132 research outputs found
Stability and Decay Rates of Non-Isotropic Attractive Bose-Einstein Condensates
Non-Isotropic Attractive Bose-Einstein condensates are investigated with
Newton and inverse Arnoldi methods. The stationary solutions of the
Gross-Pitaevskii equation and their linear stability are computed. Bifurcation
diagrams are calculated and used to find the condensate decay rates
corresponding to macroscopic quantum tunneling, two-three body inelastic
collisions and thermally induced collapse.
Isotropic and non-isotropic condensates are compared. The effect of
anisotropy on the bifurcation diagram and the decay rates is discussed.
Spontaneous isotropization of the condensates is found to occur. The influence
of isotropization on the decay rates is characterized near the critical point.Comment: revtex4, 11 figures, 2 tables. Submitted to Phys. Rev.
Electrical resistivity at large temperatures: Saturation and lack thereof
Many transition metal compounds show saturation of the resistivity at high
temperatures, T, while the alkali-doped fullerenes and the high-Tc cuprates are
usually considered to show no saturation. We present a model of transition
metal compounds, showing saturation, and a model of alkali-doped fullerenes,
showing no saturation. To analyze the results we use the f-sum rule, which
leads to an approximate upper limit for the resistivity at large T. For some
systems and at low T, the resistivity increases so rapidly that this upper
limit is approached for experimental T. The resistivity then saturates. For a
model of transition metal compounds with weakly interacting electrons, the
upper limit corresponds to a mean free path consistent with the Ioffe-Regel
condition. For a model of the high Tc cuprates with strongly interacting
electrons, however, the upper limit is much larger than the Ioffe-Regel
condition suggests. Since this limit is not exceeded by experimental data, the
data are consistent with saturation also for the cuprates. After "saturation"
the resistivity usually grows slowly. For the alkali-doped fullerenes,
"saturation" can be considered to have happened already for T=0, due to
orientational disorder. For these systems, however, the resistivity grows so
rapidly after "saturation" that this concept is meaningless. This is due to the
small band width and to the coupling to the level energies of the important
phonons.Comment: 22 pages, RevTeX, 19 eps figures, additional material available at
http://www.mpi-stuttgart.mpg.de/andersen/fullerene
Multidimensional quantum solitons with nondegenerate parametric interactions: Photonic and Bose-Einstein condensate environments
We consider the quantum theory of three fields interacting via parametric and repulsive quartic couplings. This can be applied to treat photonic chi((2)) and chi((3)) interactions, and interactions in atomic Bose-Einstein condensates or quantum Fermi gases, describing coherent molecule formation together with a-wave scattering. The simplest two-particle quantum solitons or bound-state solutions of the idealized Hamiltonian, without a momentum cutoff, are obtained exactly. They have a pointlike structure in two and three dimensions-even though the corresponding classical theory is nonsingular. We show that the solutions can be regularized with a momentum cutoff. The parametric quantum solitons have much more realistic length scales and binding energies than chi((3)) quantum solitons, and the resulting effects could potentially be experimentally tested in highly nonlinear optical parametric media or interacting matter-wave systems. N-particle quantum solitons and the ground state energy are analyzed using a variational approach. Applications to atomic/molecular Bose-Einstein condensates (BEC's) are given, where we predict the possibility of forming coupled BEC solitons in three space dimensions, and analyze superchemistry dynamics
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