673 research outputs found
Bose-Einstein-condensed gases with arbitrary strong interactions
Bose-condensed gases are considered with an effective interaction strength
varying in the whole range of the values between zero and infinity. The
consideration is based on the usage of a representative statistical ensemble
for Bose systems with broken global gauge symmetry. Practical calculations are
illustrated for a uniform Bose gas at zero temperature, employing a
self-consistent mean-field theory, which is both conserving and gapless.Comment: Latex file, 23 pages, 4 figure
Microscopic calculation of 240Pu scission with a finite-range effective force
Hartree-Fock-Bogoliubov calculations of hot fission in
have been performed with a newly-implemented code that uses the D1S
finite-range effective interaction. The hot-scission line is identified in the
quadrupole-octupole-moment coordinate space. Fission-fragment shapes are
extracted from the calculations. A benchmark calculation for
is obtained and compared to results in the literature. In
addition, technical aspects of the use of HFB calculations for fission studies
are examined in detail. In particular, the identification of scission
configurations, the sensitivity of near-scission calculations to the choice of
collective coordinates in the HFB iterations, and the formalism for the
adjustment of collective-variable constraints are discussed. The power of the
constraint-adjustment algorithm is illustrated with calculations near the
critical scission configurations with up to seven simultaneous constraints.Comment: 18 pages, 24 figures, to be published in Physical Review
Gapless Hartree-Fock-Bogoliubov Approximation for Bose Gases
A dilute Bose system with Bose-Einstein condensate is considered. It is shown
that the Hartree-Fock-Bogolubov approximation can be made both conserving as
well as gapless. This is achieved by taking into account all physical
normalization conditions, that is, the normalization condition for the
condensed particles and that for the total number of particles. Two Lagrange
multipliers, introduced for preserving these normalization conditions, make the
consideration completely self-consistent.Comment: Latex file, 22 pages, 2 figure
Bogoliubov theory of Feshbach molecules in the BEC-BCS crossover
We present the Bogoliubov theory for the Bose-Einstein condensation of
Feshbach molecules in a balanced Fermi mixture. Because the Bogoliubov theory
includes (Gaussian) fluctuations, we can in this manner accurately incorporate
both the two-body and many-body aspects of the BEC-BCS crossover that occurs
near a Feshbach resonance. We apply the theory in particular to the very broad
Feshbach resonance in atomic Li-6 at a magnetic field of B_0 = 834 G and find
good agreement with experiments in that case. The BEC-BCS crossover for more
narrow Feshbach resonances is also discussed.Comment: 13 pages of RevTex and 12 Figures. Submitted for publication in
Physical review
On second-order differential equations with highly oscillatory forcing terms
We present a method to compute efficiently solutions of systems of ordinary differential equations that possess highly oscillatory forcing terms. This approach is based on asymptotic expansions in inverse powers of the oscillatory parameter,and features two fundamental advantages with respect to standard ODE solvers: rstly, the construction of the numerical solution is more efficient when the system is highly oscillatory, and secondly, the cost of the computation is essentially independent of the oscillatory parameter. Numerical examples are provided, motivated by problems in electronic engineering
Solutions of the Klein-Gordon equation on manifolds with variable geometry including dimensional reduction
We develop the recent proposal to use dimensional reduction from the
four-dimensional space-time D=(1+3) to the variant with a smaller number of
space dimensions D=(1+d), d < 3, at sufficiently small distances to construct a
renormalizable quantum field theory. We study the Klein-Gordon equation on a
few toy examples ("educational toys") of a space-time with variable special
geometry, including a transition to a dimensional reduction. The examples
considered contain a combination of two regions with a simple geometry
(two-dimensional cylindrical surfaces with different radii) connected by a
transition region. The new technique of transforming the study of solutions of
the Klein-Gordon problem on a space with variable geometry into solution of a
one-dimensional stationary Schr\"odinger-type equation with potential generated
by this variation is useful. We draw the following conclusions: (1) The signal
related to the degree of freedom specific to the higher-dimensional part does
not penetrate into the smaller-dimensional part because of an inertial force
inevitably arising in the transition region (this is the centrifugal force in
our models). (2) The specific spectrum of scalar excitations resembles the
spectrum of the real particles; it reflects the geometry of the transition
region and represents its "fingerprints". (3) The parity violation due to the
asymmetric character of the construction of our models could be related to
violation of the CP symmetry.Comment: laTeX file, 9 pages, 8 figures. Significant corrections in the title,
abstract, text. Corrected formulas and figures. Added new references,
amendments in English. Acceptred for publication in Theoretical and
Mathematical Physics. To appear in vol. 167, may 201
A pair potential supporting a mixed mean-field / BCS- phase
We construct a Hamiltonian which in a scaling limit becomes equivalent to one
that can be diagonalized by a Bogoliubov transformation. There may appear
simultaneously a mean-field and a superconducting phase. They influence each
other in a complicated way. For instance, an attractive mean field may
stimulate the superconducting phase and a repulsive one may destroy it.Comment: 11 pages, 5 figures, LaTe
Justification of c-Number Substitutions in Bosonic Hamiltonians
The validity of substituting a c-number for the mode operator
is established rigorously in full generality, thereby verifying one aspect of
Bogoliubov's 1947 theory. This substitution not only yields the correct value
of thermodynamic quantities like the pressure or ground state energy, but also
the value of that maximizes the partition function equals the true
amount of condensation in the presence of a gauge-symmetry breaking term -- a
point that had previously been elusive.Comment: RevTeX4, 4pages; minor modifications in the text; final version, to
appear in Phys. Rev. Let
Existence of Long-Range Order for Trapped Interacting Bosons
We derive an inequality governing ``long range'' order for a localized
Bose-condensed state, relating the condensate fraction at a given temperature
with effective curvature radius of the condensate and total particle number.
For the specific example of a one-dimensional, harmonically trapped dilute Bose
condensate, it is shown that the inequality gives an explicit upper bound for
the Thomas-Fermi condensate size which may be tested in current experiments.Comment: 4 pages, 1 figure, RevTex4. Title changed at the request of editors;
to appear in Phys. Rev. Letter
Ballistic effects in a proximity induced superconducting diffusive metal
Using a Scanning Tunneling Microscope (STM), we investigate the Local Density
of States (LDOS) of artificially fabricated normal metal nano-structures in
contact with a superconductor. Very low temperature local spectroscopic
measurements (100 mK) reveal the presence of well defined subgap peaks at
energy |E|<Delta in the LDOS at various positions of the STM tip. Although no
clear correlations between the LDOS and the shape of the samples have emerged,
some of the peak features suggest they originate from quasi-particle bound
states within the normal metal structures (De Gennes St James states).
Refocusing of electronic trajectories induced by the granular srtucture of the
samples can explain the observation of spatially uncorrelated interference
effects in a non-ballistic medium.Comment: 4 pages, 4 figure
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