arXiv subjects

Y. Otsuka

Publications and source records attributed to Y. Otsuka.

12 recordsLinked to original sources

arXiv2026

Unraveling the Generation Mechanism of a Mid-Latitude Plasma Blob and the Evidence of Its Rare Interaction with a MSTID Phase Front

We report observations of two distinct nighttime F-region irregularities, plasma blob (localized density enhancement) and medium-scale traveling ionospheric disturbance (MSTID), in O(1D) 630.0 nm all-sky airglow images from Hanle (32.7{\deg}N, 78.9{\deg}E; Mlat~24.1{\deg}N), Ladakh, India, during the geomagnetically quiet (Ap=6) night of 06 July 2021. Global vertical total electron content (VTEC) maps revealed that the plasma blob developed beyond the southern edge of imager's field-of-view before appearing in airglow images and propagated predominantly westward, as confirmed from both the airglow and VTEC datasets. The existence of the plasma blob and MSTID outside the imager field-of-view was further confirmed by temporal VTEC fluctuations recorded by multiple GNSS receivers. Additionally, FORMOSAT-7/COSMIC-2 signal-to-noise ratio and ICON/MIGHTI wind profiles indicated the presence of sporadic-E (ES) layers at E-region near both the plasma blob and the MSTID. We propose that polarization electric field associated with either MSTID or ES-layers mapped along magnetic field lines to lower latitudes, driving upward plasma transport from F-peak region through vertical uplift of the F-layer. This F-layer uplift was confirmed by simultaneous in-situ O+/H+ density enhancements/reductions at LEO altitudes measured by FORMOSAT-7/COSMIC-2. Upward-transported plasma experienced reduced chemical loss at higher altitudes, producing localized VTEC enhancements (plasma blob). The plasma subsequently diffused along magnetic field lines to higher/lower latitudes/altitudes (~250 km), entering imager's field-of-view, where enhanced dissociative recombination of O2+ produced high intensity airglow region. Interestingly, interaction between the plasma blob and MSTID's plasma-depleted front caused gradual decay and bifurcation of the front due to plasma influx from the high-density blob region.

physics.space-ph↗

arXiv2025

Power-Law Suppression of Phonon Thermal Transport by Magnetic Excitations in a Molecular Quantum Spin Liquid

We present large-scale ab initio phonon calculations for the molecular quantum spin liquid X[Pd(dmit)2]2. An unusually low average phonon velocity ( 700 {m/s}) and optical modes below 10 cm^{-1} confine the Debye T^{3} regime to T < 2 K. As the transfer-integral anisotropy approaches the maximally frustrated regime (t'/t \to 1), the lattice stiffens, ruling out lattice softening as the origin of the spin-liquid state. By quantifying the additional suppression of the thermal conductivity from experimental data, we observe a power-law behavior consistent with two-dimensional magnetic excitations with a nodal, approximately linear (Dirac-like) spectrum.

cond-mat.str-el↗

arXiv2014

Search for bosonic superweakly interacting massive dark matter particles with the XMASS-I detector

Bosonic superweakly interacting massive particles (super-WIMPs) are a candidate for warm dark matter. With the absorption of such a boson by a xenon atom these dark matter candidates would deposit an energy equivalent to their rest mass in the detector. This is the first direct detection experiment exploring the vector super-WIMPs in the mass range between 40 and 120 keV. Using 165.9 days of data no significant excess above background was observed in the fiducial mass of 41 kg. The present limit for the vector super-WIMPs excludes the possibility that such particles constitute all of dark matter. The absence of a signal also provides the most stringent direct constraint on the coupling constant of pseudoscalar super-WIMPs to electrons. The unprecedented sensitivity was achieved exploiting the low background at a level $10^{-4}$ kg$^{-1}$keV$_{ee}^{-1}$day$^{-1}$ in the detector.

astro-ph.CO↗

arXiv2014

Search for inelastic WIMP nucleus scattering on $^{129}$Xe in data from the XMASS-I experiment

A search for inelastic scattering of Weakly Interacting Massive Particles (WIMPs) on the isotope $^{129}$Xe was done in data taken with the single phase liquid xenon detector XMASS at the Kamioka Observatory. Using a restricted volume containing 41 kg of LXe at the very center of our detector we observed no significant excess of events in 165.9 days of data. Our background reduction allowed us to derive our limits without explicitly subtracting the remaining events which are compatible with background expectations and derive for e.g. a 50 GeV WIMP an upper limit for its inelastic cross section on $^{129}$Xe nuclei of 3.2 pb at the 90% confidence level.

astro-ph.CO↗

arXiv2013

Search for solar axions in XMASS, a large liquid-xenon detector

XMASS, a low-background, large liquid-xenon detector, was used to search for solar axions that would be produced by bremsstrahlung and Compton effects in the Sun. With an exposure of 5.6ton days of liquid xenon, the model-independent limit on the coupling for mass $\ll$ 1keV is $|g_{aee}|< 5.4\times 10^{-11}$ (90% C.L.), which is a factor of two stronger than the existing experimental limit. The bounds on the axion masses for the DFSZ and KSVZ axion models are 1.9 and 250eV, respectively. In the mass range of 10-40keV, this study produced the most stringent limit, which is better than that previously derived from astrophysical arguments regarding the Sun to date.

astro-ph.CO↗

arXiv2013

Light WIMP search in XMASS

A search for light dark matter using low-threshold data from the single phase liquid xenon scintillation detector XMASS, has been conducted. Using the entire 835 kg inner volume as target, the analysis threshold can be lowered to 0.3 keVee (electron-equivalent) to search for light dark matter. With low-threshold data corresponding to a 5591.4 kg$\cdot$day exposure of the detector and without discriminating between nuclear-recoil and electronic events, XMASS excludes part of the parameter space favored by other experiments.

astro-ph.CO↗

arXiv2013

XMASS detector

The XMASS project aims to detect dark matter, pp and $^{7}$Be solar neutrinos, and neutrinoless double beta decay using ultra pure liquid xenon. The first phase of the XMASS experiment searches for dark matter. In this paper, we describe the XMASS detector in detail, including its configuration, data acquisition equipment and calibration system.

physics.ins-det↗

arXiv2002

Correlation effects on the Fermi surface of the two-dimensional Hubbard model

Effects of electron correlation on the Fermi surface is investigated for the two-dimensional Hubbard model by the quantum Monte Carlo method. At first, an infinitesimal doping from the half filling is focused on and the momentum dependent charge susceptibility $κ(k)=\frac {dn(k)}{dμ}$ is calculated at a finite temperature. At the temperature $T \sim \frac {t^2} U$, it shows peak structure at $(\pm π/2,\pm π/2)$ on the Fermi surface (line). It is consistent with the mean-field prediction of the d-wave pairing state or the staggerd flux state. This momentum dependent structure disappears at the high temperature $T \approx U$. After summarizing the results of the half filling case, we also discuss the effects of the doping on the momentum dependent charge susceptibility. The anisotropic structure at half filling fades out with sufficient doping.

cond-mat.str-el↗

arXiv2002

Anisotropy on the Fermi Surface of the Two-Dimensional Hubbard Model

We investigate anisotropic charge fluctuations in the two-dimensional Hubbard model at half filling. By the quantum Monte Carlo method, we calculate a momentum-resolved charge compressibility $κ(\bm{k}) = {d < n(\bm{k}) >}/{d μ}$, which shows effects of an infinitesimal doping. At the temperature $T \sim {t^2}/{U}$, $κ(\bm{k})$ shows peak structure at the $(\pm π/2,\pm π/2)$ points along the $|k_x| + |k_y| = π$ line. A similar peak structure is reproduced in the mean-filed calculation for the d-wave pairing state or the staggered flux state.

cond-mat.str-el↗

arXiv2001

Mott Transition in the Two-Dimensional Flux Phase

Effects of the electron-electron interaction in the two-dimensional flux phase are investigated. We treat the half-filled Hubbard model with a magnetic flux $π$ per plaquette by the quantum Monte Carlo method. When the interaction is small, an antiferromagnetic long-range does not exist and the charge fluctuation is different from that of the Mott insulator It suggests that the Mott transition occurs at finite strength of the interaction in the flux phase, which is in contrast to the standard Hubbard model.

cond-mat.str-el↗

arXiv2000

Numerical study of disorder effects on the three-dimensional Hubbard model

Combined effects of interactions and disorder are investigated using a finite temperature quantum Monte Carlo technique for the three-dimensional Hubbard model with random potentials of a finite range. Temperature dependence of the charge compressibility shows that the Mott gap collapses beyond a finite disorder strength. This is a quantum phase transition from an incompressible phase to a compressible phase driven by disorder. We calculate the antiferromagnetic structure factor in the presence of disorder as well. Strong antiferromagnetic correlation, which is characteristic of the Mott insulator, is destroyed by a finite amount of disorder.

cond-mat.str-el↗

arXiv1998

Collapse of Charge Gap in Random Mott Insulators

Effects of randomness on interacting fermionic systems in one dimension are investigated by quantum Monte-Carlo techniques. At first, interacting spinless fermions are studied whose ground state shows charge ordering. Quantum phase transition due to randomness is observed associated with the collapse of the charge ordering. We also treat random Hubbard model focusing on the Mott gap. Although the randomness closes the Mott gap and low-lying states are created, which is observed in the charge compressibility, no (quasi-) Fermi surface singularity is formed. It implies localized nature of the low-lying states.

cond-mat.str-el↗