arXiv subjects

S. Hayashida

Publications and source records attributed to S. Hayashida.

At least 19 recordsLinked to original sources

arXiv2026

Zr-based bulk metallic glass clamp cell for high-pressure inelastic neutron scattering

We report the fabrication and characterization of a Zr-based bulk metallic glass (Zr-BMG) clamp cell designed for high-pressure inelastic neutron scattering (INS) measurements. The INS spectra of the empty cell exhibit broad and featureless backgrounds, reflecting the amorphous structure of the Zr-BMG. Test measurements using a reference sample, CsFeCl$_{3}$, confirm that the neutron transmission of the Zr-BMG cell is significantly higher than that of a conventional monobloc CuBe clamp cell. These results demonstrate that the Zr-BMG clamp cell provides both enhanced neutron transparency and a clean background profile, thereby advancing high-pressure INS studies.

cond-mat.mtrl-sci↗

arXiv2024

Magnetic ground state of the dimer-based hexagonal perovskite Ba$_{3}$ZnRu$_{2}$O$_{9}$

We investigate the magnetic ground state of single crystals of the ruthenium-dimer-based hexagonal perovskite Ba$_{3}$ZnRu$_{2}$O$_{9}$ using magnetic susceptibility and resonant inelastic x-ray scattering (RIXS) measurements. While a previous study on powder samples exhibited intriguing magnetic behavior, questions about whether the spin state within a Ru$_{2}$O$_{9}$ dimer is a conventional $S = 3/2$ dimer or an orbital-selective $S = 1$ dimer were raised. The RIXS spectra reveal magnetic excitations from Hund's intraionic multiplet and intradimer spin-triplet transitions. The observed transition energies of the Hund's intraionic multiplets align with the $S=3/2$ ground state, contrasting with the theoretically proposed orbital-selective $S=1$ dimer state. High-temperature magnetic susceptibility analysis confirms the realization of the spin $S=3/2$ dimer state, and the extracted intradimer coupling is consistent with the spin-triplet transition energy observed in the RIXS spectra. These results highlights the ability of "spectroscopic fingerprinting" by RIXS to determine the magnetic ground states of complex materials.

cond-mat.str-el↗

arXiv2024

Braided Ising spin-tube physics in a purported kagome magnet

The magnetic insulator Nd$_3$BWO$_9$ has been previously proposed to realize the highly frustrated breathing kagome lattice model. We report a combination of single-crystal neutron scattering studies and numerical simulations that debunk this interpretation. We show that it is the inter-plane couplings that determine the physics. To explain the exotic magnetism, we derive a simple one-dimensional Ising model composed of twisted triangular spin-tubes, i.e., triple braids of Ising spin chains with almost-orthogonal anisotropy frames and competing ferro-antiferromagnetic interactions. This model can account for the ground state, excitations, the numerous field-induced fractional magnetization plateau phases and incommensurate magnetic correlations at elevated temperatures. Nd$_3$BWO$_9$ constitutes a rare example where rich magnetic phenomena can be understood and simulated quantitatively in terms of a simple classical Hamiltonian.

cond-mat.str-el↗

arXiv2024

Search for Very-Short-Baseline Oscillations of Reactor Antineutrinos with the SoLid Detector

In this letter we report the first scientific result based on antineutrinos emitted from the BR2 reactor at SCK CEN. The SoLid experiment uses a novel type of highly granular detector whose basic detection unit combines two scintillators, PVT and 6LiF:ZnS(Ag), to measure antineutrinos via their inverse-beta-decay products. An advantage of PVT is its highly linear response as a function of deposited particle energy. The full-scale detector comprises 12800 voxels and operates over a very short 6.3--8.9 m baseline from the reactor core. The detector segmentation and its 3D imaging capabilities facilitate the extraction of the positron energy from the rest of the visible energy, allowing the latter to be utilised for signal-background discrimination. We present a result based on 280 reactor-on days (55 MW mean power) and 172 reactor-off days, respectively, of live data-taking. A total of 29479 $\pm$ 603 (stat.) antineutrino candidates have been selected, corresponding to an average rate of 105 events per day and a signal-to-background ratio of 0.27. A search for disappearance of antineutrinos to a sterile state has been conducted using complementary model-dependent frequentist and Bayesian fits, providing constraints on the allowed region of the Reactor Antineutrino Anomaly.

hep-ex↗

arXiv2024

Dielectric relaxation in the quantum multiferroics Rb$_2$Cu$_2$Mo$_3$O$_{12}$ and Cs$_2$Cu$_2$Mo$_3$O$_{12}$

Motivated by the recent discovery of dielectric relaxation by quantum critical magnons in Cs$_2$Cu$_2$Mo$_3$O$_{12}$, we conduct a detailed analysis of its dielectric response and compare it to that in the isostructural compound Rb$_2$Cu$_2$Mo$_3$O$_{12}$. Measurements in the vicinity of the field-induced magnon softening show that its description in terms of 3D Bose-Einstein condensation of magnons quantum critical point is unaltered by the presence of dielectric relaxation. We also demonstrate the existence of dielectric relaxation anomalies at 19 K in Rb$_2$Cu$_2$Mo$_3$O$_{12}$ and discuss the implications for the microscopic origin of dielectric activity in two compounds.

cond-mat.str-el↗

arXiv2024

Investigation of spin excitations and charge order in bulk crystals of the infinite-layer nickelate LaNiO$_2$

Recent x-ray spectroscopic studies have revealed spin excitations and charge density waves in thin films of infinite-layer (IL) nickelates. However, clarifying whether the origin of these phenomena is intrinsic to the material class or attributable to impurity phases in the films has presented a major challenge. Here we utilize topotactic methods to synthesize bulk crystals of the IL nickelate LaNiO$_2$ with crystallographically oriented surfaces. We examine these crystals using resonant inelastic x-ray scattering (RIXS) at the Ni $L_3$-edge to elucidate the spin and charge correlations in the bulk of the material. While we detect the presence of prominent spin excitations in the crystals, fingerprints of charge order are absent at the ordering vectors identified in previous in thin-film studies. These results contribute to the understanding of the bulk properties of LaNiO$_2$ and establish topotactically synthesized crystals as viable complementary specimens for spectroscopic investigations.

cond-mat.str-el↗

arXiv2024

Excitation Spectrum and Spin Hamiltonian of the Frustrated Quantum Ising Magnet Pr$_3$BWO$_9$

We present a thorough experimental investigation on single crystals of the rare-earth based frustrated quantum antiferromagnet Pr$_3$BWO$_9$, a purported spin-liquid candidate on the breathing kagome lattice. This material possesses a disordered ground state with an unusual excitation spectrum involving a coexistence of sharp spin-waves and broad continuum excitations. Nevertheless, we show through a combination of thermodynamic, magnetometric and spectroscopic probes with detailed theoretical modeling that it should be understood in a completely different framework. The crystal field splits the lowest quasi-doublet states into two singlets moderately coupled through frustrated superexchange, resulting in a simple effective Hamiltonian of an Ising model in a transverse magnetic field. While our neutron spectroscopy data do point to significant correlations within the kagome planes, the dominant interactions are out-of-plane, forming frustrated triangular spin-tubes through two competing ferro-antiferromagnetic bonds. The resulting ground state is a simple quantum paramagnet, but with significant modifications to both thermodynamic and dynamic properties due to small perturbations to the transverse field Ising model in the form of hyperfine enhanced nuclear moments and weak structural disorder.

cond-mat.str-el↗

arXiv2023

Magnetic phase diagram of the breathing-kagome antiferromagnet Nd$_3$BWO$_9$

The highly-frustrated rare-earth based magnet Nd$_3$BWO$_9$ is a promising candidate in the search for proximate spin liquid physics. We present a thorough investigation on single crystals of this material using bulk and microscopic techniques. Magnetization data reveal a fractional magnetization plateau for three different investigated field directions. The magnetic phase diagram is mapped out from calorimetric data and exhibits several domes of magnetic order below 0.3 K. Propagation vectors for all ordered phases are presented. The results suggest complex ordering in this material, and unveil the existence of a commensuration transition of the propagation vector at zero magnetic field. A scenario where interplane exchange interactions are essential to a magnetic model of Nd$_3$BWO$_9$ is discussed.

cond-mat.str-el↗

arXiv2022

Search for the Majorana Nature of Neutrinos in the Inverted Mass Ordering Region with KamLAND-Zen

The KamLAND-Zen experiment has provided stringent constraints on the neutrinoless double-beta ($0\nu\beta\beta$) decay half-life in $^{136}$Xe using a xenon-loaded liquid scintillator. We report an improved search using an upgraded detector with almost double the amount of xenon and an ultralow radioactivity container, corresponding to an exposure of 970 kg yr of $^{136}$Xe. These new data provide valuable insight into backgrounds, especially from cosmic muon spallation of xenon, and have required the use of novel background rejection techniques. We obtain a lower limit for the $0\nu\beta\beta$ decay half-life of $T_{1/2}^{0\nu} > 2.3 \times 10^{26}$ yr at 90% C.L., corresponding to upper limits on the effective Majorana neutrino mass of 36-156 meV using commonly adopted nuclear matrix element calculations.

hep-ex↗

arXiv2021

Limits on astrophysical antineutrinos with the KamLAND experiment

We report on a search for electron antineutrinos ($\bar{\nu}_e$) 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 $\bar{\nu}_e$'s, $P_{\nu_e \rightarrow \bar{\nu}_e} < 3.5\times10^{-5}$ (90% CL) assuming an undistorted $\bar{\nu}_e$ shape. This corresponds to a solar $\bar{\nu}_e$ flux of 60 cm$^{-2}$ s$^{-1}$ (90% CL) in the analysis energy range.

astro-ph.HE↗

arXiv2021

Anomalous spin waves in CsFeCl$_{3}$ and RbFeCl$_{3}$

We investigate magnetic excitations in the $S=1$ easy-plan-type triangular antiferromagnets CsFeCl$_{3}$ and RbFeCl$_{3}$ through a combination of inelastic neutron scattering measurements and spin-wave theory calculations based on an established exchange-coupling Hamiltonian. We show that in either material the model falls short of providing an adequate description of the measured intensities and for RbFeCl$_{3}$ even fails to reproduce the measured dispersion relation. The most striking discrepancy is a very anisotropic azimuthal intensity distribution in the acoustic spin-wave branches in the long-wavelength limit, which is incompatible with spin-wave theory on a fundamental level. The observed anomalies are attributed to long-range dipolar interactions.

cond-mat.str-el↗

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

The nylon balloon for xenon loaded liquid scintillator in KamLAND-Zen 800 neutrinoless double-beta decay search experiment

The KamLAND-Zen 800 experiment is searching for the neutrinoless double-beta decay of $^{136}$Xe by using $^{136}$Xe-loaded liquid scintillator. The liquid scintillator is enclosed inside a balloon made of thin, transparent, low-radioactivity film that we call Inner Balloon (IB). The IB, apart from guaranteeing the liquid containment, also allows to minimize the background from cosmogenic muon-spallation products and $^{8}$B solar neutrinos. Indeed these events could contribute to the total counts in the region of interest around the Q-value of the double-beta decay of $^{136}$Xe. In this paper, we present an overview of the IB and describe the various steps of its commissioning minimizing the radioactive contaminations, from the material selection, to the fabrication of the balloon and its installation inside the KamLAND detector. Finally, we show the impact of the IB on the KamLAND background as measured by the KamLAND detector itself.

physics.ins-det↗

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 ($\chi^{0}$) is accompanied by a charged excited state ($\chi^{-}$) separated by a small mass difference, it can form a stable bound state with a nucleus. In supersymmetric models, the $\chi^{0}$ and the $\chi^{-}$ 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 $m_{\chi^{0}} = 100$ GeV and $\Delta m = 10$ MeV, this study sets the most stringent upper limits on the recombination cross section $\langle\sigma v\rangle$ and the decay-width of $\chi^{-}$ of $2.0 \times 10^{-31}$ ${\rm cm^3/s}$ and $1.1 \times 10^{-18}$ GeV, respectively (90\% confidence level).

hep-ex↗

arXiv2021

Inelastic neutron scattering determination of the spin Hamiltonian for BaCdVO(PO$_4$)$_2$

Single crystal inelastic neutron scattering is used to study spin wave excitations in the fully polarized state of the frustrated quantum ferro-antiferromagnet BaCdVO(PO$_4$)$_2$. The data analysis is based on a Heisenberg spin Hamiltonian that includes as many distinct nearest-neighbor and next-nearest neighbor interactions as allowed by crystal symmetry. All 8 such exchange constants are obtained in a simultaneous fit to over 150 scans across the dispersion manifold. This establishes a definitive quantitative model of this material. It turns out to be substantially different from the one assumed in numerous previous studies based on powder experiments.

cond-mat.str-el↗

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

Critical dielectric susceptibility at a magnetic BEC quantum critical point

Magnetic-field-induced phase transitions are investigated in the frustrated gapped quantum paramagnet Rb$_{2}$Cu$_{2}$Mo$_3$O$_{12}$ through dielectric and calorimetric measurements on single-crystal samples. It is clarified that the previously reported dielectric anomaly at 8~K in powder samples is not due to a chiral spin liquid state as has been suggested, but rather to a tiny amount of a ferroelectric impurity phase. Two field-induced quantum phase transitions between paraelectric and paramagnetic and ferroelectric and magnetically ordered states are clearly observed. It is shown that the electric polarization is a secondary order parameter at the lower-field (gap closure) quantum critical point but a primary one at the saturation transition. Having clearly identified the magnetic Bose-Einstein condensation (BEC) nature of the latter, we use the dielectric channel to directly measure the critical divergence of BEC susceptibility. The observed power-law behavior is in very good agreement with theoretical expectations for three-dimensional BEC. Finally, dielectric data reveal magnetic presaturation phases in this compound that may feature exotic order with unconventional broken symmetries.

cond-mat.str-el↗

arXiv2020

Magnetic phase diagram of the linear quantum ferro-antiferromagnet Cs$_{2}$Cu$_{2}$Mo$_{3}$O$_{12}$

A single-crystal sample of the frustrated quasi one-dimensional quantum magnet Cs$_{2}$Cu$_{2}$Mo$_{3}$O$_{12}$ is investigated by magnetic and thermodynamic measurements.A combination of specific heat and magnetic torque measurements maps out the entire $H$-$T$ phase diagram for three orientations.Remarkably, a new phase emerges below the saturation field, irrespective of the crystal orientation. It is suggested that the presaturation phase represents spin-nematic order or other multi-magnon condensate. The phase diagrams within the long-range ordered dome are qualitatively different for each geometry. In particular, multiple transitions are identified in the field along the chain direction.

cond-mat.str-el↗