arXiv subjects

K. Aoyama

Publications and source records attributed to K. Aoyama.

12 recordsLinked to original sources

arXiv2025

Gamma-Ray and AntiMatter Survey(GRAMS) experiment

The Gamma-Ray and AntiMatter Survey (GRAMS) is a next-generation experiment using a Liquid Argon Time Projection Chamber (LArTPC) detector to measure MeV gamma rays and antiparticles. MeV gamma-ray observations are important for understanding multi-messenger and time-domain astronomy, enabling exploration of the universe's most potent events, such as supernovae and neutron star mergers. Despite the significance of MeV gamma-rays, GRAMS could also explore the so-called 'MeV gap' region to improve MeV gamma-ray measurement sensitivity that was restricted by the challenge of accurately reconstructing Compton events. Aside from gamma-ray detection, the GRAMS proposed method also serves as an antiparticle spectrometer, targeting the low-energy range of cosmic antinuclei measurements. This work will provide updates on the current status and progress towards the prototype balloon flight with a small-scale LArTPC (pGRAMS) scheduled for early 2026, as well as the recent progress on antihelium-3 sensitivity calculation.

hep-ex↗

arXiv2025

Antihelium-3 Sensitivity for the GRAMS Experiment

The Gamma-Ray and AntiMatter Survey (GRAMS) is a next-generation balloon/satellite mission utilizing a Liquid Argon Time Projection Chamber (LArTPC) detector to measure both MeV gamma rays and antinuclei produced by dark matter annihilation or decay. The GRAMS can identify antihelium-3 events based on the measurements of X-rays and charged pions from the decay of the exotic atoms, Time of Flight (TOF), energy deposition, and stopping range. This paper shows the antihelium-3 sensitivity estimation using a GEANT4 Monte Carlo simulation. For the proposed long-duration balloon (LDB) flight program (35 days $ \times $ 3 flights) and future satellite mission (2-year observation / 10-year observation), the sensitivities become 1.47 $\times$ 10$^{-7}$ [m$^2$ s sr GeV/n]$^{-1}$ and 1.55 $\times$ 10$^{-9}$ [m$^2$ s sr GeV/n]$^{-1}$ / $3.10\times10^{-10}$ [m$^2$ s sr GeV/n]$^{-1}$, respectively. The results indicate that GRAMS can extensively investigate various dark matter models through the antihelium-3 measurements.

hep-ex↗

arXiv2025

Effects of the three-dimensional interplanar coupling on the centrosymmetric skyrmion crystal formation in the frustrated stacked-triangular Heisenberg model

Effects of the three-dimensional (3D) interplanar coupling on centrosymmetric skyrmion crystal (SkX) formation is investigated via extensive Monte Carlo simulations on the frustrated isotropic Heisenberg model on a stacked-triangular lattice in both cases of the ferromagnetic (F) and the antiferromagnetic (AF) nearest-neighbor interplanar coupling $J_{1c}$. The SkX phase is stabilized at finite fields and at finite temperatures for both F and AF $J_{1c}$, although it is destabilized by modestly weak AF $J_{1c}$. The magnetic phase diagram of the 3D short-range model is more or less similar to those of the 2D short-range model and of the 2D long-range RKKY model. We find that an intriguing phenomenon of replica-symmetry breaking, popular in glass physics and recently identified in the SkX phase of the 3D long-range RKKY model [K. Mitsumoto and H. Kawamura, Phys. Rev. B {\bf 104}, 184432 (2021)], does not arise in the 3D short-range model, suggesting that the long-range nature of interaction might be necessary to realize the RSB in centrosymmetric SkX state.

cond-mat.str-el↗

arXiv2024

First operation of LArTPC in the stratosphere as an engineering GRAMS balloon flight (eGRAMS)

GRAMS (Gamma-Ray and AntiMatter Survey) is a next-generation balloon/satellite experiment utilizing a LArTPC (Liquid Argon Time Projection Chamber), to simultaneously target astrophysical observations of cosmic MeV gamma-rays and conduct an indirect dark matter search using antimatter. While LArTPCs are widely used in particle physics experiments, they have never been operated at balloon altitudes. An engineering balloon flight with a small-scale LArTPC (eGRAMS) was conducted on July 27th, 2023, to establish a system for safely operating a LArTPC at balloon altitudes and to obtain cosmic-ray data from the LArTPC. The flight was launched from the Japan Aerospace Exploration Agency's (JAXA) Taiki Aerospace Research Field in Hokkaido, Japan. The total flight duration was 3 hours and 12 minutes, including a level flight of 44 minutes at a maximum altitude of 28.9 km. The flight system was landed on the sea and successfully recovered. The LArTPC was successfully operated throughout the flight, and about 0.5 million events of the cosmic-ray data including muons, protons, and Compton scattering gamma-ray candidates, were collected. This pioneering flight demonstrates the feasibility of operating a LArTPC in high-altitude environments, paving the way for future GRAMS missions and advancing our capabilities in MeV gamma-ray astronomy and dark matter research.

astro-ph.IM↗

arXiv2023

Signatures of a magnetic superstructure phase induced by ultrahigh magnetic fields in a breathing pyrochlore antiferromagnet

The mutual coupling of spin and lattice degrees of freedom is ubiquitous in magnetic materials and potentially creates exotic magnetic states in response to the external magnetic field. Particularly, geometrically frustrated magnets serve as a fertile playground for realizing magnetic superstructure phases. Here, we observe an unconventional two-step magnetostructural transition prior to a half-magnetization plateau in a breathing pyrochlore chromium spinel by means of state-of-the-art magnetization and magnetostriction measurements in ultrahigh magnetic fields available up to 600 T. Considering a microscopic magnetoelastic theory, the intermediate-field phase can be assigned to a magnetic superstructure with a three-dimensional periodic array of 3-up-1-down and canted 2-up-2-down spin molecules. We attribute the emergence of the magnetic superstructure to a unique combination of the strong spin-lattice coupling and large breathing anisotropy.

cond-mat.mtrl-sci↗

arXiv2022

Observation of topological $Z_2$ vortex fluctuations in the frustrated Heisenberg magnet NaCrO$_2$

Spin fluctuations in the triangular-lattice Heisenberg antiferromagnet NaCrO$_2$ are investigated by means of quasi-elastic neutron scattering with high energy resolution and wide energy band. Two components with the following features are captured separately. They are pronounced at intermediate temperatures of 20-50 K. One with quite an extended lifetime corresponding to $\sim$$0.001E_{\rm ex}$ ($E_{\rm ex}$ the exchange energy) nearly disappears at low temperature 10 K, and the other with an extended lifetime $\sim$$0.01E_{\rm ex}$ survives there, identified as free $Z_2$ vortex and $Z_2$-vortex pair, respectively, in harmony with the $Z_2$ vortex theory.

cond-mat.str-el↗

arXiv2020

Liquid argon scintillation response to electronic recoils between $2.8$--$1275~{\rm keV}$ in a high light yield single-phase detector

We measure the liquid argon scintillation response to electronic recoils in the energy range of $2.82$ to $1274.6~{\rm keV}$ at null electric field. The single-phase detector with a large optical coverage used in this measurement yields $12.8 \pm 0.3 ~ (11.2 \pm 0.3)~{\rm photoelectron/keV}$ for $511.0$-${\rm keV}$ $γ$-ray events based on a photomultiplier tube single photoelectron response modeling with a Gaussian plus an additional exponential term (with only a Gaussian term). It is exposed to a variety of calibration sources such as $^{22}{\rm Na}$ and $^{241}{\rm Am}$ $γ$-ray emitters, and a $^{252}{\rm Cf}$ fast neutron emitter that induces quasimonoenergetic $γ$ rays through a $(n, n'γ)$ reaction with $^{19}{\rm F}$ in polytetrafluoroethylene. In addition, the high light detection efficiency of the detector enables identification of the $2.82$-${\rm keV}$ peak of $^{37}{\rm Ar}$, a cosmogenic isotope in atmospheric argon. The observed light yield and energy resolution of the detector are obtained by the full-absorption peaks. We find up to approximately $25\%$ shift in the scintillation yield across the energy range and $3\%$ of the energy resolution for the $511.0$-${\rm keV}$ line. The Thomas-Imel box model with its constant parameter $ς=0.033 ^{+0.012} _{-0.008}$ is found to explain the result. For liquid argon, this is the first measurement on the energy-dependent scintillation yield down to a few ${\rm keV}$ at null field and provides essential inputs for tuning the argon response model to be used for physics experiments.

physics.ins-det↗

arXiv2020

Spin current as a probe of the $\mathbb{Z}_2$-vortex topological transition in the classical Heisenberg antiferromagnet on the triangular lattice

We have theoretically investigated transport properties of the classical Heisenberg antiferromagnet on the triangular lattice in which a binding-unbinding topological transition of $\mathbb{Z}_2$ vortices is predicted to occur at a finite temperature $T_v$. It is shown by means of the hybrid Monte-Carlo and spin-dynamics simulations that the longitudinal spin-current conductivity exhibits a divergence at $T_v$, while the thermal conductivity only shows a monotonic temperature dependence with no clear anomaly at $T_v$. The significant enhancement of the spin-current conductivity is found to be due to the rapid growth of the spin-current-relaxation time toward $T_v$, which can be understood as a manifestation of the topological nature of the free $\mathbb{Z}_2$ vortex whose lifetime gets longer toward $T_v$. The result suggests that the spin-current measurement is a promising probe to detect the $\mathbb{Z}_2$-vortex topological transition which has remained elusive in experiments.

cond-mat.str-el↗

arXiv2014

Field-induced superconducting phase of FeSe in the BCS-BEC cross-over

Fermi systems in the crossover regime between weakly coupled Bardeen-Cooper-Schrieffer (BCS) and strongly coupled Bose-Einstein-condensate (BEC) limits are among the most fascinating objects to study the behavior of an assembly of strongly interacting particles. The physics of this crossover has been of considerable interest both in the fields of condensed matter and ultracold atoms. One of the most challenging issue in this regime is the effect of large spin imbalance on a Fermi system under magnetic fields. Although several exotic physical properties have been predicted theoretically, the experimental realization of such an unusual superconducting state has not been achieved so far. Here we show that pure single crystals of superconducting FeSe offer the possibility to enter the previously unexplored realm where the three energies, Fermi energy $\varepsilon_{\rm F}$, superconducting gap $Δ$ and Zeeman energy, become comparable. Through the superfluid response, transport, thermoelectric response, and spectroscopic-imaging scanning tunneling microscopy, we demonstrate that $\varepsilon_{\rm F}$ of FeSe is extremely small, with the ratio $Δ/\varepsilon_{\rm F}\sim1 (\sim0.3)$ in the electron (hole) band. Moreover, thermal-conductivity measurements give evidence of a distinct phase line below the upper critical field, where the Zeeman energy becomes comparable to $\varepsilon_{\rm F}$ and $Δ$. The observation of this field-induced phase provides insights into previously poorly understood aspects of the highly spin-polarized Fermi liquid in the BCS-BEC crossover regime.

cond-mat.supr-con↗

arXiv2013

Orbital order and Hund's rule frustration in Kondo lattices

We analyze a microscopic origin of the Kondo effect-assisted orbital order in heavy-fermion materials. By studying the periodic two-orbital Anderson model with two local electrons, we show that frustration of Hund's rule coupling due to the Kondo effect leads to an incommensurate spiral orbital and magnetic order, which exists only inside the Kondo screened (heavy-electron) phase. This spiral state can be observed in neutron and resonant X-ray scattering measurements in ${\rm U}$- and ${\rm Pr}$-based heavy-fermion compounds, and realized in cold atomic gases, e.g. fermionic ${}^{173}{\rm Yb}$.

cond-mat.str-el↗

arXiv2013

Field-induced reentrant superconductivity in thin films of nodal superconductor

Previous works on nodal d-wave superconductors have shown that a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) like modulated superconducting (SC) state can be realized with no magnetic field when quasiparticles acquire an additional linear term in the wavevector in their dispersion. In the present work, stability of such a novel modulated SC state in an artificial film against an applied magnetic field is studied. As a reflection of the presence of the two different FFLO states, one close to zero field and the other at the high field end, in a single field v.s. temperature phase diagram of thin films, the conventional uniform SC state generally tends to appear as a reentrant ordered phase bounded by the normal phase in {\it lower} fields.

cond-mat.supr-con↗

arXiv2013

Inhomogeneous Superconducting States of Mesoscopic Thin-Walled Cylinders in External Magnetic Fields

We theoretically investigate the appearance of spatially modulated superconducting states in mesoscopic superconducting thin-wall cylinders in a magnetic field at low temperatures. Quantization of the electron motion around the circumference of the cylinder leads to a discontinuous evolution of the spatial modulation of the superconducting order parameter along the transition line T_c(H). We show that this discontinuity leads to the non-monotonic behavior of the specific heat jump at the onset of superconductivity as a function of temperature and field. We argue that this geometry provides an excellent opportunity to directly and unambiguously detect distinctive signatures of the Fulde-Ferrell-Larkin-Ovchinnikov modulation of the superconducting order.

cond-mat.supr-con↗