1,166 research outputs found
Proposal for manipulating and detecting spin and orbital states of trapped electrons on helium using cavity quantum electrodynamics
We propose to couple an on-chip high finesse superconducting cavity to the
lateral-motion and spin state of a single electron trapped on the surface of
superfluid helium. We estimate the motional coherence times to exceed 15
microseconds, while energy will be coherently exchanged with the cavity photons
in less than 10 nanoseconds for charge states and faster than 1 microsecond for
spin states, making the system attractive for quantum information processing
and cavity quantum electrodynamics experiments. Strong interaction with cavity
photons will provide the means for both nondestructive readout and coupling of
distant electrons.Comment: 4 pages, 3 figures, supplemental material
Effect of nonequilibrium phonons on hot-electron spin relaxation in n-type GaAs quantum wells
We have studied the effect of nonequilibrium longitudinal optical phonons on
hot-electron spin relaxation in -type GaAs quantum wells. The longitudinal
optical phonons, due to the finite relaxation rate, are driven to
nonequilibrium states by electrons under an in-plane electric field. The
nonequilibrium phonons then in turn influence the electron spin relaxation
properties via modifying the electron heating and drifting. The spin relaxation
time is elongated due to the enhanced electron heating and thus the
electron-phonon scattering in the presence of nonequilibrium phonons. The
frequency of spin precession, which is roughly proportional to the electron
drift velocity, can be either increased (at low electric field and/or high
lattice temperature) or decreased (at high electric field and/or low lattice
temperature). The nonequilibrium phonon effect is more pronounced when the
electron density is high and the impurity density is low.Comment: 6 pages, 3 figure
Influencia del pastoreo en la cubierta vegetal y la geomorfodinámica en el transecto Depresión del Ebro-Pirineos
Extensive grazing by sheep and goats on fallow land and abandoned fields in the Ebro basin and the Pyrenees impedes the regeneration of vegetation cover by browsing and hoof threads. An association of characteristic patterns of geomorphodynamically highly dynamic animal trails and largely stable areas develops. In the High Pyrenees and the Pre-Pyrenees, even existing matorral scrubss hows clear sign of thinning, while morphodynamic activity increases. Areas affected by erosion rills expand especially where sheep trails encourage their development by very high runoff and erosion rates. Regarding their effects on vegetation cover development and geomorphodynarnic processes, sheep breeding subsidizing programmes by the European Union have to he critically assessed.El pastoreo extensivo de campos en barbecho o abandonados entre la Depresión Central del Ebro y el Pirineo Central impide la regeneración de la cubierta vegetal. Se desarrolla un patrón típico de areas con geomorfodinárnica muy activa en los senderos del ganado, junto a otras extensas superficies estables. En el Prepirineo y el Pirineo Central, se puede incluso observar la reducción de la cubierta de matorral ya existente, de forma simultánea al incremento de la geomorfodinámica. Las zonas afectadas por fuerte erosión lineal aumentan a lo largo de los senderos, ya que éstos muestran tasas de escorrentia superficial y de erosión muy altas. A causa de estos efectos, los incentivos comunitarios a la ganadería ovina han de ser revisados de forma crítica
Positronium chemistry studied by AMOC measurements using a relativistic positron beam
Beam-based Age-Momentum Correlation (β+γΔE AMOC) measurements using an MeV positron beam have become a powerful tool to study chemical reactions of positronium by time-domain observations of the different positron states tagged by the Doppler-broadening (ΔE) characteristics of the 511 keV annihilation radiation. As an example, the investigation of the spin-conversion reaction in the system HTEMPO/methanol is reported. The experimental data for small HTEMPO concentrations can be fitted quite well to a rate-equation model containing the conversion rate as the only adjustable parameter. This model can presumably be applied not only to spin conversion but also to other chemical reactions and to inhibition of positronium. Possible effects of oxidation, complex formation, and inhibition are discussed by way of examples
Diffusion controlled initial recombination
This work addresses nucleation rates in systems with strong initial
recombination. Initial (or `geminate') recombination is a process where a
dissociated structure (anion, vortex, kink etc.) recombines with its twin
brother (cation, anti-vortex, anti-kink) generated in the same nucleation
event. Initial recombination is important if there is an asymptotically
vanishing interaction force instead of a generic saddle-type activation
barrier. At low temperatures, initial recombination strongly dominates
homogeneous recombination. In a first part, we discuss the effect in one-,
two-, and three-dimensional diffusion controlled systems with spherical
symmetry. Since there is no well-defined saddle, we introduce a threshold which
is to some extent arbitrary but which is restricted by physically reasonable
conditions. We show that the dependence of the nucleation rate on the specific
choice of this threshold is strongest for one-dimensional systems and decreases
in higher dimensions. We discuss also the influence of a weak driving force and
show that the transport current is directly determined by the imbalance of the
activation rate in the direction of the field and the rate against this
direction. In a second part, we apply the results to the overdamped sine-Gordon
system at equilibrium. It turns out that diffusive initial recombination is the
essential mechanism which governs the equilibrium kink nucleation rate. We
emphasize analogies between the single particle problem with initial
recombination and the multi-dimensional kink-antikink nucleation problem.Comment: LaTeX, 11 pages, 1 ps-figures Extended versio
Continuous Wavelets on Compact Manifolds
Let be a smooth compact oriented Riemannian manifold, and let
be the Laplace-Beltrami operator on . Say 0 \neq f
\in \mathcal{S}(\RR^+), and that . For , let
denote the kernel of . We show that is
well-localized near the diagonal, in the sense that it satisfies estimates akin
to those satisfied by the kernel of the convolution operator on
\RR^n. We define continuous -wavelets on , in such a
manner that satisfies this definition, because of its localization
near the diagonal. Continuous -wavelets on are analogous to
continuous wavelets on \RR^n in \mathcal{S}(\RR^n). In particular, we are
able to characterize the Hlder continuous functions on by
the size of their continuous wavelet transforms, for
Hlder exponents strictly between 0 and 1. If is the torus
\TT^2 or the sphere , and (the ``Mexican hat''
situation), we obtain two explicit approximate formulas for , one to be
used when is large, and one to be used when is small
Statistical Mechanics of Kinks in (1+1)-Dimensions: Numerical Simulations and Double Gaussian Approximation
We investigate the thermal equilibrium properties of kinks in a classical
\F^4 field theory in dimensions. From large scale Langevin simulations
we identify the temperature below which a dilute gas description of kinks is
valid. The standard dilute gas/WKB description is shown to be remarkably
accurate below this temperature. At higher, ``intermediate'' temperatures,
where kinks still exist, this description breaks down. By introducing a double
Gaussian variational ansatz for the eigenfunctions of the statistical transfer
operator for the system, we are able to study this region analytically. In
particular, our predictions for the number of kinks and the correlation length
are in agreement with the simulations. The double Gaussian prediction for the
characteristic temperature at which the kink description ultimately breaks down
is also in accord with the simulations. We also analytically calculate the
internal energy and demonstrate that the peak in the specific heat near the
kink characteristic temperature is indeed due to kinks. In the neighborhood of
this temperature there appears to be an intricate energy sharing mechanism
operating between nonlinear phonons and kinks.Comment: 28 pages (8 Figures not included, hard-copies available), Latex,
LA-UR-93-276
Muonium Decay
Modifications of the mu+ lifetime in matter due to muonium (M = mu+ e-)
formation and other medium effects are examined. Muonium and free mu+ decay
spectra are found to differ at O(alpha m_e/m_mu) from Doppler broadening and
O(alpha^2 m_e/m_mu) from the Coulomb bound state potential. However, both types
of corrections are shown to cancel in the total decay rate due to Lorentz and
gauge invariance respectively, leaving a very small time dilation lifetime
difference, (tau_M - tau_mu+)/tau_mu+ = alpha^2 m_e^2/ 2m_mu^2 \simeq 6\times
10^-10, as the dominant bound state effect. It is argued that other medium
effects on the stopped mu+ lifetime are similarly suppressed.Comment: 14 pages, revte
Determination of the parameters of semiconducting CdF2:In with Schottky barriers from radio-frequency measurements
Physical properties of semiconducting CdF_2 crystals doped with In are
determined from measurements of the radio-frequency response of a sample with
Schottky barriers at frequencies 10 - 10^6 Hz. The dc conductivity, the
activation energy of the amphoteric impurity, and the total concentration of
the active In ions in CdF_2 are found through an equivalent-circuit analysis of
the frequency dependencies of the sample complex impedance at temperatures from
20 K to 300 K. Kinetic coefficients determining the thermally induced
transitions between the deep and the shallow states of the In impurity and the
barrier height between these states are obtained from the time-dependent
radio-frequency response after illumination of the material. The results on the
low-frequency conductivity in CdF_2:In are compared with submillimeter (10^{11}
- 10^{12} Hz) measurements and with room-temperature infrared measurements of
undoped CdF_2. The low-frequency impedance measurements of semiconductor
samples with Schottky barriers are shown to be a good tool for investigation of
the physical properties of semiconductors.Comment: 9 pages, 7 figure
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