42,719 research outputs found
Rule Managed Reporting in Energy Controlled Wireless Sensor Networks
This paper proposes a technique to extend the network lifetime of a wireless sensor network, whereby each sensor node decides its network involvement, based on energy resources and the information in each message (ascertained through a system of rules). Results obtained from the simulation of an industrial monitoring scenario have shown that a considerable increase in the lifetime and connectivity can be obtained
Magnetoelectric Effects on Composite Nano Granular Films
Employing a new experimental technique to measure magnetoelectric response
functions, we have measured the magnetoelectric effect in composite films of
nano granular metallic iron in anatase titanium dioxide at temperatures below
50 K. A magnetoelectric resistance is defined as the ratio of a transverse
voltage to bias current as a function of the magnetic field. In contrast to the
anomalous Hall resistance measured above 50 K, the magnetoelectic resistance
below 50 K is significantly larger and exhibits an even symmetry with respect
to magnetic field reversal . The measurement technique required
attached electrodes in the plane of the film composite in order to measure
voltage as a function of bias current and external magnetic field. To our
knowledge, the composite films are unique in terms of showing magnetoelectric
effects at low temperatures, 50 K, and anomalous Hall effects at high
temperatures, 50 K.Comment: ReVTeX, 2 figures, 3 page
Birkhoff strata of the Grassmannian Gr: Algebraic curves
Algebraic varieties and curves arising in Birkhoff strata of the Sato
Grassmannian Gr are studied. It is shown that the big cell
contains the tower of families of the normal rational curves of all odd orders.
Strata , contain hyperelliptic curves of genus
and their coordinate rings. Strata , contain
plane curves for and and
curves in , respectively. Curves in the strata
have zero genus.Comment: 14 pages, no figures, improved some definitions, typos correcte
Spatial evolution of short pulses under coherent population trapping
Spatial and temporal evolution is studied of two powerful short laser pulses
having different wavelengths and interacting with a dense three-level
Lambda-type optical medium under coherent population trapping. A general case
of unequal oscillator strengths of the transitions is considered. Durations of
the probe pulse and the coupling pulse () are assumed to be
shorter than any of the relevant atomic relaxation times. We propose analytical
and numerical solutions of a self-consistent set of coupled Schr\"{o}dinger
equations and reduced wave equations in the adiabatic limit with the account of
the first non-adiabatic correction. The adiabaticity criterion is also
discussed with the account of the pulse propagation. The dynamics of
propagation is found to be strongly dependent on the ratio of the transition
oscillator strengths. It is shown that envelopes of the pulses slightly change
throughout the medium length at the initial stage of propagation. This distance
can be large compared to the one-photon resonant absorption length. Eventually,
the probe pulse is completely reemitted into the coupling pulse during
propagation. The effect of localization of the atomic coherence has been
observed similar to the one predicted by Fleischhauer and Lukin (PRL, {\bf 84},
5094 (2000).Comment: 16 pages revtex style, 7 EPS figures, accepted to Physical Review
Irreversible phase transitions induced by an oscillatory input
A novel kind of irreversible phase transitions (IPT's) driven by an
oscillatory input parameter is studied by means of computer simulations. Second
order IPT's showing scale invariance in relevant dynamic critical properties
are found to belong to the universality class of directed percolation. In
contrast, the absence of universality is observed for first order IPT's.Comment: 18 pages (Revtex); 8 figures (.ps); submitted to Europhysics Letters,
December 9th, 199
Spin ice in a field: quasi-phases and pseudo-transitions
Thermodynamics of the short-range model of spin ice magnets in a field is
considered in the Bethe - Peierls approximation. The results obtained for
[111], [100] and [011] fields agrees reasonably well with the existing
Monte-Carlo simulations and some experiments. In this approximation all
extremely sharp field-induced anomalies are described by the analytical
functions of temperature and applied field. In spite of the absence of true
phase transitions the analysis of the entropy and specific heat reliefs over
H-T plane allows to discern the "pseudo-phases" with specific character of spin
fluctuations and define the lines of more or less sharp "pseudo-transitions"
between them.Comment: 18 pages, 16 figure
Sn delta-doping in GaAs
We have prepared a number of GaAs structures delta-doped by Sn using the
well-known molecular beam epitaxy growth technique. The samples obtained for a
wide range of Sn doping densities were characterised by magnetotransport
experiments at low temperatures and in high magnetic fields up to 38 T.
Hall-effect and Shubnikov-de Haas measurements show that the electron densities
reached are higher than for other delta-dopants, like Si and Be. The maximum
carrier density determined by the Hall effect equals 8.4x10^13 cm^-2. For all
samples several Shubnikov-de Haas frequencies were observed, indicating the
population of multiple subbands. The depopulation fields of the subbands were
determined by measuring the magnetoresistance with the magnetic field in the
plane of the delta-layer. The experimental results are in good agreement with
selfconsistent bandstructure calculations. These calculation shows that in the
sample with the highest electron density also the conduction band at the L
point is populated.Comment: 11 pages text (ps), 9 figures (ps), submitted to Semicon. Science
Tech
Quantum nano-electromechanics with electrons, quasiparticles and Cooper pairs: effective bath descriptions and strong feedback effects
Using a quantum noise approach, we discuss the physics of both normal metal
and superconducting single electron transistors (SET) coupled to mechanical
resonators. Particular attention is paid to the regime where transport occurs
via incoherent Cooper-pair tunneling (either via the Josephson quasiparticle
(JQP) or double Josephson quasiparticle (DJQP) process). We show that,
surprisingly, the back-action of tunneling Cooper pairs (or superconducting
quasiparticles) can be used to significantly cool the oscillator. We also
discuss the physical origin of negative damping effects in this system, and how
they can lead to a regime of strong electro-mechanical feedback, where despite
a weak SET - oscillator coupling, the motion of the oscillator strongly effects
the tunneling of the Cooper pairs. We show that in this regime, the oscillator
is characterized by an energy-dependent effective temperature. Finally, we
discuss the strong analogy between back-action effects of incoherent
Cooper-pair tunneling and ponderomotive effects in an optical cavity with a
moveable mirror; in our case, tunneling Cooper pairs play the role of the
cavity photons.Comment: 27 pages, 7 figures; submitted to the New Journal of Physics focus
issue on Nano-electromechanical Systems; error in references correcte
Replica Symmetry Breaking and the Renormalization Group Theory of the Weakly Disordered Ferromagnet
We study the critical properties of the weakly disordered -component
ferromagnet in terms of the renormalization group (RG) theory generalized to
take into account the replica symmetry breaking (RSB) effects coming from the
multiple local minima solutions of the mean-field equations. It is shown that
for the traditional RG flows at dimensions , which are
usually considered as describing the disorder-induced universal critical
behavior, are unstable with respect to the RSB potentials as found in spin
glasses. It is demonstrated that for a general type of the Parisi RSB
structures there exists no stable fixed points, and the RG flows lead to the
{\it strong coupling regime} at the finite scale , where
is the small parameter describing the disorder. The physical concequences
of the obtained RG solutions are discussed. In particular, we argue, that
discovered RSB strong coupling phenomena indicate on the onset of a new spin
glass type critical behaviour in the temperature interval near . Possible relevance of the considered RSB effects for
the Griffith phase is also discussed.Comment: 32 pages, Late
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