3,761 research outputs found
"Gray" BCS condensate of excitons and internal Josephson effect
It has been recently suggested that the Bose-Einstein condensate formed by
excitons in the dilute limit must be dark, i.e., not coupled to photons. Here,
we show that, under a density increase, the dark exciton condensate must
acquire a bright component due to carrier exchange in which dark excitons turn
bright. This however requires a density larger than a threshold which seems to
fall in the forbidden region of the phase separation between a dilute exciton
gas and a dense electron-hole plasma. The BCS-like condensation which is likely
to take place on the dense side, must then have a dark and a bright component -
which makes it "gray". It should be possible to induce an internal Josephson
effect between these two coherent components, with oscillations of the
photoluminescence as a strong proof of the existence for this "gray" BCS-like
exciton condensate.Comment: 4 pages, typo correcte
Statistical analysis of the growth and morphology of the filamentous microbe Streptomyces coelicolor
On Koopman-von Neumann Waves II
In this paper we continue the study, started in [1], of the operatorial
formulation of classical mechanics given by Koopman and von Neumann (KvN) in
the Thirties. In particular we show that the introduction of the KvN Hilbert
space of complex and square integrable "wave functions" requires an enlargement
of the set of the observables of ordinary classical mechanics. The possible
role and the meaning of these extra observables is briefly indicated in this
work. We also analyze the similarities and differences between non selective
measurements and two-slit experiments in classical and quantum mechanics.Comment: 18+1 pages, 1 figure, misprints fixe
Topological stripelike coreless textures with inner incommensurability in two-dimensional Heisenberg antiferromagnet
For two-dimensional Heisenberg antiferromagnet we present an analysis of
topological coreless excitations having a stripe form. These textures are
characterized by singularities at boundaries. A detailed classification of the
stripe textures results in a certain analogy with the coreless excitations in
phase: Mermin-Ho and Anderson-Toulouse coreless vortices. The
excitations of the last type may have a low bulk energy. The stripe textures
may be observed as an occurrence of short-range incommensurate order in the
antiferromagnetic environment
Collective oscillations of a Fermi gas near a Feshbach resonance
A sum rule approach is used to calculate the zero temperature oscillation
frequencies of a two component trapped atomic Fermi gas in the BCS-Bose
Einstein condensation crossover region. These sum rules are evaluated using a
local density approximation which explicitly includes Feshbach molecules.
Breathing modes show non-monotonic behavior as a function of the interaction
strength, while quadrupole modes are insensitive to interactions for both
spherically symmetric and axially symmetric traps. Quantitative agreement is
found with experiments on atomic system and with other theoretical
approaches.Comment: 7 pages, 4 figure
Calculation of NMR Properties of Solitons in Superfluid 3He-A
Superfluid 3He-A has domain-wall-like structures, which are called solitons.
We calculate numerically the structure of a splay soliton. We study the effect
of solitons on the nuclear-magnetic-resonance spectrum by calculating the
frequency shifts and the amplitudes of the soliton peaks for both longitudinal
and transverse oscillations of magnetization. The effect of dissipation caused
by normal-superfluid conversion and spin diffusion is calculated. The
calculations are in good agreement with experiments, except a problem in the
transverse resonance frequency of the splay soliton or in magnetic-field
dependence of reduced resonance frequencies.Comment: 15 pages, 10 figures, updated to the published versio
Bound states in a quasi-two-dimensional Fermi gas
We consider the problem of N identical fermions of mass M and one
distinguishable particle of mass m interacting via short-range interactions in
a confined quasi-two-dimensional (quasi-2D) geometry. For N=2 and mass ratios
M/m<13.6, we find non-Efimov trimers that smoothly evolve from 2D to 3D. In the
limit of strong 2D confinement, we show that the energy of the N+1 system can
be approximated by an effective two-channel model. We use this approximation to
solve the 3+1 problem and we find that a bound tetramer can exist for mass
ratios M/m as low as 5 for strong confinement, thus providing the first example
of a universal, non-Efimov tetramer involving three identical fermions.Comment: 5 pages, 4 figure
Surface Energy in Cold Asymmetrical Fermion Superfluids
We derive the energy of the surface between the normal and superfluid
components of a mixed phase of a system composed of two particle species with
different densities. The surface energy is obtained by the integration of the
free energy density in the interface between the two phases. We show that the
mixed phase remains as the favored ground state over the gapless phase in weak
coupling. We find that the surface energy effects emerge only at strong
coupling.Comment: 12 pages, 2 figures, typos corrected, published versio
Pair condensation and bound states in fermionic systems
We study the finite temperature-density phase diagram of an attractive
fermionic system that supports two-body (dimer) and three-body (trimer) bound
states in free space. Using interactions characteristic for nuclear systems, we
obtain the critical temperature T_c2 for the superfluid phase transition and
the limiting temperature T_c3 for the extinction of trimers. The phase diagram
features a Cooper-pair condensate in the high-density, low-temperature domain
which, with decreasing density, crosses over to a Bose condensate of strongly
bound dimers. The high-temperature, low-density domain is populated by trimers
whose binding energy decreases toward the density-temperature domain occupied
by the superfluid and vanishes at a critical temperature T_c3 > T_c2.Comment: 11 pages, 4 figures, uses RevTex; v2: 12 pages, 4 figures, matches
published versio
Coherent population trapping in the stochastic limit
A 2-level atom with degenerate ground state interacting with a quantum field
is investigated. We show, that the field drives the state of the atom to a
stationary state, which is non-unique, but depends on the initial state of the
system through some conserved quantities. This non-uniqueness follows from the
degeneracy of the ground state of the atom, and when the ground subspace is
two-dimensional, the family of stationary states will depend on a
one-dimensional parameter. Only one of the stationary states in this family is
a pure state, and this state coincides with the known non-coupled population
trapped state (zero population in the excited level. Another one stationary
state corresponds to an equal weight mixture of the excited level and of the
coupled state.Comment: 13 pages, LaTe
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