7,895 research outputs found
Universal scaling in BCS superconductivity in two dimensions in non-s waves
The solutions of a renormalized BCS model are studied in two space dimensions
in , and waves for finite-range separable potentials. The gap
parameter, the critical temperature , the coherence length and the
jump in specific heat at as a function of zero-temperature condensation
energy exhibit universal scalings. In the weak-coupling limit, the present
model yields a small and large appropriate to those for high-
cuprates. The specific heat, penetration depth and thermal conductivity as a
function of temperature show universal scaling in and waves.Comment: 11 pages, LATEX, 4 postscript figures embedded using eps
Self-trapping of a binary Bose-Einstein condensate induced by interspecies interaction
The problem of self-trapping of a Bose-Einstein condensate (BEC) and a binary
BEC in an optical lattice (OL) and double well (DW) is studied using the
mean-field Gross-Pitaevskii equation. For both DW and OL, permanent
self-trapping occurs in a window of the repulsive nonlinearity of the GP
equation: . In case of OL, the critical nonlinearities
and correspond to a window of chemical potentials
defining the band gap(s) of the periodic OL. The
permanent self-trapped BEC in an OL usually represents a breathing oscillation
of a stable stationary gap soliton. The permanent self-trapped BEC in a DW, on
the other hand, is a dynamically stabilized state without any stationary
counterpart. For a binary BEC with intraspecies nonlinearities outside this
window of nonlinearity, a permanent self trapping can be induced by tuning the
interspecies interaction such that the effective nonlinearities of the
components fall in the above window
Two phase transitions in (s+id)-wave Bardeen-Cooper-Schrieffer superconductivity
We establish universal behavior in temperature dependencies of some
observables in -wave BCS superconductivity in the presence of a weak
wave. There also could appear a second second-order phase transition. As
temperature is lowered past the usual critical temperature , a less
ordered superconducting phase is created in wave, which changes to a more
ordered phase in wave at (). The presence of two phase
transitions manifest in two jumps in specific heat at and . The
temperature dependencies of susceptibility, penetration depth, and thermal
conductivity also confirm the new phase transition.Comment: 6 pages, 5 post-script figures
Thermal fluctuations in the lattice Boltzmann method for non-ideal fluids
We introduce thermal fluctuations in the lattice Boltzmann method for
non-ideal fluids. A fluctuation-dissipation theorem is derived within the
Langevin framework and applied to a specific lattice Boltzmann model that
approximates the linearized fluctuating Navier-Stokes equations for fluids
based on square-gradient free energy functionals. The obtained thermal noise is
shown to ensure equilibration of all degrees of freedom in a simulation to high
accuracy. Furthermore, we demonstrate that satisfactory results for most
practical applications of fluctuating hydrodynamics can already be achieved
using thermal noise derived in the long wavelength-limit.Comment: 15 pages, 5 figure
Long-range interactions of hydrogen atoms in excited states. III. nS-1S interactions for n >= 3
The long-range interaction of excited neutral atoms has a number of
interesting and surprising properties, such as the prevalence of long-range,
oscillatory tails, and the emergence of numerically large can der Waals C_6
coefficients. Furthermore, the energetically quasi-degenerate nP states require
special attention and lead to mathematical subtleties. Here, we analyze the
interaction of excited hydrogen atoms in nS states (3 <= n <= 12) with
ground-state hydrogen atoms, and find that the C_6 coefficients roughly grow
with the fourth power of the principal quantum number, and can reach values in
excess of 240,000 (in atomic units) for states with n = 12. The nonretarded van
der Waals result is relevant to the distance range R << a_0/alpha, where a_0 is
the Bohr radius and alpha is the fine-structure constant. The Casimir-Polder
range encompasses the interatomic distance range a_0/alpha << R << hbar c/L,
where L is the Lamb shift energy. In this range, the contribution of
quasi-degenerate excited nP states remains nonretarded and competes with the
1/R^2 and 1/R^4 tails of the pole terms which are generated by lower-lying mP
states with 2 <= m <= n-1, due to virtual resonant emission. The dominant pole
terms are also analyzed in the Lamb shift range R >> hbar c/L. The familiar
1/R^7 asymptotics from the usual Casimir-Polder theory is found to be
completely irrelevant for the analysis of excited-state interactions. The
calculations are carried out to high precision using computer algebra in order
to handle a large number of terms in intermediate steps of the calculation, for
highly excited states.Comment: 17 pages; RevTe
Virtual Resonant Emission and Oscillatory Long-Range Tails in van der Waals Interactions of Excited States: QED Treatment and Applications
We report on a quantum electrodynamic (QED) investigation of the interaction
between a ground state atom with another atom in an excited state. General
expressions, applicable to any atom, are indicated for the long-range tails
which are due to virtual resonant emission and absorption into and from vacuum
modes whose frequency equals the transition frequency to available lower-lying
atomic states. For identical atoms, one of which is in an excited state, we
also discuss the mixing term which depends on the symmetry of the two-atom wave
function (these evolve into either the gerade or the ungerade state for close
approach), and we include all nonresonant states in our rigorous QED treatment.
In order to illustrate the findings, we analyze the fine-structure resolved van
der Waals interaction for nD-1S hydrogen interactions with n=8,10,12 and find
surprisingly large numerical coefficients.Comment: 6 pages; RevTe
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