673,680 research outputs found
Spin selective transport through helical molecular systems
Highly spin selective transport of electrons through a helically shaped
electrostatic potential is demonstrated in the frame of a minimal model
approach. The effect is significant even in the case of weak spin-orbit
coupling. Two main factors determine the selectivity, an unconventional Rashba-
like spin-orbit interaction, reflecting the helical symmetry of the system, and
a weakly dispersive electronic band of the helical system. The weak electronic
coupling, associated with the small dispersion, leads to a low mobility of the
charges in the system and allows even weak spin-orbit interactions to be
effective. The results are expected to be generic for chiral molecular systems
displaying low spin-orbit coupling and low conductivity.Comment: 9 pages, 4 figures v2 (misprints corrected, new figures
Sound modes in composite incommensurate crystals
We propose a simple phenomenological model describing composite crystals,
constructed from two parallel sets of periodic inter-penetrating chains. In the
harmonic approximation and neglecting thermal fluctuations we find the
eigenmodes of the system. It is shown that at high frequencies there are two
longitudinal sound modes with standard attenuation, while in the low frequency
region there is one propagating sound mode and an over-damped phase mode. The
crossover between these two regions is analyzed numerically and the dynamical
structure factor is calculated. It is shown that the qualitative features of
the experimentally observed spectra can be consistently described by our model.Comment: 12 pages, 2 eps figures, Revtex, accepted to European Physics Journal
B, (2002
Electromagnetic Mach principle
We will introduce a gauge model which an electromagnetic coupling constant
and local mass are related to all the charge in the universe. we will use the
standard Dirac action, but where the mass and the electromagnetic coupling
constant are a function of the sum of all the charge in the universe, which
represent Mach principle for electromagnetic coupling constant. The
formalisation is not manifestly Lorentz invariant, however Lorentz invariance
can be restored by performing a phase transformation of the Dirac field.Comment: 3 page
Confining Boundary conditions from dynamical Coupling Constants
It is shown that it is possible to consistently and gauge invariantly
formulate models where the coupling constant is a non trivial function of a
scalar field . In the case the coupling to the gauge field contains a
term of the form where is an
auxiliary field and is the Dirac current. The scalar field
determines the local value of the coupling of the gauge field to the Dirac
particle. The consistency of the equations determine the condition
which implies that the Dirac current cannot have
a component in the direction of the gradient of the scalar field. As a
consequence, if has a soliton behaviour, like defining a bubble that
connects two vacuua, we obtain that the Dirac current cannot have a flux
through the wall of the bubble, defining a confinement mechanism where the
fermions are kept inside those bags. Consistent models with time dependent fine
structure constant can be also constructedComment: 4 pages, 3 figures. A new reference added and discussion expande
Dark solitons in laser radiation build-up dynamics
We reveal the existence of slowly-decaying dark solitons in the radiation
build-up dynamics of bright pulses in all-normal dispersion mode-locked fiber
lasers, numerically modeled in the framework of a generalized nonlinear
Schr\"odinger equation. The evolution of noise perturbations to
quasi-stationary dark solitons is examined, and the significance of background
shape and soliton-soliton collisions on the eventual soliton decay is
established. We demonstrate the role of a restoring force in extending soliton
interactions in conservative systems to include the effects of dissipation, as
encountered in laser cavities, and generalize our observations to other
nonlinear systems
Integrity bases for local invariants of composite quantum systems
Unitary group branchings appropriate to the calculation of local invariants
of density matrices of composite quantum systems are formulated using the
method of -function plethysms. From this, the generating function for the
number of invariants at each degree in the density matrix can be computed. For
the case of two two-level systems the generating function is . Factorisation of such series leads
in principle to the identification of an integrity basis of algebraically
independent invariants. This note replaces Appendix B of our paper\cite{us} J
Phys {\bf A33} (2000) 1895-1914 (\texttt{quant-ph/0001076}) which is incorrect.Comment: Latex, 4 pages, correcting Appendix B of quant-ph/0001076 Error in
corrected and conclusions modified accordingl
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