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T. Sato

Publications and source records attributed to T. Sato.

At least 37 records · Page 2Linked to original sources
arXiv2023

180$^\circ$-twisted bilayer ReSe$_2$ as an artificial noncentrosymmetric semiconductor

We have fabricated a 180$^\circ$-twisted bilayer ReSe$_2$ by stacking two centrosymmetric monolayer ReSe$_2$ flakes in opposite directions, which is expected to lose spatial inversion symmetry. By the second harmonic generation and angle-resolved photoemission spectroscopy, we successfully observed spatial inversion symmetry breaking and emergent band dispersions. The band calculation shows the finite lifting of spin degeneracy (~50 meV) distinct from natural monolayer and bilayer ReSe$_2$. Our results demonstrate that the spin-momentum locked state, which leads to spintronic functions and Berry-curvature-related phenomena, can be realized even with the stacking of centrosymmetric monolayers.

cond-mat.mes-hall↗
arXiv2023

Gyromagnetic bifurcation in a levitated ferromagnetic particle

We examine the mechanical rotation of a levitated magnetic particle that is induced by ferromagnetic resonance under microwave irradiation. We show that two stable solutions appear in a certain range of parameters by bifurcation when the rotation frequency is comparable to the microwave frequency. This phenomenon originates from the coexistence of the Barnett and the Einstein-de Haas effects. We also reveal that this measurement is sensitive to the strength of the spin-rotation coupling. Our work provides a platform for accessing a microscopic relaxation process from spin to macroscopic rotation.

cond-mat.mes-hall↗
arXiv2023

Quantum fluctuation in rotation velocity of a levitated magnetic particle

We consider a ferromagnetic particle levitated in air under microwave irradiation and theoretically study the noise in its rigid-body rotation induced by the gyromagnetic effect. This rotational noise includes useful information on angular momentum transfer from the magnetization to the rigid-body rotation, such as the unit of angular momentum per one spin relaxation process. We formulate the rotational noise in terms of the Lindblad equation, which describes the quantum stochastic process, and estimate it in the case of realistic experimental parameters. We show that a bifurcation phenomenon observed in our setup amplifies the noise and, therefore, can be exploited making an accurate measurement of the rotational noise.

cond-mat.mes-hall↗
arXiv2023

Pure nematic state in iron-based superconductor

Lattice and electronic states of thin FeSe films on LaAlO$_3$ substrates are investigated in the vicinity of the nematic phase transition. No evidence of structural phase transition is found by x-ray diffraction below $T^\ast \sim 90$ K, while results obtained from resistivity measurement and angle-resolved photoemission spectroscopy clearly show the appearance of a nematic state. These results indicate formation of a pure nematic state in the iron-based superconductor and provide conclusive evidence that the nematic state originates from the electronic degrees of freedom. This pure nematicity in the thin film implies difference in the electron-lattice interaction from bulk FeSe crystals. FeSe films provide valuable playgrounds for observing the pure response of "bare" electron systems free from the electron-lattice interaction, and should make important contribution to investigate nematicity and its relationship with superconductivity.

cond-mat.supr-con↗
arXiv2023

Unusual band evolution and persistence of topological surface states in high-T_C magnetic topological insulator

Understanding the mechanism of ferromagnetism in ferromagnetic topological insulators (TIs) is a key to realize exotic time-reversal-symmetry-broken quantum phases. However, electronic states relevant to the ferromagnetism are highly controversial. Here we report angle-resolved photoemission spectroscopy on (CrxSb1-x)2Te3 thin films, high-Curie-temperature (T_C) ferromagnetic TIs, spanning the non-doped (T_C=0 K) to highly-doped (T_C=192 K) region. We found that, upon Cr doping to Sb2Te3, the bulk valence-band valley exhibits filling-in behavior while retaining band inversion, leading to the formation of a nearly-flat band in high-T_C regime and evolution from a six-petal flower to a Star-of-David Fermi surface. Despite the weakening of spin-orbit coupling with Cr doping, the Dirac-cone state persists up to the highest-T_C sample, and shows a clear magnetic-gap opening below TC accompanied with an unexpected band shift, signifying its strong coupling with spontaneous ferromagnetism. The present result lays the foundation for understanding the interplay between band topology and ferromagnetism in TIs.

cond-mat.mes-hall↗
arXiv2023

Electron wave functions in beta-decay formulas revisited (II): Completion including recoil-order and induced currents

We present complete formulas of the allowed and first-forbidden transitions of the nuclear beta decay taking into account the recoil-order and induced currents up to the next-to-leading order (NLO). The longitudinal part of the vector current is cleared away by the use of the conservation of the vector current for the multipole operators of the natural-parity transitions, which makes the effect of the meson exchange current for the vector current as small as possible. The formula is transparent enough to be applied to various beta-decay processes. As a numerical demonstration, we apply our formulas to the beta decay of a neutron-rich nucleus $^{160}$Sn. We find that the NLO corrections amount to 10--20\% of the total decay rate, whereas the induced currents alter the rate at most 5\%.

nucl-th↗
arXiv2023

Search for the Pair Production of Dark Particles $X$ with $K_L^0 \to XX$, $X \to γγ$

We present the first search for the pair production of dark particles $X$ via $K_L^0\to XX$ with $X$ decaying into two photons using the data collected by the KOTO experiment. No signal was observed in the mass range of 40 - 110 MeV/c$^2$ and 210 - 240 MeV/c$^2$. This sets upper limits on the branching fractions as $\mathcal{B}(K_L^0 \to XX)$ $<$ (1-4) $\times$ 10$^{-7}$ and $\mathcal{B}(K_L^0 \to XX)$ $<$ (1-2) $\times$ 10$^{-6}$ at the 90% confidence level for the two mass regions, respectively.

hep-ex↗
arXiv2022

Theoretical tools for neutrino scattering: interplay between lattice QCD, EFTs, nuclear physics, phenomenology, and neutrino event generators

Maximizing the discovery potential of increasingly precise neutrino experiments will require an improved theoretical understanding of neutrino-nucleus cross sections over a wide range of energies. Low-energy interactions are needed to reconstruct the energies of astrophysical neutrinos from supernovae bursts and search for new physics using increasingly precise measurement of coherent elastic neutrino scattering. Higher-energy interactions involve a variety of reaction mechanisms including quasi-elastic scattering, resonance production, and deep inelastic scattering that must all be included to reliably predict cross sections for energies relevant to DUNE and other accelerator neutrino experiments. This white paper discusses the theoretical status, challenges, required resources, and path forward for achieving precise predictions of neutrino-nucleus scattering and emphasizes the need for a coordinated theoretical effort involved lattice QCD, nuclear effective theories, phenomenological models of the transition region, and event generators.

hep-ph↗
arXiv2022

Rhombic Fermi surfaces in a ferromagnetic MnGa thin film with perpendicular magnetic anisotropy

Mn$_{1-x}$Ga$_x$ (MnGa) with the $L1_0$ structure is a ferromagnetic material with strong perpendicular magneto-crystalline anisotropy. Although MnGa thin films have been successfully grown epitaxially and studied for various spintronics devices, fundamental understandings of its electronic structure are still lacking. To address this issue, we have investigated $L1_0$-MnGa thin films using angle-resolved photoemission spectroscopy (ARPES). We have observed a large Fermi surface with a rhombic shape in the $k_x$-$k_y$ plane overlapping neighboring Fermi surfaces. The $k_z$ dependence of the band structure suggests that the band dispersion observed by ARPES comes from the three-dimensional band structure of MnGa folded by a $\sqrt{2} \times \sqrt{2}$ reconstruction. The band dispersion across the corner of the rhombic Fermi surface forms an electron pocket with a weak $k_z$ dependence. The effective mass and the mobility of the bands crossing the Fermi level near the corner are estimated from the ARPES images. Based on the experimental findings, the relationship between the observed band structure and the spin-dependent properties in MnGa-based heterostructures is discussed.

cond-mat.mtrl-sci↗
arXiv2022

A new Standard DNA damage (SDD) data format

Our understanding of radiation induced cellular damage has greatly improved over the past decades. Despite this progress, there are still many obstacles to fully understanding how radiation interacts with biologically relevant cellular components to form observable endpoints. One hurdle is the difficulty faced by members of different research groups in directly comparing results. Multiple Monte Carlo codes have been developed to simulate damage induction at the DNA scale, while at the same time various groups have developed models that describe DNA repair processes with varying levels of detail. These repair models are intrinsically linked to the damage model employed in their development, making it difficult to disentangle systematic effects in either part of the modelling chain. The modelling chain typically consists of track structure Monte Carlo simulations of the physics interactions creating direct damages to the DNA; followed by simulations of the production and initial reactions of chemical species causing indirect damages. After the DNA damage induction, DNA repair models combine the simulated damage patterns with biological models to determine the biological consequences of the damage. We propose a new Standard data format for DNA Damage to unify the interface between the simulation of damage induction and the biological modelling of cell repair processes. Such a standard greatly facilitates inter model comparisons, providing an ideal environment to tease out model assumptions and identify persistent, underlying mechanisms. Through inter model comparisons, this unified standard has the potential to greatly advance our understanding of the underlying mechanisms of radiation induced DNA damage and the resulting observable biological effects.

q-bio.OT↗
arXiv2022

Integrated ICN and CDN Slice as a Service

In this article, we leverage Network Function Virtualization (NFV) and Multi-Access Edge Computing (MEC) technologies, proposing a system which integrates ICN (Information-Centric Network) with CDN (Content Delivery Network) to provide an efficient content delivery service. The proposed system combines the dynamic CDN slicing concept with the NDN (Named Data Network) based ICN slicing concept to avoid core network congestion. A dynamic CDN slice is deployed to cache content at optimal locations depending on the nature of the content and the geographical distributions of potential viewers. Virtual cache servers, along with supporting virtual transcoders, are placed across a cloud belonging to multiple-administrative domains, forming a CDN slice. The ICN slice is, in turn, used for the regional distribution of content, leveraging the name-based access and the autonomic in-network content caching. This enables the delivery of content from nearby network nodes, avoiding the duplicate transfer of content and also ensuring shorter response times. Our experiments demonstrate that integrated ICN/CDN is better than traditional CDN in almost all aspects, including service scalability, reliability, and quality of service.

cs.NI↗
arXiv2021

Respiratory Rate Estimation Based on WiFi Frame Capture

This paper presents a method that estimates the respiratory rate based on the frame capturing of wireless local area networks. The method uses beamforming feedback matrices (BFMs) contained in the captured frames, which is a rotation matrix of channel state information (CSI). BFMs are transmitted unencrypted and easily obtained using frame capturing, requiring no specific firmware or WiFi chipsets, unlike the methods that use CSI. Such properties of BFMs allow us to apply frame capturing to various sensing tasks, e.g., vital sensing. In the proposed method, principal component analysis is applied to BFMs to isolate the effect of the chest movement of the subject, and then, discrete Fourier transform is performed to extract respiratory rates in a frequency domain. Experimental evaluation results confirm that the frame-capture-based respiratory rate estimation can achieve estimation error lower than 3.2 breaths/minute.

eess.SP↗
arXiv2021

Photo-induced plasmon-phonon coupling in PbTe

We report the observation of photo-induced plasmon-phonon coupled modes in the group IV-VI semiconductor PbTe using Fourier-transform inelastic X-ray scattering at the Linac Coherent Light Source (LCLS). We measure the near-zone-center dispersion of the heavily screened longitudinal optical (LO) phonon branch as extracted from differential changes in x-ray diffuse scattering intensity following above band gap photoexcitation.

cond-mat.mtrl-sci↗
arXiv2021

Field-induced multiple metal-insulator crossovers of correlated Dirac electrons of perovskite CaIrO$_3$

The interplay between electron correlation and topology of relativistic electrons may lead to a new stage of the research on quantum materials and emergent functions. The emergence of various collective electronic orderings/liquids, which are tunable by external stimuli, is a remarkable feature of correlated electron systems, but has rarely been realized in the topological semimetals with high-mobility relativistic electrons. Here, we report that the correlated Dirac electrons with the Mott criticality in perovskite CaIrO$_3$ show unconventional field-induced successive metal-insulator-metal crossovers in the quantum limit accompanying a giant magnetoresistance (MR) with MR ratio of 3,500 % (18 T and 1.4 K). The analysis shows that the insulating state originates from the collective electronic ordering such as charge/spin density wave promoted by electron correlation, whereas it turns into the quasi-one-dimensional metal at higher fields due to the field-induced reduction of chemical potential, highlighting the highly field-sensitive character of correlated Dirac electrons.

cond-mat.str-el↗
arXiv2021

Electronic states of multilayer VTe2: quasi-one-dimensional Fermi surface and implications to charge-density waves

We have performed angle-resolved photoemission spectroscopy on epitaxial VTe2 films to elucidate the relationship between the fermiology and charge-density waves (CDW). We found that a two-dimensional triangular pocket in 1 monolayer (ML) VTe2 is converted to a strongly warped quasi-one-dimensional (1D) Fermi surface in the 6ML counterpart, likely associated with the 1T-to-1T' structural phase transition. We also revealed a metallic Fermi edge on the entire Fermi surface in 6ML at low temperature distinct from anisotropic pseudogap in 1ML, signifying a contrast behavior of CDW that is also supported by first-principles band-structure caluculations. The present result points to the importance of simultaneously controlling the structural phase and fermiology to manipulate the CDW properties in ultrathin transition-metal dichalcogenides.

cond-mat.mtrl-sci↗
arXiv2021

A Versatile Post-Doping Towards Two-Dimensional Semiconductors

We have developed a simple and straightforward way to realize controlled post-doping towards 2D transition metal dichalcogenides (TMDs). The key idea is to use low-kinetic energy dopant beams and a high-flux chalcogen beam at the same time, leading to substitutional doping with controlled dopant densities. Atomic-resolution transmission electron microscopy has revealed that dopant atoms injected toward TMDs are incorporated substitutionally into the hexagonal framework of TMDs. Electronic properties of doped TMDs (Nb-doped WSe2) have shown drastic change, p-type action with more than two orders of magnitude increase in on current. Position-selective doping has also been demonstrated by the post-doping toward TMDs with a patterned mask on the surface. The post-doping method developed in this work can be a versatile tool for 2D-based next-generation electronics in the future.

cond-mat.mtrl-sci↗
arXiv2021

Beta-decay formulas revisited (I): Gamow--Teller and spin-dipole contributions to allowed and first-forbidden transitions

We propose formulas of the nuclear beta-decay rate that are useful in a practical calculation. The decay rate is determined by the product of the lepton and hadron current densities. A widely used formula relies upon the fact that the low-energy lepton wave functions in a nucleus can be well approximated by a constant and linear to the radius for the $s$-wave and $p$-wave wave functions, respectively. We find, however, the deviation from such a simple approximation is evident for heavy nuclei with large $Z$ by numerically solving the Dirac equation. In our proposed formulas, the neutrino wave function is treated exactly as a plane wave, while the electron wave function is obtained by iteratively solving the integral equation, thus we can control the uncertainty of the approximate wave function. The leading-order approximation gives a formula equivalent to the conventional one and overestimates the decay rate. We demonstrate that the next-to-leading-order formula reproduces well the exact result for a schematic transition density as well as a microscopic one obtained by a nuclear energy-density functional method.

nucl-th↗