94 research outputs found
Inductive voltage divider modeling in Matlab
Inductive voltage dividers have the most appropriate metrological characteristics on alternative current and are widely used for converting physical signals. The model of a double-decade inductive voltage divider was designed with the help of Matlab/Simulink. The first decade is an inductive voltage divider with balanced winding, the second decade is a single-stage inductive voltage divider. In the paper, a new transfer function algorithm was given. The study shows errors and differences that appeared between the third degree reduced model and a twenty degree unreduced model. The obtained results of amplitude error differ no more than by 7 % between the reduced and unreduced model
Spin superradiance versus atomic superradiance
A comparative analysis is given of spin superradiance and atomic
superradiance. Their similarities and distinctions are emphasized. It is shown
that, despite a close analogy, these phenomena are fundamentally different. In
atomic systems, superradiance is a self-organized process, in which both the
initial cause, being spontaneous emission, as well as the collectivizing
mechanism of their interactions through the common radiation field, are of the
same physical nature. Contrary to this, in actual spin systems with dipole
interactions, the latter are the major reason for spin motion. Electromagnetic
spin interactions through radiation are negligible and can never produce
collective effects. The possibility of realizing superradiance in molecular
magnets by coupling them to a resonant circuit is discussed.Comment: Latex file, 12 pages, no figure
Correlation search for coherent pion emission in heavy ion collisions
The methods allowing to extract the coherent component of pion emission
conditioned by the formation of a quasi-classical pion source in heavy ion
collisions are suggested. They exploit a nontrivial modification of the quantum
statistical and final state interaction effects on the correlation functions of
like and unlike pions in the presence of the coherent radiation. The extraction
of the coherent pion spectrum from pi+pi-, pi+pi+ and pi-pi- correlation
functions and single--pion spectra is discussed in detail for large expanding
systems produced in ultra-relativistic heavy ion collisions.Comment: 21 pages, 3 eps figures, ReVTeX, corrected submission abstract.
Version published in PRC 65 (2002) 064904. Added is a detailed explanation of
the differences between pure coherent states and charge constrained coherent
states in the case of a simple example model. The expressions for
two-particle spectra taking into account both the final state interaction and
the coherent component of pion emission are derived in a more general and
transparent wa
Absence of spin superradiance in resonatorless magnets
A spin system is considered with a Hamiltonian typical of molecular magnets,
having dipole-dipole interactions and a single-site magnetic anisotropy. In
addition, spin interactions through the common radiation field are included. A
fully quantum-mechanical derivation of the collective radiation rate is
presented. An effective narrowing of the dipole-dipole attenuation, due to high
spin polarization is taken into account. The influence of the radiation rate on
spin dynamics is carefully analysed. It is shown that this influence is
completely negligible. No noticeable collective effects, such as superradiance,
can appear in molecular magnets, being caused by electromagnetic spin
radiation. Spin superradiance can arise in molecular magnets only when these
are coupled to a resonant electric circuit, as has been suggested earlier by
one of the authors in Laser Phys. {\bf 12}, 1089 (2002).Comment: Latex file, 14 pages, 5 figure
Fluctuations of an Atomic Ledge Bordering a Crystalline Facet
When a high symmetry facet joins the rounded part of a crystal, the step line
density vanishes as sqrt(r) with r denoting the distance from the facet edge.
This means that the ledge bordering the facet has a lot of space to meander as
caused by thermal activation. We investigate the statistical properties of the
border ledge fluctuations. In the scaling regime they turn out to be
non-Gaussian and related to the edge statistics of GUE multi-matrix models.Comment: Version with major revisions -- RevTeX, 4 pages, 2 figure
Andreev Reflection in Ferromagnet/Superconductor/Ferromagnet Double Junction Systems
We present a theory of Andreev reflection in a
ferromagnet/superconductor/ferromagnet double junction system. The spin
polarized quasiparticles penetrate to the superconductor in the range of
penetration depth from the interface by the Andreev reflection. When the
thickness of the superconductor is comparable to or smaller than the
penetration depth, the spin polarized quasiparticles pass through the
superconductor and therefore the electric current depends on the relative
orientation of magnetizations of the ferromagnets. The dependences of the
magnetoresistance on the thickness of the superconductor, temperature, the
exchange field of the ferromagnets and the height of the interfacial barriers
are analyzed. Our theory explains recent experimental results well.Comment: 8 pages, 9 figures, submitted to Phys. Rev.
Instabilities and Bifurcations of Nonlinear Impurity Modes
We study the structure and stability of nonlinear impurity modes in the
discrete nonlinear Schr{\"o}dinger equation with a single on-site nonlinear
impurity emphasizing the effects of interplay between discreteness,
nonlinearity and disorder. We show how the interaction of a nonlinear localized
mode (a discrete soliton or discrete breather) with a repulsive impurity
generates a family of stationary states near the impurity site, as well as
examine both theoretical and numerical criteria for the transition between
different localized states via a cascade of bifurcations.Comment: 8 pages, 8 figures, Phys. Rev. E in pres
Interplay of non-linear elasticity and dislocation-induced superfluidity in solid Helium-4
The mechanism of the roughening induced partial depinning of gliding
dislocations from Helium-3 impurities is proposed as an alternative to the
standard "boiling off". We give a strong argument that Helium-3 remains bound
to dislocations even at large temperatures due to very long equilibration
times. A scenario leading to the similarity between elastic and superfluid
responses of solid Helium-4 is also discussed. Its main ingredient is a strong
suppression of the superfluidity along dislocation cores by dislocation kinks
(D. Aleinikava, et. al., arXiv:0812.0983). These kinks, on one hand, determine
the temperature and Helium-3 dependencies of the generalized shear modulus and,
on the other hand, control the superfluid response. Several proposals for
theoretical and experimental studies of solid Helium-4 are suggested.Comment: final version accepted to the special JLTP issue on Supersolid, 16
pages, 6 figures: typos corrected, more explanations give
Bosonic Excitations in Random Media
We consider classical normal modes and non-interacting bosonic excitations in
disordered systems. We emphasise generic aspects of such problems and parallels
with disordered, non-interacting systems of fermions, and discuss in particular
the relevance for bosonic excitations of symmetry classes known in the
fermionic context. We also stress important differences between bosonic and
fermionic problems. One of these follows from the fact that ground state
stability of a system requires all bosonic excitation energy levels to be
positive, while stability in systems of non-interacting fermions is ensured by
the exclusion principle, whatever the single-particle energies. As a
consequence, simple models of uncorrelated disorder are less useful for bosonic
systems than for fermionic ones, and it is generally important to study the
excitation spectrum in conjunction with the problem of constructing a
disorder-dependent ground state: we show how a mapping to an operator with
chiral symmetry provides a useful tool for doing this. A second difference
involves the distinction for bosonic systems between excitations which are
Goldstone modes and those which are not. In the case of Goldstone modes we
review established results illustrating the fact that disorder decouples from
excitations in the low frequency limit, above a critical dimension , which
in different circumstances takes the values and . For bosonic
excitations which are not Goldstone modes, we argue that an excitation density
varying with frequency as is a universal
feature in systems with ground states that depend on the disorder realisation.
We illustrate our conclusions with extensive analytical and some numerical
calculations for a variety of models in one dimension
Multiband tight-binding theory of disordered ABC semiconductor quantum dots: Application to the optical properties of alloyed CdZnSe nanocrystals
Zero-dimensional nanocrystals, as obtained by chemical synthesis, offer a
broad range of applications, as their spectrum and thus their excitation gap
can be tailored by variation of their size. Additionally, nanocrystals of the
type ABC can be realized by alloying of two pure compound semiconductor
materials AC and BC, which allows for a continuous tuning of their absorption
and emission spectrum with the concentration x. We use the single-particle
energies and wave functions calculated from a multiband sp^3 empirical
tight-binding model in combination with the configuration interaction scheme to
calculate the optical properties of CdZnSe nanocrystals with a spherical shape.
In contrast to common mean-field approaches like the virtual crystal
approximation (VCA), we treat the disorder on a microscopic level by taking
into account a finite number of realizations for each size and concentration.
We then compare the results for the optical properties with recent experimental
data and calculate the optical bowing coefficient for further sizes
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