arXiv subjects

M. Okada

Publications and source records attributed to M. Okada.

12 recordsLinked to original sources

arXiv2024

Shell-cluster transition in $^{48}$Ti

Background: Whether or not the $\alpha$ ($^4$He nucleus) clustering exists in the medium-mass region of nuclear systems is a fundamental and intriguing question. However, the recent analysis of the $\alpha$ knockout reaction on $^{48}$Ti [Phys. Rev. C 103, L031305 (2021)] poses a puzzle: The microscopic wave function gives an $\alpha$ knockout cross section that is two orders of magnitude smaller than the experiment, while basic nuclear properties such as the charge radius and the electromagnetic transition probabilities are well explained. Purpose: The ground-state structure of $^{48}$Ti is investigated by using proton- and $\alpha$-nucleus elastic scattering at a few to several hundred MeV, which offers different sensitivity to the region of the nuclear density profiles. Method: Four types of density distributions, the $jj$-coupling shell model and three cluster model configurations, are generated in a single scheme by the antisymmetrized quasi-cluster model (AQCM). The angular distribution of the proton- and $\alpha$-$^{48}$Ti elastic scattering cross sections are obtained with a reliable high-energy reaction theory, the Glauber model. Results: The $jj$-coupling shell model configuration is found to best reproduce the proton-nucleus elastic scattering cross section. On the other hand, the trace of the $\alpha$ cluster structure in the tail region of the wave function is embedded in the $\alpha$-nucleus elastic scattering cross section. Conclusion: Our results suggest that the structure of the nucleus changes as a function of distance from the center, from the $jj$-coupling shell model structure in the surface region to the $\alpha$+$^{44}$Ca cluster structure in the tail region. This picture is consistent with the finding of the $\alpha$ knockout reaction on $^{48}$Ti.

nucl-th↗

arXiv2023

Efficient NMR measurement and data analysis supported by the Bayesian inference : The case of the heavy fermion compound YbCo2Zn20

We propose a data-driven technique to infer microscopic physical quantities from nuclear magnetic resonance(NMR) spectra, in which the data size and quality required for the Bayesian inference are investigated. The $^{59}$Co-NMR measurement of YbCo$_2$Zn$_{20}$ single crystal generates complex spectra with 28 peaks. By exploiting the site symmetry in the crystal structure, the isotropic Knight shift $K_{iso}$ and nuclear quadrupole resonance(NQR) frequency $\nu_Q$ were estimated to be $K_{iso} = 0.7822 \pm 0.0090 \%$, $\nu_Q = 2.008 \pm 0.016$ MHz ( T = 20 K, H $\simeq$ 10.2 T) by analyzing only 30 data points from one spectrum. The estimation of $\nu_Q$ is consistent with the precise value obtained in the NQR experiment. Our method can significantly reduce the measurement time and the computational cost of data analysis in NMR experiments.

cond-mat.mtrl-sci↗

arXiv2020

Enhanced exciton-exciton collisions in an ultra-flat monolayer MoSe2 prepared through deterministic flattening

Squeezing bubbles and impurities out of interlayer spaces by applying force through a few-layer graphene capping layer leads to van der Waals heterostructures with ultra-flat structure free from random electrostatic potential arising from charged impurities. Without the graphene capping layer, a squeezing process with an AFM tip induces applied-force-dependent charges of n ~ 2 x 10^12 cm^-2 uN^-1, resulting in strong intensity of trions in photoluminescence spectra of MoSe2 at low temperature. We found that a hBN/MoSe2/hBN prepared with the "modified nano-squeezing method" shows a strong excitonic emission with negligible trion peak, and the residual linewidth of the exciton peak is only 2.2 meV, which is comparable to the homogeneous limit. Furthermore, in this high-quality sample, we found that formation of biexciton occurs even at extremely low excitation power (Phi ~ 2.3 x 10^19 cm^-2 s^-1) due to the enhanced collisions between excitons.

cond-mat.mes-hall↗

arXiv2017

Randomness-induced quantum spin liquid on honeycomb lattice

We present a quantu spin liquid state in a spin-1/2 honeycomb lattice with randomness in the exchange interaction. That is, we successfully introduce randomness into the organic radial-based complex and realize a random-singlet (RS) state. All magnetic and thermodynamic experimental results indicate the liquid-like behaviors, which are consistent with those expected in the RS state. These results demonstrate that the randomness or inhomogeneity in the actual systems stabilize the RS state and yield liquid-like behavior.

cond-mat.str-el↗

arXiv2016

Ultrafast Melting of Spin Density Wave Order in BaFe$_{2}$As$_{2}$ Observed by Time- and Angle-Resolved Photoemission Spectroscopy with Extreme-Ultraviolet Higher Harmonic Generation

Transient single-particle spectral function of BaFe$_{2}$As$_{2}$, a parent compound of iron-based superconductors, has been studied by time- and angle-resolved photoemission spectroscopy with an extreme-ultraviolet laser generated by higher harmonics from Ar gas, which enables us to investigate the dynamics in the entire Brillouin zone. We observed electronic modifications from the spin-density-wave (SDW) ordered state within $\sim$ 1 ps after the arrival of a 1.5 eV pump pulse. We observed optically excited electrons at the zone center above $E_{F}$ at 0.12 ps, and their rapid decay. After the fast decay of the optically excited electrons, a thermalized state appears and survives for a relatively long time. From the comparison with the density-functional theory band structure for the paramagnetic and SDW states, we interpret the experimental observations as the melting of the SDW. Exponential decay constants for the thermalized state to recover back to the SDW ground state are $\sim$ 0.60 ps both around the zone center and the zone corner.

cond-mat.supr-con↗

arXiv2014

Correspondence between phase oscillator network and classical XY model with the same infinite-range interaction in statics

We study the phase oscillator networks with distributed natural frequencies and classical XY models both of which have a class of infinite-range interactions in common. We find that the integral kernel of the self-consistent equations (SCEs) for oscillator networks correspond to that of the saddle point equations (SPEs) for XY models, and that the quenched randomness (distributed natural frequencies) corresponds to thermal noise. We find a sufficient condition that the probability density of natural frequency distributions is one-humped in order that the kernel in the oscillator network is strictly decreasing as that in the XY model. Furthermore, taking the uniform and Mexican-hat type interactions, we prove the one to one correspondence between the solutions of the SCEs and SPEs. As an application of the correspondence, we study the associative memory type interaction. In the XY model with this interaction, there exists a peculiar one-parameter family of solutions. For the oscillator network, we find a non-trivial solution, i.e., a limit cycle oscillation.

cond-mat.stat-mech↗

arXiv2009

Statistical Mechanical Study on a Neural Network Model with Time Dependent Interactions

We study a neural network model in which both neurons and synaptic interactions evolve in time simultaneously. The time evolution of synaptic interactions is described by a Langevin equation including a Hebbian learning term, and a bias term which is the interactions of the Hopfield model. We assume that synaptic interactions change much slower than neurons and study the stationary states of synaptic interactions by the replica method. We find that the order of the phase transition changes from the second to the first and that the existence regions of the Hopfield attractor and mixed states increase as the coefficient of the learning term increases. We also study the AT stability of solutions and find that the temperature region in which the Hopfield attractor is stable increases as the learning coefficient increases. Theoretical results are confirmed by the direct numerical integration of the Langevin equation. Further, we study the characteristics of the resultant synaptic interactions by partial annealing and find that the stability of the attractor which emerges after partial annealing is enhanced and those of the coexistent attractors are reduced.

physics.bio-ph↗

arXiv2005

Dynamical replica theoretic analysis of CDMA detection dynamics

We investigate the detection dynamics of the Gibbs sampler for code-division multiple access (CDMA) multiuser detection. Our approach is based upon dynamical replica theory which allows an analytic approximation to the dynamics. We use this tool to investigate the basins of attraction when phase coexistence occurs and examine its efficacy via comparison with Monte Carlo simulations.

cond-mat.dis-nn↗

arXiv2005

Search for optimal measure for discriminating spike trains with different randomness

We wish to discriminate spike sequences based on the degree of irregularity. For this purpose, we search for a rational expressions of quadratic functions of consecutive interspike intervals that efficiently measures spiking irregularity. Under natural assumptions, the functional form of the coefficient can be parameterized by a single parameter. The parameter is determined so as to maximize the mutual information between the distributions of coefficients computed for spike sequences derived from different renewal point processes. We find that the local variation of interspike intervals, LV (Neural Comput. Vol. 15, pp. 2823-42, 2003), is nearly optimal for whose intrinsic irregularity is close to that of experimental data.

q-bio.NC↗

arXiv2004

Photo-Luminescence and Possible Forsterite nanoparticles Model of Extended Red Emission

Possible forsterite nanoparticle model of the Extended Red Emission(ERE) is proposed on the basis of photo-luminescence of forsterite after gamma-ray and neutron irradiation. Forsterite exhibits interesting thermoluminescence spectrum similar to ERE of Red Rectangle after irradiation in low temperature. It is shown that the forsterite after thermoluminescence is over exhibits photo-luminescence(PL) when Ultraviolet ray is irradiated. The structure of PL spectrum is almost similar to that of thermoluminescence. In order to explain small variations of the peak position of wavelength of ERE spectrum, possible nanoparticle model of forsterite is investigated. Our model is consistent to the ISO observation data in near and middle infrared region, which suggest the existence of forsterite.

astro-ph↗

arXiv2004

Retrieval Properties of Hopfield and Correlated Attractors in an Associative Memory Model

We examine a previouly introduced attractor neural network model that explains the persistent activities of neurons in the anterior ventral temporal cortex of the brain. In this model, the coexistence of several attractors including correlated attractors was reported in the cases of finite and infinite loading. In this paper, by means of a statistical mechanical method, we study the statics and dynamics of the model in both finite and extensive loading, mainly focusing on the retrieval properties of the Hopfield and correlated attractors. In the extensive loading case, we derive the evolution equations by the dynamical replica theory. We found several characteristic temporal behaviours, both in the finite and extensive loading cases. The theoretical results were confirmed by numerical simulations.

cond-mat.dis-nn↗

arXiv2002

Thermoluminescence of Simulated Interstellar Matter after Gamma-ray Irradiation

Interstellar matter is known to be strongly irradiated by radiation and several types of cosmic ray particles. Simulated interstellar matter, such as forsterite $\rm Mg_{2}SiO_{4}$, enstatite $\rm MgSiO_{3}$ and magnesite $\rm MgCO_{3}$ has been irradiated with the $\rm ^{60}Co$ gamma-rays in liquid nitrogen, and also irradiated with fast neutrons at 10 K and 70 K by making use of the low-temperature irradiation facility of Kyoto University Reactor (KUR-LTL. Maximum fast neutron dose is $10^{17}n_f{\rm /cm^{2}}$). After irradiation, samples are stored in liquid nitrogen for several months to allow the decay of induced radioactivity. We measured the luminescence spectra of the gamma ray irradiated samples during warming to 370K using a spectrophotometer. For the forsterite and magnesite, the spectra exhibit a rather intense peak at about 645 -- 655 nm and 660 nm respectively, whereas luminescence scarcely appeared in olivine sample. The spectra of forsterite is very similar to the ERE of the Red Rectangle.

astro-ph↗