8,613 research outputs found
New selection rules for resonant Raman scattering on quantum wires
The bosonisation technique is used to calculate the resonant Raman spectrum
of a quantum wire with two electronic sub-bands occupied. Close to resonance,
the cross section at frequencies in the region of the inter sub-band
transitions shows distinct peaks in parallel polarisation of the incident and
scattered light that are signature of collective higher order spin density
excitations. This is in striking contrast to the conventional selection rule
for non-resonant Raman scattering according to which spin modes can appear only
in perpendicular polarisation. We predict a new selection rule for the
excitations observed near resonance, namely that, apart from charge density
excitations, only spin modes with positive group velocities can appear as peaks
in the spectra in parallel configuration close to resonance. The results are
consistent with all of the presently available experimental data.Comment: 7 pages, 2 figure
Magnetic Fourier Integral Operators
In some previous papers we have defined and studied a 'magnetic'
pseudodifferential calculus as a gauge covariant generalization of the Weyl
calculus when a magnetic field is present. In this paper we extend the standard
Fourier Integral Operators Theory to the case with a magnetic field, proving
composition theorems, continuity theorems in 'magnetic' Sobolev spaces and
Egorov type theorems. The main application is the representation of the
evolution group generated by a 1-st order 'magnetic' pseudodifferential
operator (in particular the relativistic Schr\"{o}dinger operator with magnetic
field) as such a 'magnetic' Fourier Integral Operator. As a consequence of this
representation we obtain some estimations for the distribution kernel of this
evolution group and a result on the propagation of singularities
Delineation of the Native Basin in Continuum Models of Proteins
We propose two approaches for determining the native basins in off-lattice
models of proteins. The first of them is based on exploring the saddle points
on selected trajectories emerging from the native state. In the second
approach, the basin size can be determined by monitoring random distortions in
the shape of the protein around the native state. Both techniques yield the
similar results. As a byproduct, a simple method to determine the folding
temperature is obtained.Comment: REVTeX, 6 pages, 5 EPS figure
A review of implant provision for hypodontia patients within a Scottish referral centre
Background: Implant treatment to replace congenitally missing teeth often involves multidisciplinary input in a secondary care environment. High quality patient care requires an in-depth knowledge of treatment requirements.
Aim: This service review aimed to determine treatment needs, efficiency of service and outcomes achieved in hypodontia patients. It also aimed to determine any specific difficulties encountered in service provision, and suggest methods to overcome these.
Methods: Hypodontia patients in the Unit of Periodontics of the Scottish referral centre under consideration, who had implant placement and fixed restoration, or review completed over a 31 month period, were included. A standardised data collection form was developed and completed with reference to the patient's clinical record. Information was collected with regard to: the indication for implant treatment and its extent; the need for, complexity and duration of orthodontic treatment; the need for bone grafting and the techniques employed and indicators of implant success.
Conclusion: Implant survival and success rates were high for those patients reviewed. Incidence of biological complications compared very favourably with the literature
Statistical properties of contact vectors
We study the statistical properties of contact vectors, a construct to
characterize a protein's structure. The contact vector of an N-residue protein
is a list of N integers n_i, representing the number of residues in contact
with residue i. We study analytically (at mean-field level) and numerically the
amount of structural information contained in a contact vector. Analytical
calculations reveal that a large variance in the contact numbers reduces the
degeneracy of the mapping between contact vectors and structures. Exact
enumeration for lengths up to N=16 on the three dimensional cubic lattice
indicates that the growth rate of number of contact vectors as a function of N
is only 3% less than that for contact maps. In particular, for compact
structures we present numerical evidence that, practically, each contact vector
corresponds to only a handful of structures. We discuss how this information
can be used for better structure prediction.Comment: 20 pages, 6 figure
Dynamical chaos and power spectra in toy models of heteropolymers and proteins
The dynamical chaos in Lennard-Jones toy models of heteropolymers is studied
by molecular dynamics simulations. It is shown that two nearby trajectories
quickly diverge from each other if the heteropolymer corresponds to a random
sequence. For good folders, on the other hand, two nearby trajectories may
initially move apart but eventually they come together. Thus good folders are
intrinsically non-chaotic. A choice of a distance of the initial conformation
from the native state affects the way in which a separation between the twin
trajectories behaves in time. This observation allows one to determine the size
of a folding funnel in good folders. We study the energy landscapes of the toy
models by determining the power spectra and fractal characteristics of the
dependence of the potential energy on time. For good folders, folding and
unfolding trajectories have distinctly different correlated behaviors at low
frequencies.Comment: 8 pages, 9 EPS figures, Phys. Rev. E (in press
Folding in two-dimenensional off-lattice models of proteins
Model off-lattice sequences in two dimensions are constructed so that their
native states are close to an on-lattice target. The Hamiltonian involves the
Lennard-Jones and harmonic interactions. The native states of these sequences
are determined with a high degree of certainty through Monte Carlo processes.
The sequences are characterized thermodynamically and kinetically. It is shown
that the rank-ordering-based scheme of the assignment of contact energies
typically fails in off-lattice models even though it generates high stability
of on-lattice sequences. Similar to the on-lattice case, Go-like modeling, in
which the interaction potentials are restricted to the native contacts in a
target shape, gives rise to good folding properties. Involving other contacts
deteriorates these properties.Comment: REVTeX, 9 pages, 8 EPS figure
Geometric Path Integrals. A Language for Multiscale Biology and Systems Robustness
In this paper we suggest that, under suitable conditions, supervised learning
can provide the basis to formulate at the microscopic level quantitative
questions on the phenotype structure of multicellular organisms. The problem of
explaining the robustness of the phenotype structure is rephrased as a real
geometrical problem on a fixed domain. We further suggest a generalization of
path integrals that reduces the problem of deciding whether a given molecular
network can generate specific phenotypes to a numerical property of a
robustness function with complex output, for which we give heuristic
justification. Finally, we use our formalism to interpret a pointedly
quantitative developmental biology problem on the allowed number of pairs of
legs in centipedes
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