831 research outputs found
Interactions between macromolecules and membranes: their effects on beer quality
When filtering beer at pore sizes of 0.45 μm and below, some desirable components
may be lost, even though they are orders of magnitude smaller than the pores. In this
work, a model beer solution of pure components has been filtered through 0.2 μm
membranes to investigate this problem. Starch (a model for the long chain
carbohydrates) at a concentration of 1500 mg.l-1 and casein (a model for the protein
fraction) at a concentration of 150 mg.l-1 were found to result in reduced permeate
fluxes in the region of 20–40 l.m-2.h-1. [Continues.
Polaron and bipolaron dispersion curves in one dimension for intermediate coupling
Bipolaron energies are calculated as a function of wave vector by a
variational method of Gurari appropriate for weak or intermediate coupling
strengths, for a model with electron-phonon interactions independent of phonon
wave vectors and a short-ranged Coulomb repulsion. It is assumed that the bare
electrons have a constant effective mass. A two-parameter trial function is
taken for the relative motion of the two electrons in the bipolaron. Energies
of bipolarons are compared with those of two single polarons as a function of
wave vector for various parameter values. Results for effective masses at the
zone center are also obtained. Comparison is made with data of other authors
for bipolarons in the Hubbard-Holstein model, which differs mainly from the
present model in that it has a tight-binding band structure for the bare
electrons.Comment: 11 pages including six figures. Physical Review B, to be publishe
Klein-Gordon Equation for Quark Pairs in Color Superconductor
The wave equation is derived for quark pairs in color superconductor in the
regime of low density / strong coupling.Comment: 4 pages, no figure
Invasive Signal Crayfish in the UK: Survey Methods to Inform Evidence-based Management
With invasive crayfish becoming increasingly widespread, evidence-based management is crucial to
protect freshwater ecosystems. Knowledge of the structure and function of invasive crayfish
populations allows for an effective evaluation of management efforts. Recent methodological
developments have enabled the first truly quantitative studies of UK invasive crayfish populations in
the field. This was achieved by the ‘triple drawdown’ (TDD) survey approach. In this article, we
explore current survey approaches and their limitations, and we introduce the TDD method with its
implications for crayfish survey, policy development and management
Optical conductivity of polaronic charge carriers
The optical conductivity of charge carriers coupled to quantum phonons is
studied in the framework of the one-dimensional spinless Holstein model. For
one electron, variational diagonalisation yields exact results in the
thermodynamic limit, whereas at finite carrier density analytical
approximations based on previous work on single-particle spectral functions are
obtained. Particular emphasis is put on deviations from weak-coupling,
small-polaron or one-electron theories occurring at intermediate coupling
and/or finite carrier density. The analytical results are in surprisingly good
agreement with exact data, and exhibit the characteristic polaronic excitations
observed in experiments on manganites.Comment: 23 pages, 11 figure
Mobile small polaron
Extending the Froehlich polaron problem to a discrete ionic lattice we study
a polaronic state with a small radius of the wave function but a large size of
the lattice distortion. We calculate the energy dispersion and the effective
mass of the polaron with the 1/\lambda perturbation theory and with the exact
Monte Carlo method in the nonadiabatic and adiabatic regimes, respectively. The
``small'' Froehlich polaron is found to be lighter than the small Holstein
polaron by one or more orders of magnitude.Comment: 4 pages, 4 figures, published versio
Modification of a charged-Bose-gas model for observed room-temperature superconductivity in narrow channels through films of oxidised atactic polypropylene
Reasons have been found for thinking that the minimum diameter of channels of
a given length to support superconductivity at room temperature through films
of oxidised atactic polypropylene (OAPP) is considerably larger than found in a
model for Bose condensation in an array of nanofilaments [D.M. Eagles, Phil.
Mag. 85, 1931 (2005)] used previously. This model was introduced to interpret
experimental results dating from 1988 on OAPP. The channels are thought to be
of larger diameter than believed before because, for an N-S-N system where the
superconductor consists of an array of single-walled carbon nanotubes, the
resistance, for good contacts, is R_Q/2N, where N is the number of nanotubes
and R_Q=12.9 kOhm [See e.g. M. Ferrier et al., Solid State Commun. 131, 615
(2004)]. We assume this would be 2R_Q/N for a triplet superconductor with all
spins in the same direction and no orbital degeneracy, which may be the case
for nanofilaments in OAPP. Hence one may infer a minimum number of filaments
for a given resistance. In the present model, the E(K) curve for the bosons is
taken to be of a Bogoliubov form, but with a less steep initial linear term in
the dispersion at T_c than occurs at low T. This form is different from the
simple linear plus quadratic dispersion, with a steeper initial slope, used in
my 2005 paper. A combination of theory and experimental data has been used to
find approximate constraints on parameters appearing in the theory.Comment: Publication details added. Correction to bracketing in Eq. (1), which
was incorrect in v1 of preprint and in the published articl
Evaluation of the BCS Approximation for the Attractive Hubbard Model in One Dimension
The ground state energy and energy gap to the first excited state are
calculated for the attractive Hubbard model in one dimension using both the
Bethe Ansatz equations and the variational BCS wavefunction. Comparisons are
provided as a function of coupling strength and electron density. While the
ground state energies are always in very good agreement, the BCS energy gap is
sometimes incorrect by an order of magnitude, particularly at half-filling.
Finite size effects are also briefly discussed for cases where an exact
solution in the thermodynamic limit is not possible. In general, the BCS result
for the energy gap is poor compared to the exact result.Comment: 25 pages, 5 Postscript figure
- …