Search Results (315 total)

We have measured the branching ratio of the three-body process in the nonmesonic weak decay of _Λ¹²C to be 0.29±0.13. This result was obtained by reproducing the nucleon and the nucleon pair yields introducing a measured final state interaction. At the same time, we have determined the absolute decay widths, Γ_n and Γ_p, along with Γ_2N, whose relative ratio has been a long-standing puzzle. Including the three-body process, we have successfully reproduced the nucleon energy distribution, the coincidence two-nucleon angular correlation, and the momentum sum distribution simultaneously.

We investigate the energy-loss spectra of high-energy electrons incident on Ar atoms using linear density response theory with a self-interaction correction. The energy region of the spectra covers from 24.5 to 30.0 eV, where many ns, np, and nd autoionizing resonant states from the 3s orbital of Ar exist with the continuum background from the 3p orbital. We compare the calculated spectra to recent energy-loss spectra measurements and identify the observed autoionizing resonant states. We also discuss the dependence of the spectra on the scattering angles and their physical meanings.

We propose a general method to calculate the full Green’s function of multielectron atomic ions. The key point exists in the usage of L² integrable functions as a complete basis set in a finite region together with an optical potential to guaranty the outgoing scattering boundary condition. In such a way, the cumbersome procedure of adjusting boundary conditions in solving the differential Schrödinger equation is avoided. To show the validity of the method, we studied the radiative recombination involving dielectronic recombinations of Be-like Hg (Z=80) ions. The radiative damping effect is taken into account naturally in the present method. The calculated results reproduce well the asymmetric line profile observed in the experiments.

We studied the effect of nonmagnetic and magnetic impurities on superconductivity in Lu₂Fe₃Si₅ by small-amount substitution of the Lu site and investigated structural, magnetic, and electrical properties of nonmagnetic (Lu_1−xSc_x)₂Fe₃Si₅, (Lu_1−xY_x)₂Fe₃Si₅, and magnetic (Lu_1−xDy_x)₂Fe₃Si₅. The rapid depression of T_c by nonmagnetic impurities in accordance with the increase in the residual resistivity reveals the strong pair breaking dominated by disorder.

We study the spin-state transition and the phase separation involved in this transition based on the multiorbital Hubbard model. Multiple spin states are realized by changing the energy separation between two orbitals and the on-site Hund coupling. By utilizing the variational Monte-Carlo simulation, we analyze the electronic and magnetic structures in hole doped and undoped states. Electronic phase separation occurs between the low-spin band insulating state and the high-spin ferromagnetic metallic state. The difference in the band widths of the two orbitals is of prime importance for the spin-state transition and the phase separation.

Using density functional theory, we examine a recently discovered structure for di-interstitial oxygen clusters in UO_2+x in which three oxygen ions share one lattice site. This di-interstitial cluster exhibits a fast diffusion pathway; the migration barrier for these clusters is approximately half of that for mono-interstitials. Using kinetic Monte Carlo, we calculate the diffusivity of oxygen with and without the di-interstitial mechanism as a function of x and find that oxygen transport is significantly increased for higher values of x when the di-interstitial mechanism is included, agreeing much more closely with experimental data. These results emphasize the importance of clustering phenomena in UO_2+x and have implications for the evolution of UO_2+x.

Asymmetric profiles have been observed in the recombination cross section of Be-like Bi obtained by measuring the electron energy dependence of the ion abundance ratio in an electron-beam ion trap. In contrast to the previous x-ray measurements, the present measurement gives the integrated recombination cross section with higher statistical quality, which provides a benchmark to test the relativistic theory involving the interference between the resonant and continuum states. The comparison with our theoretical study shows that the Breit interaction plays an important role in this case.

Potential energy distribution of interstitial O₂ molecule in the vicinity of SiO₂/Si(001) interface is investigated by means of classical molecular simulation. A 4-nm-thick SiO₂ film model is built by oxidizing a Si(001) substrate, and the potential energy of an O₂ molecule is calculated at Cartesian grid points with an interval of 0.05 nm in the SiO₂ film region. The result shows that the potential energy of the interstitial site gradually rises with approaching the interface. The potential gradient is localized in the region within about 1 nm from the interface, which coincides with the experimental thickness of the interfacial strained layer. The potential energy is increased by about 0.62 eV at the SiO₂/Si interface. The result agrees with a recently proposed kinetic model for dry oxidation of silicon [Phys. Rev. Lett. 96, 196102 (2006)], which argues that the oxidation rate is fully limited by the oxidant diffusion.

Ultrasound velocity measurements of cubic spinel GeCo₂O₄ in single crystal were performed for the investigation of shear and compression moduli. The shear moduli in the paramagnetic state reveal the absence of Jahn-Teller activity despite the presence of orbital degeneracy in the Co²⁺ ions. Such a Jahn-Teller inactivity indicates that the intersite orbital-orbital interaction is much stronger than the Jahn-Teller coupling. The compression moduli in the paramagnetic state near the Néel temperature T_N reveal that the most relevant exchange path for the antiferromagnetic transition lies in the [111] direction. This exchange-path anisotropy is consistent with the antiferromagnetic structure with the wave vector q∥[111], suggesting the presence of bond frustration due to competition among a direct ferromagnetic interaction and several distant-neighbor antiferromagnetic interactions. In the Jahn-Teller-inactive condition, the bond frustration can be induced by geometrical orbital frustration of t_2g-t_2g interaction between the Co²⁺ ions, which can be realized in the pyrochlore lattice of the high-spin Co²⁺ with t_2g-orbital degeneracy. In GeCo₂O₄, the tetragonal elongation below T_N releases the orbital frustration by quenching the orbital degeneracy.

We study static properties of attractively interacting Bose-Fermi mixtures of uniform atomic gases at zero temperature. Using the Green’s function formalism, we calculate the boson-fermion scattering amplitude and the fermion self-energy in the medium to lowest order of the hole-line expansion. We study the ground-state energy and pressure as a function of the scattering length for a few values of the boson-fermion mass ratio m_b∕m_f and the number ratio N_b∕N_f. We find that the attractive contribution to the energy is greatly enhanced for small values of the mass ratio. We study the role of Bose-Fermi pair correlations in the mixture by calculating the pole of the boson-fermion scattering amplitude in the medium. The pole shows a standard quasiparticle dispersion for a Bose-Fermi pair. In addition, we also study the fermion dispersion relation. We find two dispersion branches with the possibility of avoided crossings. This strongly depends on the number ratio N_b∕N_f.

The photoproduction process of neutral kaons on a liquid deuterium target is investigated near the threshold region, E_γ=0.8-1.1 GeV. K⁰ events are reconstructed from positive and negative pions, and differential cross sections are derived. Experimental momentum spectra are compared with those calculated in the spectator model using a realistic deuteron wave function. Elementary amplitudes as given by recent isobar models and a simple phenomenological model are used to study the effect of the new data on the angular behavior of the elementary cross section. The data favor a backward-peaked angular distribution of the elementary n(γ,K⁰)Λ process, which provides additional constraints on current models of kaon photoproduction. The present study demonstrates that the n(γ,K⁰)Λ reaction can provide key information on the mechanism of the photoproduction of strangeness.

By using an optimization variational Monte Carlo method, we study the half-filled-band Hubbard model on anisotropic triangular lattices, as a continuation of the preceding study [T. Watanabe, H. Yokoyama, Y. Tanaka, and J. Inoue, J. Phys. Soc. Jpn. 75, 074707 (2006)]. We introduce two new trial states: (i) A coexisting state of Ψ_Q^co-antiferromagnetic (AF) and a d-wave singlet gaps, in which we allow for a band renormalization effect, and (ii) a state with an AF order of 120° spin structure. In both states, a first-order metal-to-insulator transition occurs at smaller U/t than that of the pure d-wave state. In insulating regimes, magnetic orders always exist; an ordinary (π,π)-AF order survives up to t^′/t∼0.9 (U/t=12), and a 120°-AF order becomes dominant for t^′/t. The regimes of the robust superconductor and of the nonmagnetic insulator the preceding study proposed give way to these magnetic domains.

Gyrokinetic Vlasov simulations of the ion temperature gradient turbulence are performed in order to investigate effects of helical magnetic configurations on turbulent transport and zonal flows. The obtained results confirm the theoretical prediction that helical configurations optimized for reducing neoclassical ripple transport can simultaneously reduce the turbulent transport with enhancing zonal-flow generation. Stationary zonal-flow structures accompanied with transport reduction are clearly identified by the simulation for the neoclassically optimized helical geometry. The generation of the stationary zonal flow explains a physical mechanism for causing the confinement improvement observed in the inward-shifted plasma in the Large Helical Device [O. Motojima , Nucl. Fusion 43, 1674 (2003)].

As recently shown, HgBa₂Ca_n−1Cu_nO_y (n≥6) cuprate superconductors have a critical temperature of about 100 K independent of n. This remarkable property can be explained by the very imbalanced distribution of carriers among the inequivalent CuO₂ planes in the unit cell. We discovered that these materials also have a common vortex melting line that resembles the theoretical melting lines of magnetically coupled pancake vortices. We suggest that there are two types of pancake pairs situated in the superconducting CuO₂ outer planes: those separated by the thin charge reservoir layer are strongly (Josephson) coupled, while those separated by the thick block of (n−2) CuO₂ nonsuperconducting inner planes are weakly (magnetically) coupled, forming magnetically coupled pancake vortex molecules.

The Θ⁺ was searched for via the K⁺p→π⁺X reaction using the 1.2 GeV/c K⁺ beam at the K6 beam line of the KEK-PS 12 GeV Proton Synchrotron. In the missing mass spectrum of the K⁺p→π⁺X reaction, no clear peak structure was observed. Therefore a 90% C.L. upper limit of 3.5 μb/sr was derived for the differential cross section averaged over 2^° to 22^° in the laboratory frame of the K⁺p→π⁺Θ⁺ reaction. This upper limit is much smaller than the theoretical calculation for the t-channel process where a K^0* is exchanged. From the present result, either the t-channel process is excluded or the coupling constant of g_K*NΘ is quite small.

Through the coincidence measurement of x rays with secondary electrons, x-ray spectra, and their yields have been measured in the interaction of H-like and bare highly charged ions (I⁵²⁺ and I⁵³⁺) with different target materials (Be, C, Cu, and W). K x-ray yields are independent of the material, where the K vacancies are almost filled through K x-ray emission. L x-ray yields are affected by the electronic state of the targets and the interacting projectiles, because the states involved in the electron capture depend on the level structure of the projectile and the target and the possible number of electrons captured by the highly charged ions is different depending on the number of electrons in the target states (2n²) which are resonant transferred to the projectile states in the dynamics of below-surface neutralization and deexcitation.

We have performed temperature- (T-)dependent laser-photoemission spectroscopy of the antiferromagnetic (AF) superconductor ErNi₂B₂C to study the electronic-structure evolution reflecting the interplay between antiferromagnetism and superconductivity. The spectra at the superconducting (SC) phase show a very broad spectral shape. A T-dependent SC gap shows a sudden deviation from the BCS prediction just below T_N. This observation can be explained well by the theoretical model and thus represents the characteristic bulk electronic structure of the AF SC phase for the first time.

We have studied the multiharmonic ac susceptibility response of HgBa₂Ca₄Cu₅O_y (Hg:1245), a multilayered high-temperature superconductor (HTS) having two crystallographically inequivalent CuO₂ planes in a unit cell with very imbalanced carrier concentrations. Vortex melting lines are well described by the commonly accepted model of melting with moderate anisotropy factors of 40–50, depending on the doping level. The diamagnetic response with applied fields parallel to superconducting (a,b) planes also shows a quite robust supercurrent along the c axis. Our results are also discussed in connection with contradictory models and experiments regarding the coexistence of superconductivity and antiferromagnetism in HTSs.

We apply the linear density response theory to the electron-impact ionization including autoionizing resonances of atoms. Such a single-electron-like method takes into account the electron-electron dynamic correlation effect through the first-order perturbation theory. The method allows us to study the dynamic correlation in the process of electron-impact excitation with ionization background. We take the autoionizing resonances of Ar atoms as an example to show the simpleness and effectiveness of the present method. Our calculated electron impact ionization cross sections including autoionizing resonances are in reasonable agreement with the experimental measurements.

The local and nonlocal characteristics of triad enstrophy transfer in the enstrophy inertial range of generalized two-dimensional turbulence, so-called α turbulence, are investigated using direct numerical simulations, with a special emphasis on α=1, 2, and 3. The enstrophy transfer via nonlocal triad interactions dominates the transfer dynamics in the enstrophy inertial range, irrespective of α. However, the contributions from more local interactions to the total enstrophy transfer increase as α decreases. The results are discussed in connection with the local and nonlocal transition of the enstrophy transfer at α=2 expected from the phenomenological scaling theory. The specific nature of the enstrophy transfer in surface quasigeostrophic turbulence (α=1) is also discussed.

We have demonstrated nonlinear cross-phase modulation in electro-optic crystals using intense, single-cycle terahertz (THz) radiation. Individual THz pulses, generated by coherent transition radiation emitted by subpicosecond electron bunches, have peak energies of up to 100  μJ per pulse. The time-dependent electric field of the intense THz pulses induces cross-phase modulation in electro-optic crystals through the Pockels effect, leading to spectral shifting, broadening, and modulation of copropagating laser pulses. The observed THz-induced cross-phase modulation agrees well with a time-dependent phase-shift model.

A Reply to the Comment by R. Bonifacio et al..

We report ^63,65Cu- and ¹⁹F-NMR studies on a four-layered high-temperature superconductor Ba₂Ca₃Cu₄O₈F₂((0234F(2.0)) with apical fluorine (F^-1), an undoped 55 K superconductor with a nominal Cu²⁺ valence on average. We reveal that this compound exhibits the antiferromagnetism (AFM) with a Néel temperature T_N=100  K despite being a T_c=55 K superconductor. Through a comparison with a related trilayered cuprate Ba₂Ca₂Cu₃O₆F₂(0233F(2.0)), it is demonstrated that electrons are transferred from the inner plane (IP) to the outer plane (OP) in 0234F(2.0) and 0223F(2.0), confirming the self-doped high-temperature superconductivity (HTSC) having electron and hole doping in a single compound. Remarkably, uniform mixing of AFM and HTSC takes place in both the electron-doped OPs and the hole-doped IPs in 0234F(2.0).

The nucleon direct-semidirect (DSD) capture cross sections are obtained by calculating a transition amplitude to the Hartree-Fock-BCS bound states. The radial matrix elements in the DSD amplitudes are calculated from the radial part of the single-particle wave functions. For deformed nuclei the single-particle states are expanded in the cylindrical harmonic-oscillator basis and then projected on the spherical harmonic-oscillator basis. The pairing correlations are treated in the BCS approach and the calculated spectroscopic factors are in fairly good agreement with experimental data in the even tin isotopes from ¹¹⁶Sn to ¹²⁴Sn. The resulting DSD cross sections for the neutron capture by ²⁰⁸Pb and ²³⁸U are found to be in good agreement with the available experimental data. The calculations are also performed for the neutron capture on ¹²²Sn and ¹³²Sn isotopes that are important for the r-process in astrophysics.