2,248 research outputs found
Integrable two-channel p_x+ip_y-wave superfluid model
We present a new two-channel integrable model describing a system of spinless
fermions interacting through a p-wave Feshbach resonance. Unlike the BCS-BEC
crossover of the s-wave case, the p-wave model has a third order quantum phase
transition. The critical point coincides with the deconfinement of a single
molecule within a BEC of bound dipolar molecules. The exact many-body
wavefunction provides a unique perspective of the quantum critical region
suggesting that the size of the condensate wavefunction, that diverges
logarithmically with the chemical potential, could be used as an experimental
indicator of the phase transition.Comment: 4 pages, 4 figure
Exact solution of the spin-isospin proton-neutron pairing Hamiltonian
The exact solution of proton-neutron isoscalar-isovector (T=0,1) pairing
Hamiltonian with non-degenerate single-particle orbits and equal pairing
strengths (g_{T=1}= g_{T=0}) is presented for the first time. The Hamiltonian
is a particular case of a family of integrable SO(8) Richardson-Gaudin (RG)
models. The exact solution of the T=0,1 pairing Hamiltonian is reduced to a
problem of 4 sets of coupled non linear equations that determine the spectral
parameters of the complete set of eigenstates. The microscopic structure of
individual eigenstates is analyzed in terms of evolution of the spectral
parameters in the complex plane for system of A=80 nucleons. The spectroscopic
trends of the exact solutions are discussed in terms of generalized rotations
in isospace.Comment: 4 pages, 2 figure
Exactly-solvable models of proton and neutron interacting bosons
We describe a class of exactly-solvable models of interacting bosons based on
the algebra SO(3,2). Each copy of the algebra represents a system of neutron
and proton bosons in a given bosonic level interacting via a pairing
interaction. The model that includes s and d bosons is a specific realization
of the IBM2, restricted to the transition regime between vibrational and
gamma-soft nuclei. By including additional copies of the algebra, we can
generate proton-neutron boson models involving other boson degrees of freedom,
while still maintaining exact solvability. In each of these models, we can
study not only the states of maximal symmetry, but also those of mixed
symmetry, albeit still in the vibrational to gamma-soft transition regime.
Furthermore, in each of these models we can study some features of F-spin
symmetry breaking. We report systematic calculations as a function of the
pairing strength for models based on s, d, and g bosons and on s, d, and f
bosons. The formalism of exactly-solvable models based on the SO(3,2) algebra
is not limited to systems of proton and neutron bosons, however, but can also
be applied to other scenarios that involve two species of interacting bosons.Comment: 8 pages, 3 figures. Submitted to Phys.Rev.
Exact Solution of the Isovector Proton Neutron Pairing Hamiltonian
The complete exact solution of the T=1 neutron-proton pairing Hamiltonian is
presented in the context of the SO(5) Richardson-Gaudin model with
non-degenerate single-particle levels and including isospin-symmetry breaking
terms. The power of the method is illustrated with a numerical calculation for
Ge for a model space which is out of reach of modern
shell-model codes.Comment: To be published by Physical Review Letter
A schematic model for QCD I: Low energy meson states
A simple model for QCD is presented, which is able to reproduce the meson
spectrum at low energy. The model is a Lipkin type model for quarks coupled to
gluons. The basic building blocks are pairs of quark-antiquarks coupled to a
definite flavor and spin. These pairs are coupled to pairs of gluons with spin
zero. The multiplicity problem, which dictates that a given experimental state
can be described in various manners, is removed when a particle-mixing
interaction is turned on. In this first paper of a series we concentrates on
the discussion of meson states at low energy, the so-called zero temperature
limit of the theory. The treatment of baryonic states is indicated, also.Comment: 29 pages, 6 figures. submitted to Phys. Rev.
Exactly solvable pairing Hamiltonian for heavy nuclei
We present a new exactly solvable Hamiltonian with a separable pairing
interaction and non-degenerate single-particle energies. It is derived from the
hyperbolic family of Richardson-Gaudin models and possesses two free
parameters, one related to an interaction cutoff and the other to the pairing
strength. These two parameters can be adjusted to give an excellent
reproduction of Gogny self-consistent mean-field calculations in the canonical
basis.Comment: 4 pages, 3 figure
Electroconvection in a Suspended Fluid Film: A Linear Stability Analysis
A suspended fluid film with two free surfaces convects when a sufficiently
large voltage is applied across it. We present a linear stability analysis for
this system. The forces driving convection are due to the interaction of the
applied electric field with space charge which develops near the free surfaces.
Our analysis is similar to that for the two-dimensional B\'enard problem, but
with important differences due to coupling between the charge distribution and
the field. We find the neutral stability boundary of a dimensionless control
parameter as a function of the dimensionless wave number .
, which is proportional to the square of the applied voltage, is
analogous to the Rayleigh number. The critical values and
are found from the minimum of the stability boundary, and its
curvature at the minimum gives the correlation length . The
characteristic time scale , which depends on a second dimensionless
parameter , analogous to the Prandtl number, is determined from the
linear growth rate near onset. and are coefficients in the
Ginzburg-Landau amplitude equation which describes the flow pattern near onset
in this system. We compare our results to recent experiments.Comment: 36 pages, 7 included eps figures, submitted to Phys Rev E. For more
info, see http://mobydick.physics.utoronto.ca
Boundary Limitation of Wavenumbers in Taylor-Vortex Flow
We report experimental results for a boundary-mediated wavenumber-adjustment
mechanism and for a boundary-limited wavenumber-band of Taylor-vortex flow
(TVF). The system consists of fluid contained between two concentric cylinders
with the inner one rotating at an angular frequency . As observed
previously, the Eckhaus instability (a bulk instability) is observed and limits
the stable wavenumber band when the system is terminated axially by two rigid,
non-rotating plates. The band width is then of order at small
() and agrees well with
calculations based on the equations of motion over a wide -range.
When the cylinder axis is vertical and the upper liquid surface is free (i.e.
an air-liquid interface), vortices can be generated or expelled at the free
surface because there the phase of the structure is only weakly pinned. The
band of wavenumbers over which Taylor-vortex flow exists is then more narrow
than the stable band limited by the Eckhaus instability. At small
the boundary-mediated band-width is linear in . These results are
qualitatively consistent with theoretical predictions, but to our knowledge a
quantitative calculation for TVF with a free surface does not exist.Comment: 8 pages incl. 9 eps figures bitmap version of Fig
Thermally Induced Fluctuations Below the Onset of Rayleigh-B\'enard Convection
We report quantitative experimental results for the intensity of
noise-induced fluctuations below the critical temperature difference for Rayleigh-B\'enard convection. The structure factor of the fluctuating
convection rolls is consistent with the expected rotational invariance of the
system. In agreement with predictions based on stochastic hydrodynamic
equations, the fluctuation intensity is found to be proportional to
where . The
noise power necessary to explain the measurements agrees with the prediction
for thermal noise. (WAC95-1)Comment: 13 pages of text and 4 Figures in a tar-compressed and uuencoded file
(using uufiles package). Detailed instructions of unpacking are include
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