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K. Arai

Publications and source records attributed to K. Arai.

246 records · Page 14Linked to original sources

A microscopic cluster model study of $^3$He+$p$ scatterings

We calculate $^3$He+$p$ scattering phase shifts in two different microscopic cluster models, Model T and Model C, in order to show the effects of tensor force as well as $D$-wave components in the cluster wave function. Model T employs a realistic nucleon-nucleon potential and includes the $D$-wave, whereas Model C employs an effective potential in which the tensor-force effect is considered to be renormalized into the central force and includes only the $S$-wave for the cluster intrinsic motion. The $S$- and $P$-wave elastic scattering phase shifts are obtained in the \{$^3$He+$p$\}+\{$d$ + 2$p$\} coupled-channels calculation. In Model T, the $d$ + 2$p$ channel plays a significant role in producing the $P$-wave resonant phase shifts but hardly affects the $S$-wave non-resonant phase shifts. In Model C, however, the effect of the $d$ + 2$p$ channel is suppressed in both of the $S$- and $P$-wave phase shifts, suggesting that it is renormalized mostly as the $^3$He(1/2$^+$)+$p$ channel in the resonance region.

nucl-th↗

Global-Vector Representation of the Angular Motion of Few-Particle Systems II

The angular motion of a few-body system is described with global vectors which depend on the positions of the particles. The previous study using a single global vector is extended to make it possible to describe both natural and unnatural parity states. Numerical examples include three- and four-nucleon systems interacting via nucleon-nucleon potentials of AV8 type and a 3$α$ system with a nonlocal $αα$ potential. The results using the explicitly correlated Gaussian basis with the global vectors are shown to be in good agreement with those of other methods. A unique role of the unnatural parity component, caused by the tensor force, is clarified in the $0^-_1$ state of $^4$He. Two-particle correlation function is calculated in the coordinate and momentum spaces to show different characteristics of the interactions employed.

nucl-th↗

NMR Observation of Rattling Phonons in the Pyrochlore Superconductor KOs2O6

We report nuclear magnetic resonance studies on the beta-pyrochlore oxide superconductor KOs2O6. The nuclear relaxation at the K sites is entirely caused by fluctuations of electric field gradient, which we ascribe to highly anharmonic low frequency oscillation (rattling) of K ions. A phenomenological analysis shows a crossover from overdamped to underdamped behavior of the rattling phonons with decreasing temperature and its sudden sharpening below the superconducting transition temperature Tc. Absence of the Hebel-Slichter peak in the relaxation rate at the O sites below Tc also indicates strong electron-phonon coupling.

cond-mat.supr-con↗

Big-bang nucleosynthesis in Brans-Dicke cosmology with a varying $Λ$ term related to WMAP

We investigate the big-bang nucleosynthesis in a Brans-Dicke model with a varying $Λ$ term using the Monte-Carlo method and likelihood analysis. It is found that the cosmic expansion rate differs appreciably from that of the standard model. The produced abundances of $^4$He, D, and barely Li are consistent with the observed ones within the uncertainties in nuclear reaction rates when the baryon to photon ratio $η= (5.47 - 6.64)\times 10^{-10}$, which is in agreement with the value deduced from WMAP.

astro-ph↗

Synchrony of limit-cycle oscillators induced by random external impulses

The mechanism of phase synchronization between uncoupled limit-cycle oscillators induced by common external impulsive forcing is analyzed. By reducing the dynamics of the oscillator to a random phase map, it is shown that phase synchronization generally occurs when the oscillator is driven by weak external impulses in the limit of large inter-impulse intervals. The case where the inter-impulse intervals are finite is also analyzed perturbatively for small impulse intensity. For weak Poissonian impulses, it is shown that the phase synchronization persists up to the first order approximation.

nlin.AO↗

Nucleosynthesis inside Gamma-Ray Burst Accretion Disks

We investigate nucleosynthesis inside both a gamma-ray burst accretion disk and a wind launched from an inner region of the disk using one-dimensional models of the disk and wind and a nuclear reaction network. Far from a central black hole, the composition of accreting gas is taken to be that of an O-rich layer of a massive star before core collapse. We find that the disk consists of five layers characterized by dominant elements: O16, Si28, Fe54 (and Ni56), He4, and nucleons, and the individual layers shift inward with keeping the overall profiles of compositions as the accretion rate decreases. Ni56 are abundantly ejected through the wind from the inner region of the disk with the electron fraction \simeq 0.5. In addition to iron group, elements heavier than Cu, in particular Cu63 and Zn64, are massively produced through the wind. Various neutron-rich nuclei can be also produced in the wind from neutron-rich regions of the disk, though the estimated yields have large uncertainties.

astro-ph↗

P-Process Nucleosynthesis inside Supernova-Driven Supercritical Accretion Disks

We investigate p-process nucleosynthesis in a supercritical accretion disk around a compact object of 1.4 M_solar, using the self-similar solution of an optically thick advection dominated flow. Supercritical accretion is expected to occur in a supernova with fallback material accreting onto a new-born compact object. It is found that appreciable amounts of p-nuclei are synthesized via the p-process in supernova-driven supercritical accretion disks (SSADs) when the accretion rate m_dot = M_dot c^2/(16 L_Edd) >10^5, where L_Edd is the Eddington luminosity. Abundance profiles of p-nuclei ejected from SSADs have similar feature to those of the oxygen/neon layers in Type II supernovae when the abundance of the fallback gas far from the compact object is that of the oxygen/neon layers in the progenitor. The overall abundance profile is in agreement with that of the solar system. Some p-nuclei, such as Mo, Ru, Sn, and La, are underproduced in the SSADs as in Type II supernovae. If the fallback gas is mixed with a small fraction of proton through Rayleigh-Taylor instability during the explosion, significant amounts of Mo92 are produced inside the SSADs. Ru96 and La138 are also produced when the fallback gas contains abundant proton though the overall abundance profile of p-nuclei is rather different from that of the solar system. The p-process nucleosynthesis in SSADs contributes to chemical evolution of p-nuclei, in particular Mo92, if several percents of fallback matter are ejected via jets and/or winds.

astro-ph↗

Magnetic and superconducting properties of Cd2Re2O7: Cd NMR and Re NQR

We report Cd NMR and Re NQR studies on Cd2Re2O7, the first superconductor among pyrochlore oxides Tc=1 K. Re NQR spectrum at zero magnetic field below 100 K rules out any magnetic or charge order. The spin-lattice relaxation rate below Tc exhibits a pronounced coherence peak and behaves within the weak-coupling BCS theory with nearly isotropic energy gap. Cd NMR results point to moderate ferromagnetic enhancement at high temperatures followed by rapid decrease of the density of states below the structural transition temperature of 200 K.

cond-mat.supr-con↗

Microscopic description of light unstable nuclei with the stochastic variational method

The structure of the light proton and neutron rich nuclei is studied in a microscopic multicluster model using the stochastic variational method. This approach enables us to describe the weakly bound nature of these nuclei in a consistent way. Applications for various nuclei $^{6-9}$Li, $^7$Be, $^8$B, $^9$C, $^{9-10}$Be, $^{9-10}$B presented. The paper discusses the relation of this model to other models as well as the possible extension for p and sd shell nuclei.

nucl-th↗

Occupation probability of harmonic-oscillator quanta for microscopic cluster-model wave functions

We present a new and simple method of calculating the occupation probability of the number of total harmonic-oscillator quanta for a microscopic cluster-model wave function. Examples of applications are given to the recent calculations including $α+n+n$-model for $^6$He, $α+t+n+n$-model for $^9$Li, and $α+α+n$-model for $^9$Be as well as the classical calculations of $α+p+n$-model for $^6$Li and $α+α+α$-model for $^{12}$C. The analysis is found to be useful for quantifying the amount of excitations across the major shell as well as the degree of clustering. The origin of the antistretching effect is discussed.

nucl-th↗

Structure of the mirror nuclei $^9$Be and $^9$B in a microscopic cluster model

The structure of the mirror nuclei $^9$Be and $^9$B is studied in a microscopic $α+ α+ n$ and $α+ α+ p$ three-cluster model using a fully antisymmetrized 9-nucleon wave function. The two-nucleon interaction includes central and spin-orbit components and the Coulomb potential. The ground state of $^9$Be is obtained accurately with the stochastic variational method, while several particle-unbound states of both $^9$Be and $^9$B are investigated with the complex scaling method.The calculation for $^9$Be supports the recent identification for the existence of two broad states around 6.5 MeV, and predicts the $\frac{3}{2}^{-}_2$ and $\frac{5}{2}^{-}_2$ states at about 4.5 MeV and 8 MeV, respectively. The similarity of the calculated spectra of $^9$Be and $^9$B enables one to identify unknown spins and parities of the $^9$B states. Available data on electromagnetic moments and elastic electron scatterings are reproduced very well. The enhancement of the $E$1 transition of the first excited state in $^9$Be is well accounted for. The calculated density of $^9$Be is found to reproduce the reaction cross section on a Carbon target. The analysis of the beta decay of $^9$Li to $^9$Be clearly shows that the wave function of $^9$Be must contain a small component that cannot be described by the simple $α+ α+ n$ model. This small component can be well accounted for by extending a configuration space to include the distortion of the $α$-particle to $t+p$ and $h+n$ partitions.

nucl-th↗