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91.
Using previously determined interatomic potentials, the activation energy for migration of a single substitutional helium atom in copper and tungsten has been determined. The mechanism of migration involves the jump of the helium atom out of the vacancy concurrent with the jump of a host atom into that vacancy. The helium then occupies the vacant site created by the jumping host atom, resulting in a substitutional helium at a distance of √2 r 0 (√3r 0) from its original site in copper (tungsten). The rate-limiting step in the process is found to be the jump of the helium out of the vacancy, the activation energy for which is 2.15 eV in copper and 4.69 eV in tungsten.  相似文献   
92.
The U A(1) problem of QCD is inevitably tied to the infrared behaviour of quarks and gluons with its most visible effect being the -mass. A dimensional argument of Kogut and Susskind showed that the mixing of the pseudoscalar flavour-singlet mesons with gluons can provide a screening of the Goldstone pole in this channel if the full quark-quark interaction is strongly infrared singular as ∼ 1/k 4 . We investigate this idea using previously obtained results for the Landau gauge ghost and gluon propagator, together with recent determinations for the singular behaviour of the quark-gluon vertex. We find that, even with an infrared vanishing gluon propagator, the singular structure of the quark-gluon vertex for certain kinematics is apposite for yielding a non-zero screening mass.  相似文献   
93.
The even-parity autoionizing resonance series 3p5np'[3/2]1,2, 3p5np'[1/2]1, and 3p5nf'[5/2]3 of Ar have been investigated exciting from the two metastable states 3p54s[3/2]2 and 3p54s'[1/2]0 in the photon energy range of 32500-35600 cm-1 with an experimental bandwidth of ~0.1 cm-1. The excitation spectra of the even-parity autoionizing resonance series show typical asymmetric line shapes. New level energies, quantum defects, line profile index and resonance widths, resonance lifetime and reduced widths of the autoionizing resonances are derived by a Fano-type line-shape analysis. The line profile index q and the resonance widths Γ are shown to be approximately proportional to the effective principal quantum number n*. The line separation of the 3p5np' autoionizing resonances is discussed.  相似文献   
94.
Gamow-Teller (GT) and spin-dipole (SD) states in 208Bi are studied by using self-consistent Hartree-Fock + Tamm-Dancoff approximation taking into account the coupling to the continuum. Most of SD strength is found at the excitation energy E x≈ 25MeV with a very broad width, which agrees with recent experimental observations. It is shown that Landau damping effect is responsible for the large width of SD peak, while the escape width is found to be at most 1MeV. We study also electric dipole (E1) transitions between GT and SD states in 208Bi. Main E1 transitions for 0- and 1- states are found near excitation energy expected from Brink's hypothesis in which SD states are considered as E1 resonances built on the GT state. Calculated E1 transition strengths between GT and SD states are compared with the analytic sum rules within one-particle one-hole (1p-1h) configuration space and within both 1p-1h and 2p-2h model space. Received: 11 April 2000 / Accepted: 30 August 2000  相似文献   
95.
Recent experimental studies of the giant electric resonance region in 58Ni and 40Ca with inelastically scattered α-particles of energy Eα= 240 MeV are analyzed within a microscopic nuclear structure model. The model includes the continuum RPA and more complex 1p1h⊗phonon configurations. By superimposing the contributions of different multipoles up to L = 4 we obtain good agreement with the newest (reanalyzed) data for the isoscalar monopole strength and for the total (α,α′) cross section in 58Ni. Agreement with experiment for the isoscalar monopole resonance in 40Ca is obtained too. We emphasize the necessity of using microscopic transition densities and discuss consequences for the analyses of such experiments in light and medium mass nuclei. It is shown that the gross structure of the isoscalar monopole resonance in 40Ca is caused by the 1p1h⊗phonon configurations. Received: 23 December 1999 / Revised version: 28 February 2000  相似文献   
96.
Magnetism and transport properties of the semi-Heusler compound NiMnSb are re-examined in great details. A wide set of experiments (elastic and inelastic neutron scattering, static magnetic measurements, magnetoresistance, Hall effect, thermopower, FMR) have been performed on polycrystals, single crystals or single-crystalline thin films, and the results are analysed. Special emphasis is given to the magnetic excitations and to the relaxation mechanisms in this metallic ferromagnet. At low temperatures, all experimental results hint at the existence of a fully spin-polarized conduction band (half metallic state). At higher temperature (T > 80 K), but well below the Curie temperature (730 K), a cross-over to a usual metallic ferromagnetic state is evidenced and discussed. Received 10 January 2000  相似文献   
97.
We report on the results obtained from the study of the 32S +74Ge deep inelastic reaction at incident energy E= 320 MeV. High-energy γ-rays were detected in an array of 6 seven-pack BaF2 clusters. Coincidence with complex fragments detected in 12 three-stage telescopes ensured the selection of peripheral reaction events. In order to investigate the pre-equilibrium dipole strength excitation two independent analyses were performed. In the first analysis the energy spectra of the γ-rays were evaluated in the statistical model framework while in the second one the γ-ray fragment angular correlation with respect to the nuclear spin vector of the composite system was studied. Both methods indicate the excitation of dipole strength in the highly deformed dinucleus and provide dipole resonance parameter sets that are in good agreement with each other. Received: 24 March 1999 / Revised version: 17 May 1999  相似文献   
98.
We perform a calculation of the absolute charged current neutrino-oxygen events rates relevant in the atmospheric neutrino experiments. The inclusive reaction cross-section is split into exclusive channels, which are classified according to the number of Čerenkov rings they produce. The model includes the effects of residual interaction in a RPA scheme with both nucleon-hole and Delta-hole excited states and the effects of (np-nh) excitations (n=2,3). Our result is that although the flavor ratio μ/e remains almost unaffected by the nuclear effects considered here and often neglected in the Monte-Carlo simulations, the absolute events rates are subject to important modifications. Received: 15 January 1999  相似文献   
99.
The influence of a soliton on the properties of the M?ssbauer effect resulting from the gamma-active nuclei situated at lattice sites in quasi-one-dimensional anharmonically organic molecular chains, in which the active nuclei emit gamma-photons, has been theoretically studied by a quantum vibrational model. An expression for the gamma-radiated M?ssbauer transition probability as a function of the temperature and non-linear interaction has been obtained. Finally we give some new properties of this effect from numerical calculation in such a case, which present a way to experimentally confirm the soliton existence in molecular crystals. Received 9 April 1998 and Received in final form 11 December 1998  相似文献   
100.
Raghavan Rangarajan 《Pramana》1999,53(6):1061-1067
We review various attempts to create the observed baryon asymmetry of the Universe. In particular, we consider models of GUT baryogenesis, baryogenesis via leptogenesis, the Affleck-Dine mechanism, electroweak baryogenesis and baryogenesis via topological defects and primordial black holes.  相似文献   
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