522,177 research outputs found
Diffusion Processes and Coherent States
It is shown that stochastic processes of diffusion type possess, in all
generality, a structure of uncertainty relations and of coherent and squeezed
states. This fact is used to obtain, via Nelson stochastic formulation of
quantum mechanics, the harmonic-oscillator coherent and squeezed states. The
method allows to derive new minimum uncertainty states in time-dependent
oscillator potentials and for the Caldirola-Kanai model of quantum damped
oscillator.Comment: 11 pages, plain LaTe
Quaternions and Special Relativity
We reformulate Special Relativity by a quaternionic algebra on reals. Using
{\em real linear quaternions}, we show that previous difficulties, concerning
the appropriate transformations on the space-time, may be overcome. This
implies that a complexified quaternionic version of Special Relativity is a
choice and not a necessity.Comment: 17 pages, latex, no figure
Quantum Groups, Coherent States, Squeezing and Lattice Quantum Mechanics
By resorting to the Fock--Bargmann representation, we incorporate the quantum
Weyl--Heisenberg (-WH) algebra into the theory of entire analytic functions.
The main tool is the realization of the --WH algebra in terms of finite
difference operators. The physical relevance of our study relies on the fact
that coherent states (CS) are indeed formulated in the space of entire analytic
functions where they can be rigorously expressed in terms of theta functions on
the von Neumann lattice. The r\^ole played by the finite difference operators
and the relevance of the lattice structure in the completeness of the CS system
suggest that the --deformation of the WH algebra is an essential tool in the
physics of discretized (periodic) systems. In this latter context we define a
quantum mechanics formalism for lattice systems.Comment: 22 pages, TEX file, DFF188/9/93 Firenz
Testing Modified Newtonian Dynamics with Low Surface Brightness Galaxies --Rotation curve fits-
We present MOND (Modified Newtonian Dynamics) fits to 15 rotation curves of
LSB galaxies. Good fits are readily found, although for a few galaxies minor
adjustments to the inclination are needed. Reasonable values for the stellar
mass-to-light ratios are found, as well as an approximately constant value for
the total (gas and stars) mass-to-light ratio. We show that the LSB galaxies
investigated here lie on the one, unique Tully-Fisher relation, as predicted by
MOND. The scatter on the Tully-Fisher relation can be completely explained by
the observed scatter in the total mass-to-light ratio. We address the question
of whether MOND can fit any arbitrary rotation curve by constructing a
plausible fake model galaxy. While MOND is unable to fit this hypothetical
galaxy, a normal dark halo fit is readily found, showing that dark matter fits
are much less selective in producing fits. The good fits to rotation curves of
LSB galaxies support MOND, especially as these are galaxies with large mass
discrepancies deep in the MOND regime.Comment: Accepted for publication in Astrophysical Journal 14 page
On the interface polaron formation in organic field-effect transistors
A model describing the low density carrier state in an organic single crystal
FET with high- gate dielectrics is studied. The interplay between
charge carrier coupling with inter-molecular vibrations in the bulk of the
organic material and the long-range interaction induced at the interface with a
polar dielectric is investigated. This interplay is responsible for the
stabilization of a polaronic state with an internal structure extending on few
lattice sites, at much lower coupling strengths than expected from the polar
interaction alone. This effect could give rise to polaron self-trapping in
high- organic FET's without invoking unphysically large values of the
carrier interface interaction.Comment: 9 pages, 9 figure
Paired composite fermion wavefunctions
We construct a family of BCS paired composite fermion wavefunctions that
generalize, but remain in the same topological phase as, the Moore-Read
Pfaffian state for the half-filled Landau level. It is shown that for a wide
range of experimentally relevant inter-electron interactions the groundstate
can be very accurately represented in this form.Comment: 4 pages, 2 figure
Charge, Orbital and Magnetic Order in Nd0.5Ca0.5MnO3
In the manganite Nd0.5Ca0.5MnO3, charge ordering occurs at much higher
temperature than the antiferromagnetic order (TCO=250K,TN=160K).
The magnetic behavior of the phase TN<T<TCO is puzzling: its magnetization
and susceptibility are typical of an antiferromagnet while no magnetic order is
detected by neutron diffraction.We have undertaken an extensive study of the
cristallographic, electric and magnetic properties of Nd0.5Ca0.5MnO3 and
established its phase diagram as a function of temperature and magnetic field.
The charge disordered, paramagnetic phase above TCO present ferromagnetic
correlations. An antiferromagnetic CE phase prevails below TN, with complete
charge and orbital ordering. In the intermediate temperature range, charge
ordering occurs while orbital ordering sets in progressively, with no magnetic
order. Strong magnetic fields destroy the charge ordered phases in a fisrt
order transition towards a ferromagnetic state.Comment: 17 pages, 11 Figures to appear in Phys. Rev.
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