arXiv subjects

A. Nakamura

Publications and source records attributed to A. Nakamura.

At least 19 recordsLinked to original sources

arXiv2026

Photo- and thermally-induced huge layer decoupling in twisted bilayer WSe$_2$

Twisted bilayer systems host a wealth of emergent phenomena, such as flat-band superconductivity, ferromagnetism, and ferroelectricity, arising from moir\'e superlattices and unconventional interlayer coupling. Despite their central role, direct and quantitative access to the three-dimensional atomic arrangement in these systems has remained elusive due to their nanoscale dimensions. Here, we introduce an automated dark-field electron tomography technique that enables quantitative three-dimensional structural analysis of atomically thin materials with sub-\r{A} precision. By applying this method to twisted bilayer WSe$_2$, we precisely visualize the twist-angle-dependent structural relaxation appearing as the AB/BA stacking domains separated by 10-20 nm domain walls.In the marginally twisted region ($\theta \leq 0.1^\circ$), we uncover a significant expansion of the interlayer spacing compared to the bulk configuration, exceeding 0.1 \r{A}, along with a remarkable temperature-driven interlayer decoupling. Ultrafast measurements further reveal optically induced interlayer separation of ~0.2 \r{A} on the picosecond timescale, attributed to transient exciton formation. These findings not only establish a powerful approach for visualizing hidden out-of-plane structures in atomically thin micro-flake materials, but also uncover the intrinsic fragility and dynamical tunability of interlayer coupling in moir\'e-engineered 2-dimensional materials.

cond-mat.mes-hall↗

arXiv2024

Quark Density in Lattice QC$_2$D at Imaginary and Real Chemical Potential

We study lattice two-color QCD (QC$_2$D) with two flavors of staggered fermions at imaginary and real quark chemical potential $μ_q$ and $T>T_c$. We employ various methods of extrapolation of the quark number density from imaginary to real quark chemical potentials $μ_q$, including series expansions as well as analytic continuation based on phenomenological models, and study their accuracy by comparing the results to the lattice data. Below the Roberge-Weiss temperature, $T T_{RW}$ we show that the analytic continuation to the real values of $μ_q$ based on trigonometric functions works equally well with the conventional method based on the Taylor expansion in powers of $μ_q$.

hep-lat↗

arXiv2024

Topological Hall effect enhanced at magnetic transition fields in a frustrated magnet EuCd$_2$

Emergent magnetic fields exerted by topological spin textures of magnets lead to an additional Hall response of itinerant carriers called the topological Hall effect (THE). While THE as a bulk effect has been widely studied, THE driven by magnetic domain boundaries (DBs) has been elusive. Here, we report rich Hall responses characterized by multiple peak structures and a hysteresis loop in films of EuCd$_2$, where Eu layers form a geometrically frustrated lattice of Heisenberg spins. We uncover a THE component sharply enhanced at magnetic transition fields, indicating a giant contribution from non-trivial spin textures possibly formed at the DBs.

cond-mat.mtrl-sci↗

arXiv2024

Unusual photoinduced crystal structure dynamics in TaTe$_2$ with double zigzag chain superstructure

Transition metal dichalcogenides with superperiodic lattice distortions have been widely investigated as the platform of ultrafast structural phase manipulations. Here we performed ultrafast electron diffraction on room-temperature TaTe$_2$, which exhibits peculiar double zigzag chain pattern of Ta atoms. From the time-dependent electron diffraction pattern, we revealed a photoinduced change in the crystal structure occurring within <0.5 ps, though there is no corresponding high-temperature equilibrium phase. We further clarified the slower response (~1.5 ps) reflecting the lattice thermalization. Our result suggests the unusual ultrafast crystal structure dynamics specific to the non-equilibrium transient process in TaTe$_2$.

cond-mat.mtrl-sci↗

arXiv2023

Change of superconducting character in UTe2induced by magnetic field

UTe2 is a recently discovered spin-triplet superconductor. One of the characteristic features of UTe2 is a magnetic field (H)-boosted superconductivity above 16 T when H is applied exactly parallel to the b axis. To date, this superconducting (SC) state has not been thoroughly investigated, and the SC properties as well as the spin state of this high-H SC (HHSC) phase are not well understood. In this study, we performed AC magnetic susceptibility and nuclear magnetic resonance (NMR) measurements and found that, up to 24.8 T, the HHSC state is intrinsic to UTe2 and quite sensitive to the H angle, and that its SC character is different from that in the low-H SC (LHSC) state. The dominant spin component of the spin-triplet pair is along the a axis in the LHSC state but is changed in the HHSC state along the b axis. Our results indicate that H-induced multiple SC states originate from the remaining spin degrees of freedom.

cond-mat.supr-con↗

arXiv2022

Numerical Study of the Roberge-Weiss Transition

We study the Roberge-Weiss phase transition numerically. The phase transition is associated with the discontinuities in the quark-number density at specific values of imaginary quark chemical potential. We parameterize the quark number density $ρ_q$ by the polynomial fit function to compute the canonical partition functions. We demonstrate that this approach provides a good framework for analyzing lattice QCD data at finite density and a high temperature. We show numerically that at high temperature, the Lee-Yang zeros lie on the negative real semi-axis provided that the high-quark-number contributions to the grand canonical partition function are taken into account. These Lee-Yang zeros have nonzero linear density, which signals the Roberge-Weiss phase transition. We demonstrate that this density agrees with the quark density discontinuity at the transition line.

hep-lat↗

arXiv2022

Zero-field current-induced Hall effect in ferrotoroidic metal

We have performed precise Hall measurements for the ferrotoroidic candidate material UNi4B. Below Neel temperature TN ~ 20 K (corresponding to the ferrotoroidic transition temperature), a Hall voltage becomes finite even at zero field and changes proportional to the square of current density; by contrast, it is almost zero above TN. Moreover, we have found that a current-induced magnetization estimated from our Hall effect measurements is qualitatively consistent with the previous directly measured value. These results provide strong evidence for a magnetoelectric phenomenon uniquely in ferrotoroidic metals -- a zero-field nonlinear Hall effect resulting from the current-induced magnetization connecting the ferrotoroidal moments.

cond-mat.str-el↗

arXiv2022

Study of two color QCD on large lattices

We study two colors lattice QCD (QC$_2$D) with two flavors of staggered fermions on $40^4$ and $32^4$ lattices with lattice spacing $a =0.048$~fm in the wide range of the quark chemical potential $μ_q$. Our focus is on the confinement-deconfinement transition in this theory. Thus we compute the string tension from the Wilson loops and the static quark free energy from the Polyakov loops. We find that the deconfinement transition found earlier in the range $μ_q \approx 800 - 1000$ MeV is shifted to higher values. This shift is attributed to decreasing of the lattice spacing used in our simulations in comparison with the earlier study.

hep-lat↗

arXiv2022

Coherent photoproduction of the neutral pion and eta meson on the deuteron at incident energies below 1.15 GeV

Coherent photoproduction of the neutral pion and eta meson on the deuteron, $γ{d}${$\to$}$π^0η{d}$, has been experimentally studied at incident photon energies ranging from the reaction threshold to 1.15 GeV. The total cross section demonstrates a rapid rise below 1 GeV. The data are underestimated by the existing theoretical calculations based on quasi-free $π^0 η$ photoproduction on the nucleon followed by deuteron coalescence. At the same time, the data are rather well reproduced by the calculations taking into account the final-state interaction. We have also measured for the first time the differential cross sections: the $π^0 η$ invariant-mass distribution $dσ/dM_{πη}$, the $π^0 d$ invariant-mass distribution $dσ/dM_{πd}$, the $ηd$ invariant-mass distribution $dσ/dM_{ηd}$, and the distribution over the deuteron emission angle $dσ/dΩ_d$ in the overall center-of-mass frame. The measured cross section $dσ/dΩ_d$ does not exhibit strongly backward-peaking behavior predicted by the calculations. At all incident energies, an increase in $dσ/dM_{ηd}$ near the $ηd$ threshold is observed, which indicates a bound or virtual $ηd$ state resulting from a strong attraction between $η$ and a deuteron. The possibilities of using coherent $π^0η$ photoproduction on a nucleus to study the $η$-nuclear interaction are also discussed.

nucl-ex↗

arXiv2021

Resonance-like structure near the $ηd$ threshold in the $γ{d}${$\to$}$π^0η{d}$ reaction

To investigate the interaction between the nucleon $N$ and nucleon resonance $N(1535)1/2^-$, the $ηd$ threshold structure connected to the isoscalar $S$-wave $N$-$N(1535)1/2^-$ system has been experimentally studied in the $γ{d}${$\to$}$π^0η{d}$ reaction at incident photon energies ranging from the reaction threshold to 1.15 GeV. A strong enhancement is observed near the $ηd$ threshold over the three-body phase-space contribution in the $ηd$ invariant-mass distribution. An analysis incorporating the known isovector resonance $\mathcal{D}_{12}$ with a spin-parity of $2^+$ in the $π^0d$ channel reveals the existence of a narrow isoscalar resonance-like structure with $1^-$ in the $ηd$ system. Using a Flatté parametrization, the mass is found to be $2.427_{-0.006}^{+0.013}$ GeV, close to the $ηd$ threshold, and the width is $\left(0.029_{-0.029}^{+0.006}{\rm\ GeV}\right)+\left(0.00_{-0.00}^{+0.41}\right) p_ηc$, where $p_η$ denotes the $η$ momentum in the rest frame of the $ηd$ system. The observed structure would be attributed to a predicted isoscalar $1^-$ $ηNN$ bound state from $ηNN$ and $πNN$ coupled-channel calculation, or an $ηd$ virtual state owing to strong $ηd$ attraction.

nucl-ex↗

arXiv2021

Disappearance of spin glass behavior in ThCr2Si2-type intermetallic PrAu2Si2

It is unexpected that a spin-glass transition, which generally occurs only in the system with some form of disorder, was observed in the ThCr2Si2-type compound PrAu2Si2 at a temperature of ~3 K. This puzzling phenomenon was later explained based on a novel dynamic frustration model that does not involve static disorder. We present the results of re-verification of the reported spin-glass behaviors by measuring the physical properties of three polycrystalline PrAu2Si2 samples annealed under different conditions. Indeed, in the sample annealed at 827 C for one week, a spin-glass transition does occur at a temperature of Tf=2.8 K as that reported previously in the literature. However, it is newly found that the spin-glass effect is actually more pronounced in the as-cast sample, and almost completely disappears in the well-annealed (at 850 C for 4 weeks) sample. The apparent sample dependence of the magnetic characteristics of PrAu2Si2 is discussed by comparing it with similar phenomena observed in the isomorphic compounds URh2Ge2 and CeAu2Si2. Our experimental results strongly suggest that the spin-glass behavior observed in the as cast and insufficient annealed samples is most likely due to the presence of small amount of crystalline impurities and/or partial site disorder on the Au and Si sublattices, and thus is not the inherent characteristic of ideal ThCr2Si2-type PrAu2Si2. The perfectly ordered PrAu2Si2 should be regarded as a paramagnetic system with obvious crystal-field effects.

cond-mat.str-el↗

arXiv2021

Nano-to-micro spatiotemporal imaging of magnetic skyrmion's life cycle

Magnetic skyrmions are the self-organized topological spin textures behaving like particles. Because of their fast creation and typically long lifetime, experimental verification of skyrmion's creation/annihilation processes has been challenging. Here we successfully track skyrmions dynamics in defect-introduced Co9Zn9Mn2, by using pump-probe Lorentz transmission electron microscope. Following the nanosecond-photothermal excitation, we resolve 160-nm-skyrmion's proliferation at <1 ns, contraction at 5 ns, drift from 10 ns to 4 microsecond and coalescence at 5 microsecond. These motions relay the multiscale arrangement and relaxation of skyrmion clusters in a repeatable cycle of 20 kHz. Such repeatable dynamics of skyrmions, arising from the weakened but still persistent topological protection around defects, enables us to visualize the whole life of the skyrmions, as well as demonstrating the possible high-frequency manipulations of topological charges brought by skyrmions.

cond-mat.mes-hall↗

arXiv2020

Nanoscale imaging of unusual photo-acoustic waves in thin flake VTe$_2$

Controlling acoustic phonons, the carriers of sound and heat, has been attracting great attention toward the manipulation of sonic and thermal properties in nanometric devices. In particular, the photo-acoustic effect using ultrafast optical pulses has a promising potential to optically manipulate phonons in picoseconds time regime. However, its mechanism has been so far mostly based on the commonplace thermoelastic expansion in isotropic media, limiting the spectrum of potential applications. We investigate a conceptually new mechanism of photo-acoustic effect involving the structural instability, by utilizing a transition-metal dichalcogenide VTe$_2$ with the ribbon-type charge-density-wave (CDW). Ultrafast electron microscope imaging and diffraction measurements reveal the generation and propagation of unusual acoustic waves in the nanometric thin plate associated with the optically induced instantaneous charge-density-wave dissolution. Our results highlight the capability of photo-induced structural instability as a source of coherent acoustic waves.

cond-mat.mtrl-sci↗

arXiv2020

Suppressing dynamical diffraction artefacts in differential phase contrast scanning transmission electron microscopy of long-range electromagnetic fields via precession

In differential phase contrast scanning transmission electron microscopy (DPC-STEM), variability in dynamical diffraction resulting from changes in sample thickness and local crystal orientation (due to sample bending) can produce contrast comparable to that arising from the long-range electromagnetic fields probed by this technique. Through simulation we explore the scale of these dynamical diffraction artefacts and introduce a metric for the magnitude of their confounding contribution to the contrast. We show that precession over an angular range of a few milliradian can suppress this confounding contrast by one-to-two orders of magnitude. Our exploration centres around a case study of GaAs near the [011] zone-axis orientation using a probe-forming aperture semiangle on the order of 0.1 mrad at 300 keV, but the trends found and methodology used are expected to apply more generally.

cond-mat.mtrl-sci↗

arXiv2019

New way of collision experiment data analysis based on Grand Canonical Distribution and Lattice QCD data

We propose new way of heavy ion collisions experiment data analysis. We analyze physical parameters of fireball created in RHIC experiment based on Grand Canonical Distribution and different Lattice QCD data available at the moment. Our results on chemical potential are in agreement with previous model estimations and do not depend on Lattice setup. At same time, we found possible T(V) states of fireball and estimated the most probable temperature and volume of fireball as function of collision energy. We conclude that hadrom matter at RHIC experiment is thermalized and described by Grand Canonical Distribution.

hep-lat↗

arXiv2019

$ωN$ scattering length from $ω$ photoproduction on the proton near the threshold

Photoproduction of the $ω$ meson on the proton has been experimentally studied near the threshold. The total cross sections are determined at incident energies ranging from 1.09 to 1.15 GeV. The 1/2 and 3/2 spin-averaged scattering length $a_{ωp}$ and effective range $r_{ωp}$ between the $ω$ meson and proton are estimated from the shape of the total cross section as a function of the incident photon energy: $a_{ωp} = \left(-0.97^{+0.16_{\rm stat}}_{-0.16_{\rm stat}}{}^{+0.03_{\rm syst}}_{-0.00_{\rm syst}}\right)+i \left(0.07^{+0.15_{\rm stat}}_{-0.14_{\rm stat}}{}^{+0.17_{\rm syst}}_{-0.09_{\rm syst}}\right)$ fm and $r_{ωp}=\left(+2.78^{+0.68_{\rm stat}}_{-0.54_{\rm stat}}{}^{+0.11_{\rm syst}}_{-0.13_{\rm syst}}\right)+i\left(-0.01^{+0.46_{\rm stat}}_{-0.50_{\rm stat}}{}^{+0.07_{\rm syst}}_{-0.00_{\rm syst}}\right)$ fm, resulting in a repulsive force. The real and imaginary parts for $a_{ωp}$ and $r_{ωp}$ are determined separately for the first time. A small $P$-wave contribution does not affect the obtained values.

nucl-ex↗

arXiv2019

Ultrafast nematic-orbital excitation in FeSe

The electronic nematic phase is an unconventional state of matter that spontaneously breaks the rotational symmetry of electrons. In iron-pnictides/chalcogenides and cuprates, the nematic ordering and fluctuations have been suggested to have as-yet-unconfirmed roles in superconductivity. However, most studies have been conducted in thermal equilibrium, where the dynamical property and excitation can be masked by the coupling with the lattice. Here we use femtosecond optical pulse to perturb the electronic nematic order in FeSe. Through time-, energy-, momentum- and orbital-resolved photo-emission spectroscopy, we detect the ultrafast dynamics of electronic nematicity. In the strong-excitation regime, through the observation of Fermi surface anisotropy, we find a quick disappearance of the nematicity followed by a heavily-damped oscillation. This short-life nematicity oscillation is seemingly related to the imbalance of Fe 3dxz and dyz orbitals. These phenomena show critical behavior as a function of pump fluence. Our real-time observations reveal the nature of the electronic nematic excitation instantly decoupled from the underlying lattice.

cond-mat.str-el↗

arXiv2018

Non-strange Dibaryon Resonances Observed in the $γd\to π^0π^0 d$ Reaction

Coherent double neutral-pion photoproduction on the deuteron, $γ{d}${$\to$}$π^0π^0{d}$, has been experimentally studied at incident photon energies ranging from 0.75 to 1.15 GeV. The total cross section as a function of the $γ{d}$ center-of-mass energy shows resonance-like behavior, which peaks at approximately 2.47 and 2.63 GeV. The measured angular distribution of deuteron emission is rather flat, which cannot be reproduced by the kinematics of quasi-free $π^0π^0$ production with deuteron coalescence. In $π^0d $ invariant-mass distributions, a clear peak is observed at $2.14{\pm}0.01$ GeV$/c^2$ with a width of $0.09{\pm}0.01$ GeV$/c^2$. The spin-parity of this state is restricted to $1^+$, $2^+$ or $3^-$ from the angular distributions of the two $π^0$s. The present work shows strong evidence for the existence of an isovector dibaryon resonance with a mass of 2.14 GeV$/c^2$. The $2^+$ assignment is consistent with the theoretically predicted ${\cal{D}}_{12}$ state, and also with the energy dependence of the $π{d}$ partial-wave amplitude $^3\!P_2$ for the $π^{\pm}d${$\to$}$π^{\pm}d$ and $π^+d${$\to$}${pp}$ reactions.

nucl-ex↗