4,017 research outputs found
On structure of solutions of 1-dimensional 2-body problem in Wheeler-Feynman electrodynamics
The problem of 1-dimensional ultra-relativistic scattering of 2 identical
charged particles in classical electrodynamics with retarded and advanced
interactions is investigated.Comment: 16 pages, 14 figure
Exotic solutions in string theory
Solutions of classical string theory, correspondent to the world sheets,
mapped in Minkowsky space with a fold, are considered. Typical processes for
them are creation of strings from vacuum, their recombination and annihilation.
These solutions violate positiveness of square of mass and Regge condition. In
quantum string theory these solutions correspond to physical states |DDF>+|sp>
with non-zero spurious component.Comment: accepted in Il Nuovo Cimento A for publication in 199
Phase locking below rate threshold in noisy model neurons
The property of a neuron to phase-lock to an oscillatory stimulus before adapting its spike rate to the stimulus frequency plays an important role for the auditory system. We investigate under which conditions neurons exhibit this phase locking below rate threshold. To this end, we simulate neurons employing the widely used leaky integrate-and-fire (LIF) model. Tuning parameters, we can arrange either an irregular spontaneous or a tonic spiking mode. When the neuron is stimulated in both modes, a significant rise of vector strength prior to a noticeable change of the spike rate can be observed. Combining analytic reasoning with numerical simulations, we trace this observation back to a modulation of interspike intervals, which itself requires spikes to be only loosely coupled. We test the limits of this conception by simulating an LIF model with threshold fatigue, which generates pronounced anticorrelations between subsequent interspike intervals. In addition we evaluate the LIF response for harmonic stimuli of various frequencies and discuss the extension to more complex stimuli. It seems that phase locking below rate threshold occurs generically for all zero mean stimuli. Finally, we discuss our findings in the context of stimulus detection
Curve crossing in linear potential grids: the quasidegeneracy approximation
The quasidegeneracy approximation [V. A. Yurovsky, A. Ben-Reuven, P. S.
Julienne, and Y. B. Band, J. Phys. B {\bf 32}, 1845 (1999)] is used here to
evaluate transition amplitudes for the problem of curve crossing in linear
potential grids involving two sets of parallel potentials. The approximation
describes phenomena, such as counterintuitive transitions and saturation
(incomplete population transfer), not predictable by the assumption of
independent crossings. Also, a new kind of oscillations due to quantum
interference (different from the well-known St\"uckelberg oscillations) is
disclosed, and its nature discussed. The approximation can find applications in
many fields of physics, where multistate curve crossing problems occur.Comment: LaTeX, 8 pages, 8 PostScript figures, uses REVTeX and psfig,
submitted to Physical Review
Atlas of Experimental and Theoretical High Temperature Methane Cross Sections from T = 295 to 1000K in the Near-Infrared
Spectra of hot methane were recorded using a tube furnace and a high-resolution Fourier transform spectrometer. We obtained experimental absorption spectra in the 1.85-1.11 μm near-infrared region at eight temperatures ranging from 295 K up to 1000 K. We have converted these into an atlas of absorption cross sections at each temperature for the methane tetradecad, icosad and triacontad polyads, excluding some spectral intervals either strongly contaminated by water or due to baseline fringes. On the theoretical side, the spectra were simulated from the ab initio-based Reims-Tomsk TheoReTS line list for the same experimental conditions. This line list has been constructed by global variational calculations from potential energy and dipole moment surfaces followed by empirical line position corrections deduced from previously published analyses. The comparisons showed very good overall agreement between observations and theory at high spectral resolution for the tetradecad and icosad and at medium or low resolution above this range. A full set of the theoretical absorption cross sections is also included. Detailed temperature dependence of the methane absorption enables the efficient method of remotely probing the temperature of distant astronomical objects based on a comparison of relative signals in carefully selected spectral intervals. This first combined experimental and theoretical easy-to-use cross-section library in the near-infrared should be of major interest for the interpretation of current and future astronomical observations up to a resolving power of 100,000-300,000 in the range 6400-7600 cm-1 and a resolving power of 5000-10,000 in the higher wavenumber range up to 9000 cm-1
Observation of enhanced transmission for s-polarized light through a subwavelength slit
Enhanced optical transmission (EOT) through subwavelength apertures is
usually obtained for p-polarized light. The present study experimentally
investigates EOT for s-polarized light. A subwavelength slit surrounded on each
side by periodic grooves has been fabricated in a gold film and covered by a
thin dielectric layer. The excitation of s-polarized dielectric waveguide modes
inside the dielectric film strongly increases the s-polarized transmission.
Transmission measurements are compared with a coupled mode model and show good
qualitative agreement. Adding a waveguide can improve light transmission
through subwavelength apertures, as both s and p-polarization can be
efficiently transmitted.Comment: 11 pages, 3 figures, submitted to Applied Physics Letter
Pauli equation and the method of supersymmetric factorization
We consider different variants of factorization of a 2x2 matrix
Schroedinger/Pauli operator in two spatial dimensions. They allow to relate its
spectrum to the sum of spectra of two scalar Schroedinger operators, in a
manner similar to one-dimensional Darboux transformations. We consider both the
case when such factorization is reduced to the ordinary 2-dimensional SUSY QM
quasifactorization and a more general case which involves covariant
derivatives. The admissible classes of electromagnetic fields are described and
some illustrative examples are given.Comment: 18 pages, Late
Magnetic and spectral properties of multi-sublattice oxides SrY2O4:Er3+ and SrEr2O4
SrEr2O4 is a geometrically frustrated magnet which demonstrates rather
unusual properties at low temperatures including a coexistence of long- and
short-range magnetic order, characterized by two different propagation vectors.
In the present work, the effects of crystal fields (CF) in this compound
containing four magnetically inequivalent erbium sublattices are investigated
experimentally and theoretically. We combine the measurements of the CF levels
of the Er3+ ions made on a powder sample of SrEr2O4 using neutron spectroscopy
with site-selective optical and electron paramagnetic resonance measurements
performed on single crystal samples of the lightly Er-doped nonmagnetic
analogue, SrY2O4. Two sets of CF parameters corresponding to the Er3+ ions at
the crystallographically inequivalent lattice sites are derived which fit all
the available experimental data well, including the magnetization and dc
susceptibility data for both lightly doped and concentrated samples.Comment: 14 pages, 9 figure
Mechanism of Electrochemical Synthesis of Phosphonium and Quasiphosphonium Salts
The radical cations generated on the anode in the course of electrochemical oxidation of tripropylphosphine in the presence of toluene and water are established to react with the aromatic compound and initial phosphine rather than with water molecules. The experimental findings made it possible to propose a new method of electrochemical synthesis of arylphosphonium salts. The electrochemical synthesis of quasiphosphonium salts, performed by electrochemical oxidation of tertiary phosphines in the presence of alcohols, amines, phenols, thiols, and disulfides, is assumed to involve a diphosphonium intermediate, while the synthesis of arylphosphonium cations proceeds by the mechanism of free-radical aromatic substitution
Tunable hybrid surface waves supported by a graphene layer
We study surface waves localized near a surface of a semi-infinite dielectric
medium covered by a layer of graphene in the presence of a strong external
magnetic field. We demonstrate that both TE-TM hybrid surface plasmons can
propagate along the graphene surface. We analyze the effect of the Hall
conductivity on the disper- sion of hybrid surface waves and suggest a
possibility to tune the plasmon dispersion by the magnetic field.Comment: 3 pages, 3 figure
- …