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Results are presented for simulations of electron-positron pair production in relativistic heavy-ion collisions leading to electron capture and positron ejection. We apply a two-center relativistic continuum distorted-wave model to represent the electron/positron dynamics during the collision process. The results are compared with experimental cross-section data for ${\rm La}^{57+}$ and ${\rm Au}^{79+}$ impact on Gold, Silver and Copper targets. The theory is in very good agreement with experiment, verifying the result that the process increases in importance with both collision energy and target atomic number, and improves upon previous simulations of this process., Comment: 4 pages 2 figures. Submitted to Phys Rev A

The effect of an applied magnetic field on the temperature at the maximum of the ZFC magnetization, $M_{ZFC}$, is studied using the recently obtained analytic results of Coffey et al. (Phys. Rev. Lett. {\bf 80}(1998) 5655) for the prefactor of the N\'{e}el relaxation time which allow one to precisely calculate the prefactor in the N\'{e}el-Brown model and thus the blocking temperature as a function of the coefficients of the Taylor series expansion of the magnetocrystalline anisotropy. The present calculations indicate that even a precise determination of the prefactor in the N\'{e}el-Brown theory, which always predicts a monotonic decrease of the relaxation time with increasing field, is insufficient to explain the effect of an applied magnetic field on the temperature at the maximum of the ZFC magnetization. On the other hand, we find that the non linear field-dependence of the magnetization along with the magnetocrystalline anisotropy appears to be of crucial importance to the existence of this maximum., Comment: 14 LaTex209 pages, 6 EPS figures. To appear in J. Phys.: Condensed Matter

Classical escape rates of uniaxial spin systems are characterized by a prefactor differing from and much smaller than that of the particle problem, since the maximum of the spin energy is attained everywhere on the line of constant latitude: theta=const, 0 =< phi =< 2*pi. If a transverse field is applied, a saddle point of the energy is formed, and high, moderate, and low damping regimes (similar to those for particles) appear. Here we present the first analytical and numerical study of crossovers between the uniaxial and other regimes for spin systems. It is shown that there is one HD-Uniaxial crossover, whereas at low damping the uniaxial and LD regimes are separated by two crossovers., Comment: 4 PR pages, 3 figures, final published version

The energy dependence of the nonradiative electron capture cross-section is discussed in the relativistic domain. A simple analytic expression is obtained for inner-shell transitions using second-order perturbation theory. We have confirmed that the leading-order term is found to have the following energy dependence: Q ~ E^{-1}ln^2 E. This is attributed to a combination of kinematic features of the process and retardation effects. Electron capture without change of spin is the dominant transition., Comment: LaTeX, 12 pages (Submitted to J.Phys.B - under review)