arXiv subjects

Yasuhiro Shimizu

Publications and source records attributed to Yasuhiro Shimizu.

At least 37 records · Page 2Linked to original sources

arXiv2021

Local Observations of Orbital Diamagnetism and Excitation in Three-Dimensional Dirac Fermion Systems Bi$_{1-x}$Sb$_x$

Dirac fermions display a singular response against magnetic and electric fields. A distinct manifestation is large diamagnetism originating in the interband effect of Bloch bands, as observed in bismuth alloys. Through $^{209}$Bi NMR spectroscopy, we extract diamagnetic orbital susceptibility inherent to Dirac fermions in the semiconducting bismuth alloys Bi$_{1-x}$Sb$_x$ ($x = 0.08 - 0.16$). The $^{209}$Bi hyperfine coupling constant provides an estimate of the effective orbital radius. In addition to the interband diamagnetism, Knight shift includes an anomalous temperature-independent term originating in the enhanced intraband diamagnetism under strong spin-orbit coupling. The nuclear spin-lattice relaxation rate $1/T_1$ is dominated by orbital excitation and follows cubic temperature dependence in the extensive temperature range. The result demonstrates the robust diamagnetism and low-lying orbital excitation against the small gap opening, whereas $x$-dependent spin excitation appears at low temperatures.

cond-mat.str-el↗

arXiv2020

Muon and Electron $g-2$ and the Origin of Fermion Mass Hierarchy

We present a model that gives a natural explanation to the charged lepton mass hierarchy and study the contributions to the electron and the muon $g-2$. In the model, we introduce lepton-flavor-dependent $U(1)_F$ symmetry and three additional Higgs doublets with $U(1)_F$ charges, to realize that each generation of charged leptons couples to one of the three additional Higgs doublets. The $U(1)_F$ symmetry is softly broken by $+1$ charges, and the smallness of the soft breaking naturally gives rise to the hierarchy of the Higgs VEVs, which then accounts for the charged lepton mass hierarchy. Since electron and muon couple to different scalar particles, each scalar contributes to the electron and the muon $g-2$ differently. We survey the space of parameters of the Higgs sector and find that there are sets of parameters that explain the muon $g-2$ discrepancy. On the other hand, we cannot find the parameter sets that can explain $g-2$ discrepancy within 2$σ$. Here the $U(1)_F$ symmetry suppresses charged lepton flavor violation.

hep-ph↗

arXiv2020

Occupation switching of $d$ orbitals probed via hyperfine interactions in vanadium dioxide

Metal-insulator transition was microscopically investigated by orbital-resolved nuclear magnetic resonance (OR-NMR) spectroscopy in a single crystal of vanadium dioxide VO$_2$. Observations of the anisotropic $^{51}$V Knight shift and the nuclear quadrupole frequency allow us to evaluate orbital-dependent spin susceptibility and $d$ orbital occupations. The result is consistent with the degenerated $t_{2g}$ orbitals in a correlated metallic phase and the $d$ orbital ordering in a nonmagnetic insulating phase. The predominant orbital pointing along the chain facilitates a spin-singlet formation triggering metal-insulator transition. The asymmetry of magnetic and electric hyperfine tensors suggests the $d$ orbital reformation favored by a low-symmetry crystal field, forming a localized molecular orbital. The result highlights the cooperative electron correlation and electron-phonon coupling in Mott transition with orbital degrees of freedom.

cond-mat.str-el↗

arXiv2019

A molecular diamond lattice antiferromagnet as a Dirac semimetal candidate

The ground state of a molecular diamond-lattice compound (ET)Ag$_4$(CN)$_5$ is investigated by the magnetization and nuclear magnetic resonance spectroscopy. We found that the system exhibits antiferromagnetic long-range ordering with weak ferromagnetism at a high temperature of 102 K owing to the strong electron correlation. The spin susceptibility is well fitted into the diamond-lattice Heisenberg model with a nearest neighbor exchange coupling of 230 K, indicating the less frustrated interactions. The transition temperature elevates up to $\sim$195 K by applying pressure of 2 GPa, which records the highest temperature among organic molecular magnets. The first-principles band calculation suggests that the system is accessible to a three-dimensional topological semimetal with nodal Dirac lines, which has been extensively searched for a half-filling diamond lattice.

cond-mat.str-el↗

arXiv2018

Physical properties of near-Earth asteroids with a low delta-${\it v}$: Survey of target candidates for the Hayabusa2 mission

Sample return from the near-Earth asteroid known as 25143 Itokawa was conducted as part of the Hayabusa mission, with a large number of scientific findings being derived from the returned samples. Following the Hayabusa mission, Hayabusa2 was planned, targeting sample return from a primitive asteroid. The primary target body of Hayabusa2 was asteroid 162173 Ryugu; however, it was also necessary to gather physical information for backup target selection. Therefore, we examined five asteroids spectroscopically, 43 asteroids spectrophotometrically, and 41 asteroids through periodic analysis. Hence, the physical properties of 74 near-Earth asteroids were obtained, which helped the Hayabusa2 backup target search and, also, furthered understanding of the physical properties of individual asteroids and their origins.

astro-ph.EP↗

arXiv2018

Log-aesthetic Curves as Similarity Geometric Analogue of Euler's Elasticae

In this paper we consider the log-aesthetic curves and their generalization which are used in CAGD. We consider those curves under similarity geometry and characterize them as stationary integrable flow on plane curves which is governed by the Burgers equation. We propose a variational formulation of those curves whose Euler-Lagrange equation yields the stationary Burgers equation. Our result suggests that the log-aesthetic curves and their generalization can be regarded as the similarity geometric analogue of Euler's elasticae.

nlin.SI↗

arXiv2017

Symmetry Preservation and Critical Fluctuations in a Pseudospin Crossover Perovskite LaCoO$_3$

Spin-state crossover beyond a conventional ligand-field theory has been a fundamental issue in condensed matter physics. Here, we report microscopic observations of spin states and low-energy dynamics through orbital-resolved NMR spectroscopy in the prototype compound LaCoO$_3$. The $^{59}$Co NMR spectrum shows the preserved crystal symmetry across the crossover, inconsistent with $d$ orbital ordering due to the Jahn-Teller distortion. The orbital degeneracy results in a pseudospin ($\tilde{J} = 1$) excited state with an orbital moment observed as $^{59}$Co hyperfine coupling tensors. We found that the population of the excited state evolves above the heart crossover temperature. The crossover involves critical spin-state fluctuations emerging under the magnetic field. These results suggest that the spin-state crossover can be mapped into a statistical problem, analogous to the supercritical liquid in liquid-gas transition.

cond-mat.str-el↗

arXiv2016

Pressure-Tuned Exchange Coupling of a Quantum Spin Liquid in the Molecular Triangular Lattice $κ$-(ET)$_2$Ag$_2$(CN)$_3$

The effects of pressure on a quantum spin liquid are investigated in an organic Mott insulator $κ$-(ET)$_2$Ag$_2$(CN)$_3$ with a spin-1/2 triangular lattice. The application of negative chemical pressure to $κ$-(ET)$_2$Cu$_2$(CN)$_3$, which is a well-known sister Mott insulator, allows for extensive tuning of antiferromagnetic exchange coupling, with $J/k_{\rm B} = 175 - 310$ K, under hydrostatic pressure. Based on $^{13}$C nuclear magnetic resonance measurements under pressure, we uncover universal scaling in the static and dynamic spin susceptibilities down to low temperatures $\sim 0.1k_{\rm B}T/J$. The persistent fluctuations and residual specific heat coefficient are consistent with the presence of gapless low-lying excitations. Our results thus demonstrate fundamental finite-temperature properties of quantum spin liquid in a wide parameter range.

cond-mat.str-el↗

arXiv2016

Natural Split Mechanism for Sfermions: $N$=2 Supersymmetry in Phenomenology

We suggest a natural split mechanism for sfermions based on $N$=2 supersymmetry (SUSY). $N$=2 SUSY protects a sfermion in an $N$=2 multiplet from gaining weight by SUSY breaking. Therefore, if partly $N$=2 SUSY is effectively obtained, a split spectrum can be realized naturally. As an example of the natural split mechanism, we build a gauge-mediated SUSY breaking-like model assuming $N$=2 SUSY is partly broken in a UV theory. The model explains the Higgs boson mass and the muon anomalous magnetic dipole moment within $1~σ$ level with a splitting sfermion spectrum. The model has seven light sparticles described by three free parameters and predicts a new chiral multiplet, sb: the $N$=2 partner of the ${\rm U(1)_Y}$ vector multiplet. The bini, the fermion component of the sb, weighs MeVs. We mention the experimental and the cosmological aspects of the model.

hep-ph↗

arXiv2015

Site-Selective Mott Transition in a Quasi-One-Dimensional Vanadate V6O13

The microscopic mechanism of the metal-insulator transition is studied by orbital-resolved 51V NMR spectroscopy in a prototype of the quasi-one-dimensional system V6O13. We uncover that the transition involves a site-selective d orbital order lifting twofold orbital degeneracy in one of the two VO6 chains. The other chain leaves paramagnetic moments on the singly occupied dxy orbital across the transition. The two chains respectively stabilize an orbital-assisted spin-Peierls state and an antiferromagnetic long-range order in the ground state. The site-selective Mott transition may be a source of the anomalous metal and the Mott-Peierls duality.

cond-mat.str-el↗

arXiv2015

Search for new physics via photon polarization of $b \rightarrow s γ$

We suggest a discriminant analysis of new physics beyond the standard model through a detection of photon polarization in a radiative B meson decay. This analysis is investigated in SUSY SU(5) GUT with right-handed neutrino and left-right symmetric models. New physics search via CP asymmetry in the same process are also evaluated in each model for comparison. We show that new physics can be found via detecting the photon polarization in a parameter space of TeV energy scale.

hep-ph↗

arXiv2015

Five New Exoplanets Orbiting Three Metal-Rich, Massive Stars: Two-Planet Systems Including Long-Period Planets, and an Eccentric Planet

We report detections of new exoplanets from a radial velocity (RV) survey of metal-rich FGK stars by using three telescopes. By optimizing our RV analysis method to long time-baseline observations, we have succeeded in detecting five new Jovian-planets around three metal-rich stars HD 1605, HD 1666, and HD 67087 with the masses of $1.3 M_{\odot}$, $1.5 M_{\odot}$, and $1.4 M_{\odot}$, respectively. A K1 subgiant star HD 1605 hosts two planetary companions with the minimum masses of $ M_p \sin i = 0.96 M_{\mathrm{JUP}}$ and $3.5 M_{\mathrm{JUP}}$ in circular orbits with the planets' periods $P = 577.9$ days and $2111$ days, respectively. HD 1605 shows a significant linear trend in RVs. Such a system consisting of Jovian planets in circular orbits has rarely been found and thus HD 1605 should be an important example of a multi-planetary system that is likely unperturbed by planet-planet interactions. HD 1666 is a F7 main sequence star which hosts an eccentric and massive planet of $ M_p \sin i = 6.4 M_{\mathrm{JUP}}$ in the orbit with $a_{\rm p} = 0.94$ AU and an eccentricity $e=0.63$. Such an eccentric and massive planet can be explained as a result of planet-planet interactions among Jovian planets. While we have found the large residuals of $\mathrm{rms} = 35.6\ \mathrm{m\ s^{-1}}$, the periodogram analysis does not support any additional periodicities. Finally, HD 67087 hosts two planets of $ M_p \sin i = 3.1 M_{\mathrm{JUP}}$ and $4.9 M_{\mathrm{JUP}}$ in orbits with $P=352.2$ days and $2374$ days, and $e=0.17$ and $0.76$, respectively. Although the current RVs do not lead to accurate determinations of its orbit and mass, HD 67087 c can be one of the most eccentric planets ever discovered in multiple systems.

astro-ph.EP↗

arXiv2015

Weight function method for precise determination of top quark mass at Large Hadron Collider

We propose a new method to measure a theoretically well-defined top quark mass at the LHC. This method is based on the "weight function method," which we proposed in our preceding paper. It requires only lepton energy distribution and is basically independent of the production process of the top quark. We perform a simulation analysis of the top quark mass reconstruction with $t\bar{t}$ pair production and lepton+jets decay channel at the leading order. The estimated statistical error of the top quark mass is about $0.4$ GeV with an integrated luminosity of $100$ fb$^{-1}$ at $\sqrt{s}=14$ TeV. We also estimate some of the major systematic uncertainties and find that they are under good control.

hep-ph↗

arXiv2014

Multi-band, Multi-epoch Observations of the Transiting Warm Jupiter WASP-80b

WASP-80b is a warm Jupiter transiting a bright late-K/early-M dwarf, providing a good opportunity to extend the atmospheric study of hot Jupiters toward the lower temperature regime. We report multi-band, multi-epoch transit observations of WASP-80b by using three ground-based telescopes covering from optical (g', Rc, and Ic bands) to near-infrared (NIR; J, H, and Ks bands) wavelengths. We observe 5 primary transits, each of which in 3 or 4 different bands simultaneously, obtaining 17 independent transit light curves. Combining them with results from previous works, we find that the observed transmission spectrum is largely consistent with both a solar abundance and thick cloud atmospheric models at 1.7$σ$ discrepancy level. On the other hand, we find a marginal spectral rise in optical region compared to the NIR region at 2.9$σ$ level, which possibly indicates the existence of haze in the atmosphere. We simulate theoretical transmission spectra for a solar abundance but hazy atmosphere, finding that a model with equilibrium temperature of 600 K can explain the observed data well, having a discrepancy level of 1.0$σ$. We also search for transit timing variations, but find no timing excess larger than 50 s from a linear ephemeris. In addition, we conduct 43 day long photometric monitoring of the host star in the optical bands, finding no significant variation in the stellar brightness. Combined with the fact that no spot-crossing event is observed in the five transits, our results confirm previous findings that the host star appears quiet for spot activities, despite the indications of strong chromospheric activities.

astro-ph.EP↗

arXiv2014

Spin disorder in an Ising honeycomb chain cobaltate

We report on a member of the spin-disordered honeycomb lattice antiferromagnet in a quasi-one-dimensional cobaltate Ba_3Co_2O_6(CO_3)_0.7. Resistivity exhibits as semimetallic along the face-sharing CoO6 chains. Magnetic susceptibility shows strongly anisotropic Ising-spin character with the easy axis along the chain due to significant spin-orbit coupling and a trigonal crystal field. Nevertheless, ^135Ba NMR detects no indication of the long-range magnetic order down to 0.48 K. Marginally itinerant electrons possess large entropy and low-lying excitations with a Wilson ratio R_W = 116, which highlight interplays of charge, spin, and orbital in the disordered ground state.

cond-mat.str-el↗

arXiv2013

Lightcurve Survey of V-type Asteroids in the Inner Asteroid Belt

We have observed the lightcurves of 13 V-type asteroids ((1933) Tinchen, (2011) Veteraniya, (2508) Alupka, (3657) Ermolova, (3900) Knezevic, (4005) Dyagilev, (4383) Suruga, (4434) Nikulin, (4796) Lewis, (6331) 1992 $\mathrm{FZ_{1}}$, (8645) 1998 TN, (10285) Renemichelsen, and (10320) Reiland). Using these observations we determined the rotational rates of the asteroids, with the exception of Nikulin and Renemichelsen. The distribution of rotational rates of 59 V-type asteroids in the inner main belt, including 29 members of the Vesta family that are regarded as ejecta from the asteroid (4) Vesta, is inconsistent with the best-fit Maxwellian distribution. This inconsistency may be due to the effect of thermal radiation Yarkovsky--O'Keefe--Radzievskii--Paddack (YORP) torques, and implies that the collision event that formed V-type asteroids is sub-billion to several billion years in age.

astro-ph.EP↗

arXiv2013

Optical-to-Near-Infrared Simultaneous Observations for the Hot Uranus GJ3470b: A Hint for Cloud-free Atmosphere

We present optical (g', R_c, and I_c) to near-infrared (J) simultaneous photometric observations for a primary transit of GJ3470b, a Uranus-mass transiting planet around a nearby M dwarf, by using the 50-cm MITSuME telescope and the 188-cm telescope, both at Okayama Astrophysical Observatory. From these data, we derive the planetary mass, radius, and density as 14.1 \pm 1.3 M_earth, 4.32^{+0.21}_{-0.10} R_earth, and 0.94 \pm 0.12 g cm^{-3}, respectively, thus confirming the low density that was reported by Demory et al. based on the Spitzer/IRAC 4.5-micron photometry (0.72^{+0.13}_{-0.12} g cm^{-3}). Although the planetary radius is about 10% smaller than that reported by Demory et al., this difference does not alter their conclusion that the planet possesses a hydrogen-rich envelope whose mass is approximately 10% of the planetary total mass. On the other hand, we find that the planet-to-star radius ratio (R_p/R_s) in the J band (0.07577^{+0.00072}_{-0.00075}) is smaller than that in the I_c (0.0802 \pm 0.0013) and 4.5-micron (0.07806^{+0.00052}_{-0.00054}) bands by 5.9% \pm 2.0% and 3.0% \pm 1.2%, respectively. A plausible explanation for the differences is that the planetary atmospheric opacity varies with wavelength due to absorption and/or scattering by atmospheric molecules. Although the significance of the observed R_p/R_s variations is low, if confirmed, this fact would suggest that GJ3470b does not have a thick cloud layer in the atmosphere. This property would offer a wealth of opportunity for future transmission-spectroscopic observations of this planet to search for certain molecular features, such as H2O, CH4, and CO, without being prevented by clouds.

astro-ph.EP↗

arXiv2013

Orbital reformation with vanadium trimerization in $d^2$ triangular lattice LiVO$_2$ revealed by $^{51}$V NMR

LiVO$_2$ is a model system of the valence bond solid (VBS) in $3d^2$ triangular lattice. The origin of the VBS formation has remained controversial. We investigate the microscopic mechanism by elucidating the $d$ orbital character via on-site $^{51}$V NMR measurements in a single crystal up to 550 K across a structural transition temperature $T_c$. The Knight shift, $K$, and nuclear quadrupole frequency, $δν$, show that the 3d orbital with local trigonal symmetry are reconstructed into a $d_{yz}d_{zx}$ orbital order below $T_c$. Together with the NMR spectra with three-fold rotational symmetry, we confirm a vanadium trimerization with $d$-$d$ $σ$ bonds. The Knight shift extracts the large Van-Vleck orbital susceptibility, $χ^{\rm VV} = 3.6 \times 10^{-4}$, in a paramagnetic state above $T_c$, which is comparable to the spin susceptibility. The results suggest that orbitally induced Peierls transition in the proximity of the frustrated itinerant state is the dominant driving force of the trimerization transition.

cond-mat.str-el↗