971 research outputs found
Fluorescence from a few electrons
Systems containing few Fermions (e.g., electrons) are of great current
interest. Fluorescence occurs when electrons drop from one level to another
without changing spin. Only electron gases in a state of equilibrium are
considered. When the system may exchange electrons with a large reservoir, the
electron-gas fluorescence is easily obtained from the well-known Fermi-Dirac
distribution. But this is not so when the number of electrons in the system is
prevented from varying, as is the case for isolated systems and for systems
that are in thermal contact with electrical insulators such as diamond. Our
accurate expressions rest on the assumption that single-electron energy levels
are evenly spaced, and that energy coupling and spin coupling between electrons
are small. These assumptions are shown to be realistic for many systems.
Fluorescence from short, nearly isolated, quantum wires is predicted to drop
abruptly in the visible, a result not predicted by the Fermi-Dirac
distribution. Our exact formulas are based on restricted and unrestricted
partitions of integers. The method is considerably simpler than the ones
proposed earlier, which are based on second quantization and contour
integration.Comment: 10 pages, 3 figures, RevTe
Scattering of slow-light gap solitons with charges in a two-level medium
The Maxwell-Bloch system describes a quantum two-level medium interacting
with a classical electromagnetic field by mediation of the the population
density. This population density variation is a purely quantum effect which is
actually at the very origin of nonlinearity. The resulting nonlinear coupling
possesses particularly interesting consequences at the resonance (when the
frequency of the excitation is close to the transition frequency of the
two-level medium) as e.g. slow-light gap solitons that result from the
nonlinear instability of the evanescent wave at the boundary. As nonlinearity
couples the different polarizations of the electromagnetic field, the
slow-light gap soliton is shown to experience effective scattering whith
charges in the medium, allowing it for instance to be trapped or reflected.
This scattering process is understood qualitatively as being governed by a
nonlinear Schroedinger model in an external potential related to the charges
(the electrostatic permanent background component of the field).Comment: RevTex, 14 pages with 5 figures, to appear in J. Phys. A: Math. Theo
Origin of Pure Spin Superradiance
The question addressed in this paper is: What originates pure spin
superradiance in a polarized spin system placed inside a resonator? The term
"pure" means that no initial coherence is imposed on spins, and its appearance
manifests a purely self-organized collective effect. The consideration is based
on a microscopic model with dipole spin interactions. An accurate solution of
evolution equations is given. The results show that the resonator Nyquist noise
does not play, contrary to the common belief, any role in starting spin
superradiance, but the emergence of the latter is initiated by local spin
fluctuations. The decisive role of nonsecular dipole interactions is stressed.Comment: 1 file, 13 pages, RevTe
Factors associated with diversity, quantity and zoonotic potential of ectoparasites on urban mice and voles
Wild rodents are important hosts for tick larvae but co-infestations with other mites and insects are largely neglected. Small rodents were trapped at four study sites in Berlin, Germany, to quantify their ectoparasite diversity. Host-specific, spatial and temporal occurrence of ectoparasites was determined to assess their influence on direct and indirect zoonotic risk due to mice and voles in an urban agglomeration. Rodent-associated arthropods were diverse, including 63 species observed on six host species with an overall prevalence of 99%. The tick Ixodes ricinus was the most prevalent species, found on 56% of the rodents. The trapping location clearly affected the presence of different rodent species and, therefore, the occurrence of particular host-specific parasites. In Berlin, fewer temporary and periodic parasite species as well as non-parasitic species (fleas, chiggers and nidicolous Gamasina) were detected than reported from rural areas. In addition, abundance of parasites with low host-specificity (ticks, fleas and chiggers) apparently decreased with increasing landscape fragmentation associated with a gradient of urbanisation. In contrast, stationary ectoparasites, closely adapted to the rodent host, such as the fur mites Myobiidae and Listrophoridae, were most abundant at the two urban sites. A direct zoonotic risk of infection for people may only be posed by Nosopsyllus fasciatus fleas, which were prevalent even in the city centre. More importantly, peridomestic rodents clearly supported the life cycle of ticks in the city as hosts for their subadult stages. In addition to trapping location, season, host species, body condition and host sex, infestation with fleas, gamasid Laelapidae mites and prostigmatic Myobiidae mites were associated with significantly altered abundance of I. ricinus larvae on mice and voles. Whether this is caused by predation, grooming behaviour or interaction with the host immune system is unclear. The present study constitutes a basis to identify interactions and vector function of rodent-associated arthropods and their potential impact on zoonotic diseases
Sequence Effects on DNA Entropic Elasticity
DNA stretching experiments are usually interpreted using the worm-like chain
model; the persistence length A appearing in the model is then interpreted as
the elastic stiffness of the double helix. In fact the persistence length
obtained by this method is a combination of bend stiffness and intrinsic bend
effects reflecting sequence information, just as at zero stretching force. This
observation resolves the discrepancy between the value of A measured in these
experiments and the larger ``dynamic persistence length'' measured by other
means. On the other hand, the twist persistence length deduced from
torsionally-constrained stretching experiments suffers no such correction. Our
calculation is very simple and analytic; it applies to DNA and other polymers
with weak intrinsic disorder.Comment: LaTeX; postscript available at
http://dept.physics.upenn.edu/~nelson/index.shtm
Bending and Base-Stacking Interactions in Double-Stranded Semiflexible Polymer
Simple expressions for the bending and the base-stacking energy of
double-stranded semiflexible biopolymers (such as DNA and actin) are derived.
The distribution of the folding angle between the two strands is obtained by
solving a Schr\"{o}dinger equation variationally. Theoretical results based on
this model on the extension versus force and extension versus degree of
supercoiling relations of DNA chain are in good agreement with the experimental
observations of Cluzel {\it et al.} [Science {\bf 271}, 792 (1996)], Smith {\it
et al.} [{\it ibid.} {\bf 271}, 795 (1996)], and Strick {\it et al.} [{\it
ibid.} {\bf 271}, 1835 (1996)].Comment: 8 pages in Revtex format, with 4 EPS figure
Electronic Coherence Dephasing in Excitonic Molecular Complexes: Role of Markov and Secular Approximations
We compare four different types of equations of motion for reduced density
matrix of a system of molecular excitons interacting with thermodynamic bath.
All four equations are of second order in the linear system-bath interaction
Hamiltonian, with different approximations applied in their derivation. In
particular we compare time-nonlocal equations obtained from so-called
Nakajima-Zwanzig identity and the time-local equations resulting from the
partial ordering prescription of the cummulant expansion. In each of these
equations we alternatively apply secular approximation to decouple population
and coherence dynamics from each other. We focus on the dynamics of intraband
electronic coherences of the excitonic system which can be traced by coherent
two-dimensional spectroscopy. We discuss the applicability of the four
relaxation theories to simulations of population and coherence dynamics, and
identify features of the two-dimensional coherent spectrum that allow us to
distinguish time-nonlocal effects.Comment: 14 pages, 8 figure
Variant of TYR and Autoimmunity Susceptibility Loci in Generalized Vitiligo.
BACKGROUND
Generalized vitiligo is an autoimmune disease characterized by melanocyte loss, which results in patchy depigmentation of skin and hair, and is associated with an elevated risk of other autoimmune diseases.
METHODS
To identify generalized vitiligo susceptibility loci, we conducted a genomewide association study. We genotyped 579,146 single-nucleotide polymorphisms (SNPs) in 1514 patients with generalized vitiligo who were of European-derived white (CEU) ancestry and compared the genotypes with publicly available control genotypes from 2813 CEU persons. We then tested 50 SNPs in two replication sets, one comprising 677 independent CEU patients and 1106 CEU controls and the other comprising 183 CEU simplex trios with generalized vitiligo and 332 CEU multiplex families.
RESULTS
We detected significant associations between generalized vitiligo and SNPs at several loci previously associated with other autoimmune diseases. These included genes encoding major-histocompatibility-complex class I molecules (P=9.05×10−23) and class II molecules (P=4.50×10−34), PTPN22 (P=1.31×10−7), LPP (P=1.01×10−11), IL2RA (P=2.78×10−9), UBASH3A (P=1.26×10−9), and C1QTNF6 (P=2.21×10−16). We also detected associations between generalized vitiligo and SNPs in two additional immune-related loci, RERE (P=7.07×10−15) and GZMB (P=3.44×10−8), and in a locus containing TYR (P=1.60×10−18), encoding tyrosinase.
CONCLUSIONS
We observed associations between generalized vitiligo and markers implicating multiple genes, some associated with other autoimmune diseases and one (TYR) that may mediate target-cell specificity and indicate a mutually exclusive relationship between susceptibility to vitiligo and susceptibility to melanoma
Nonlinear spin relaxation in strongly nonequilibrium magnets
A general theory is developed for describing the nonlinear relaxation of spin
systems from a strongly nonequilibrium initial state, when, in addition, the
sample is coupled to a resonator. Such processes are characterized by nonlinear
stochastic differential equations. This makes these strongly nonequilibrium
processes principally different from the spin relaxation close to an
equilibrium state, which is represented by linear differential equations. The
consideration is based on a realistic microscopic Hamiltonian including the
Zeeman terms, dipole interactions, exchange interactions, and a single-site
anisotropy. The influence of cross correlations between several spin species is
investigated. The critically important function of coupling between the spin
system and a resonant electric circuit is emphasized. The role of all main
relaxation rates is analyzed. The phenomenon of self-organization of transition
coherence in spin motion, from the quantum chaotic stage of incoherent
fluctuations, is thoroughly described. Local spin fluctuations are found to be
the triggering cause for starting the spin relaxation from an incoherent
nonequilibrium state. The basic regimes of collective coherent spin relaxation
are studied.Comment: Latex file, 31 page
Thermoluminescence of zircon: a kinetic model
The mineral zircon, ZrSiO4, belongs to a class of promising materials for geochronometry by means of thermoluminescence (TL) dating. The development of a reliable and reproducible method for TL dating with zircon requires detailed knowledge of the processes taking place during exposure to ionizing radiation, long-term storage, annealing at moderate temperatures and heating at a constant rate (TL measurements). To understand these processes one needs a kinetic model of TL. This paper is devoted to the construction of such amodel. The goal is to study the qualitative behaviour of the system and to determine the parameters and processes controlling TL phenomena of zircon. The model considers the following processes: (i) Filling of electron and hole traps at the excitation stage as a function of the dose rate and the dose for both (low dose rate) natural and (high dose rate) laboratory irradiation. (ii) Time dependence of TL fading in samples irradiated under laboratory conditions. (iii) Short time annealing at a given temperature. (iv) Heating of the irradiated sample to simulate TL experiments both after laboratory and natural irradiation.
The input parameters of the model, such as the types and concentrations of the TL centres and the energy distributions of the hole and electron traps, were obtained by analysing the experimental data on fading of the TL-emission spectra of samples from different geological locations. Electron paramagnetic resonance (EPR) data were used to establish the nature of the TL centres. Glow curves and 3D TL emission spectra are simulated and compared with the experimental data on time-dependent TL fading. The saturation and annealing behaviour of filled trap concentrations has been considered in the framework of the proposed kinetic model and comparedwith the EPR data associated with the rare-earth ions Tb3+ and Dy3+, which play a crucial role as hole traps and recombination centres. Inaddition, the behaviour of some of the SiOmn− centres has been compared with simulation results.
- …