) search. The AXEL has a unique readout system called ELCC which has a cellular structure and photosensors to detect electroluminescence light produced by ionization electrons. We demonstrated the performance of the ELCC with a small prototype detector (AXEL-HP10L). The obtained energy resolution corresponds to 0.82 -- 1.74% (FWHM) at the 0n2b Q-value. We are constructing a new prototype (AXEL-HP180L) in order to study the energy resolution at the Q-value of 0n2b with a new design of ELCC with unit structure, newly developed electronics board, field-shaping electrodes, and Cockcroft-Walton-type high voltage power supply. For a future 1-ton scale large AXEL detector, we are developing new background-reduction techniques; topology identification with machine learning, positive-ion detection, and active-shield options.

physics.ins-det↗

arXiv2019

Mottness versus unit-cell doubling as the driver of the insulating state in 1T-TaS2

If a material with an odd number of electrons per unit cell turns out to be insulating, Mott localisation may be invoked as an explanation. This is widely accepted for the layered compound 1T-TaS2, which has a low-temperature insulating phase comprising charge order clusters with 13 unpaired orbitals each. But if the stacking of layers doubles up the unit cell to include an even number of orbitals, the nature of the insulating state is ambiguous. Here, scanning tunnelling microscopy (STM) reveals two distinct terminations of the charge order in 1T-TaS2, the sign of such a double-layer stacking pattern. However, spectroscopy at both terminations allows us to disentangle unit-cell doubling effects and determine that Mott localisation alone is enough to drive gap formation. We also observe the collapse of Mottness at an extrinsically restacked termination, demonstrating that the microscopic mechanism of insulator-metal transitions lies in degrees of freedom of interlayer stacking.

cond-mat.str-el↗

arXiv2019

51V-NMR study on the S=1/2 square lattice antiferromagnet K2V3O8

Static and dynamic properties of the quasi-two-dimensional antiferromagnet K$_2$V$_3$O$_8$ have been investigated by $^{51}$V-NMR experiments on nonmagnetic V$^{5+}$ sites. Above the structural transition temperature $T_{\rm{S}}$ = 115 K, NMR spectra are fully compatible with the $P4bm$ space group symmetry. The formation of superstructure below $T_{\rm{S}}$ causes splitting of the NMR lines, which get broadened at lower temperatures so that individual peaks are not well resolved. Evolution of NMR spectra with magnetic field along $c$-axis below the magnetic transition temperature $T_{\rm{N}} \sim 4$ K is qualitatively consistent with a simple Néel order and a spin flop transition. However, broad feature of the spectra does not rule out possible incommensurate spin structure. The spin-lattice relaxation rate $1/T_1$ below $T_{\rm{N}}$ shows huge enhancement for a certain range of magnetic field, which is independent of temperature and attributed to cross relaxation due to anomalously large nuclear spin-spin coupling between V$^{5+}$ and magnetic V$^{4+}$ sites. The results indicate strong gapless spin fluctuations, which could arise from incommesurate orders or complex spin textures.

cond-mat.str-el↗