Results 1 - 10 of 34841
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[en] The time delay, as defined via phase shifts, does not apply to atom-atom collisions because of the semiclassical nature of the system. In this limit the contribution of one partial wave to the cross section is negligible. Therefore, we analyze time delay for the scattering amplitude and show that some new phenomena may occur, which cannot be explained by the time delay for a single phase shift. The time delay, which is averaged over all scattering angles, shows structure corresponding to several delay mechanisms. We also show that the lifetime of a resonance state, formed in a collision, may be considerably shorter than expected from the theory of resonance scattering
[en] We study the dynamics of a two-component Bose-Einstein condensate where the two components are coupled via an optical lattice. In particular, we focus on the dynamics as one drives the system through a critical point of a first-order phase transition characterized by a jump in the internal populations. Solving the time-dependent Gross-Pitaevskii equation, we analyze the breakdown of adiabaticity, impact of nonlinear atom-atom scattering, and role of a harmonic trapping potential. Our findings demonstrate that the phase transition is resilient to both contact interaction between atoms and external trapping confinement.
[en] Three recently proposed semiclassical methods for nonadiabatic scattering are numerically compared for a one-dimensional curve crossing model. The generalized surface hopping approach has been found to be in excellent agreement with quantum results over a wide range of energies. The simpler variant of the self-consistent eikonal method follows the quantum results well at low energies. At higher energies it still follows the correct overall trend of decreasing transition probability, but it does not quantitatively reproduce the oscillations in the transition probability. The classical analog technique provides relatively good agreement with the quantum results over the entire range of energies examined. It is found in the application of this last method that there are typically four to eight stationary phase contributions at each energy and it is crucial to correctly describe the interference between these terms. It is also necessary to include uniform and analytically continued contributions at many energies, complicating the application of the method
[en] An analytical model is considered to the e → c change of Hund's coupling cases in the X(J=1) + Y(1S0) collision of polarized atoms. Deviation from a widely used approximation of a sudden change of the coupling scheme is shown to exceed 10% for realistic interatomic potentials. The proposed analytical model for population redistribution in quasimolecular states that results from passage through a region where the type of coupling changes is in good agreement with the reported numerical calculations for the dispersion interatomic interaction. 27 refs., 4 figs., 2 tabs
[en] We model collisionless collective conversion of a degenerate Fermi gas of atoms into bosonic molecules via a Feshbach resonance, treating the bosonic molecules as a classical field and seeding the pairing amplitudes with random phases. A dynamical instability of the Fermi sea against association with molecules drives the conversion. The model qualitatively reproduces several experimental observations [Regal et al., Nature (London) 424, 47 (2003)]. We predict that the initial temperature of the Fermi gas sets the limit for the efficiency of atom-molecule conversion
[en] The dependence of two-level systems in disordered atomic chain on pressure, both positive and negative was studied numerically. The disorder was produced through the use of interatomic pair potentials having more than one energy minimum. It was found that there exists a correlation between the energy separation of the minima of two-level systems Δ and the variation of this separation with pressure. The correlation may have either positive or negative sign, implying that the asymmetry of two-level systems may in average increase or decrease with pressure depending on the interplay of different interactions between atoms in disordered state. The values of Δ depend on the sign of pressure.
[en] SIGMA is a computer code for generating multi-group, neutral-plasma cross sections for input to the neutron transport codes DTFX and TWOTRAN. Only cross sections for hydrogen plasmas are considered at present. At the option of the user, either 1/v, P-O or non-1/v, P-1 cross sections can be generated
[en] A simple model for the study of anisotropy is proposed. It is found that the analysis of the anisotropy can be made by a direct examination of the alignment signal. This led to a simple determination of the relaxation coefficients. In the case of isotropy the results obtained are identical with the classical results already known. Different cases are examined according to the direction of the external applied magnetic field and the direction of the detected signal. (author)
[en] A numerical study is performed for the locking of an electronic angular momentum j = 1 to the molecular axis during a collision of two atoms interacting via potential proportional to an inverse power of interatomic distance, R. Limitations to the notion of the locking radius and slipping probability are discussed in connection with the steepness of the interaction governed by the exponent n. Numerical calculations confirm our earlier analytical result: the optimal criterion for determination of the locking radius is a condition for the accumulated phase difference between two molecular states. (author)