11,026 research outputs found
Beam normal spin asymmetry in the quasi-RCS approximation
The two-photon exchange contribution to the single spin asymmetries with the
spin orientation normal to the reaction plane is discussed for elastic
electron-proton scattering in the equivalent photon approximation. In this
case, hadronic part of the two-photon exchange amplitude describes real Compton
scattering (RCS). We show that in the case of the beam normal spin asymmetry,
this approximation selects only the photon helicity flip amplitudes of RCS. At
low energies, we make use of unitarity and estimate the contribution of the
multipoles to the photon helicity flip amplitudes. In the Regge regime,
QRCS approximation allows for a contribution from two pion exchange, and we
provide an estimate of such contributions. We furthermore discuss the
possibility of the quark and gluon GPD's contributions in the QRCS kinematics.Comment: 10 pages, 5 figures, revtex, submitted to Phys. Rev. C; new version:
references adde
From the Kubo formula to variable range hopping
Consider a multichannel closed ring with disorder. In the semiclassical
treatment its conductance is given by the Drude formula. Quantum mechanics
challenge this result both in the limit of strong disorder (eigenstates are not
quantum-ergodic in real space) and in the limit of weak disorder (eigenstates
are not quantum-ergodic in momentum space). Consequently the analysis of
conductance requires going beyond linear response theory, leading to a resistor
network picture of transitions between energy levels. We demonstrate that our
semi-linear response theory provides a firm unified framework from which the
"hopping" phenomenology of Mott can be derived.Comment: 5 pages, published version with an extended concluding paragrap
On the origin of unusual transport properties observed in densely packed polycrystalline CaAl_{2}
A possible origin of unusual temperature behavior of transport coefficients
observed in densely packed polycrystalline CaAl_{2} compound [M. Ausloos et
al., J. Appl. Phys. 96, 7338 (2004)] is discussed, including a power-like
dependence of resistivity with and N-like form of the
thermopower. All these features are found to be in good agreement with the
Shklovskii-Efros localization scenario assuming polaron-mediated hopping
processes controlled by the Debye energy
Metal-Insulator transitions in the periodic Anderson model
We solve the Periodic Anderson model in the Mott-Hubbard regime, using
Dynamical Mean Field Theory. Upon electron doping of the Mott insulator, a
metal-insulator transition occurs which is qualitatively similar to that of the
single band Hubbard model, namely with a divergent effective mass and a first
order character at finite temperatures. Surprisingly, upon hole doping, the
metal-insulator transition is not first order and does not show a divergent
mass. Thus, the transition scenario of the single band Hubbard model is not
generic for the Periodic Anderson model, even in the Mott-Hubbard regime.Comment: 5 pages, 4 figure
Spin-Hall effect in a [110] quantum well
A self-consistent treatment of the spin-Hall effect requires consideration of
the spin-orbit coupling and electron-impurity scattering on equal footing. This
is done here for the experimentally relevant case of a [110] GaAs quantum well
[Sih {\it et al.}, Nature Physics 1, 31 (2005)]. Working within the framework
of the exact linear response formalism we calculate the spin-Hall conductivity
including the Dresselhaus linear and cubic terms in the band structure, as well
as the electron-impurity scattering and electron-electron interaction to all
orders. We show that the spin-Hall conductivity naturally separates into two
contributions, skew-scattering and side-jump, and we propose an experiment to
distinguish between them.Comment: The connection with the recent experiment on [110] quantum wells is
emphasize
Lithium abundance and 6Li/7Li ratio in the active giant HD123351 I. A comparative analysis of 3D and 1D NLTE line-profile fits
Current three-dimensional (3D) hydrodynamical model atmospheres together with
NLTE spectrum synthesis, permit to derive reliable atomic and isotopic chemical
abundances from high-resolution stellar spectra. Not much is known about the
presence of the fragile 6Li isotope in evolved solar-metallicity RGB stars, not
to mention its production in magnetically active targets like HD123351. From
fits of the observed CFHT spectrum with synthetic line profiles based on 1D and
3D model atmospheres, we seek to estimate the abundance of the 6Li isotope and
to place constraints on its origin. We derive A(Li) and the 6Li/7Li isotopic
ratio by fitting different synthetic spectra to the Li-line region of a
high-resolution CFHT spectrum (R=120 000, S/R=400). The synthetic spectra are
computed with four different line lists, using in parallel 3D hydrodynamical
CO5BOLD and 1D LHD model atmospheres and treating the line formation of the
lithium components in non-LTE (NLTE). We find A(Li)=1.69+/-0.11 dex and
6Li/7Li=8.0+/-4.4 % in 3D-NLTE, using the line list of Mel\'endez et al.
(2012), updated with new atomic data for V I, which results in the best fit of
the lithium line profile of HD123351. Two other line lists lead to similar
results but with inferior fit qualities. Our 2-sigma detection of the 6Li
isotope is the result of a careful statistical analysis and the visual
inspection of each achieved fit. Since the presence of a significant amount of
6Li in the atmosphere of a cool evolved star is not expected in the framework
of standard stellar evolution theory, non-standard, external lithium production
mechanisms, possibly related to stellar activity or a recent accretion of rocky
material, need to be invoked to explain the detection of 6Li in HD123351.Comment: 16 pages, 11 figures. Accepted for publication in A&
Wigner-Mott scaling of transport near the two-dimensional metal-insulator transition
Electron-electron scattering usually dominates the transport in strongly
correlated materials. It typically leads to pronounced resistivity maxima in
the incoherent regime around the coherence temperature , reflecting the
tendency of carriers to undergo Mott localization following the demise of the
Fermi liquid. This behavior is best pronounced in the vicinity of
interaction-driven (Mott-like) metal-insulator transitions, where the
decreases, while the resistivity maximum increases. Here we show
that, in this regime, the entire family of resistivity curves displays a
characteristic scaling behavior while
the and assume a powerlaw dependence on the
quasi-particle effective mass . Remarkably, precisely such trends are
found from an appropriate scaling analysis of experimental data obtained from
diluted two-dimensional electron gases in zero magnetic fields. Our analysis
provides strong evidence that inelastic electron-electron scattering -- and not
disorder effects -- dominates finite temperature transport in these systems,
validating the Wigner-Mott picture of the two-dimensional metal-insulator
transition.Comment: 7 page
Micrometeorological processes driving snow ablation in an Alpine catchment
Mountain snow covers typically become patchy over the course of a melting season. The snow pattern during melt is mainly governed by the end of winter snow depth distribution and the local energy balance. The objective of this study is to investigate micrometeorological processes driving snow ablation in an Alpine catchment. For this purpose we combine a meteorological model (ARPS) with a fully distributed energy balance model (Alpine3D). Turbulent fluxes above melting snow are further investigated by using data from eddy-correlation systems. We compare modelled snow ablation to measured ablation rates as obtained from a series of Terrestrial Laser Scanning campaigns covering a complete ablation season. The measured ablation rates indicate that the advection of sensible heat causes locally increased ablation rates at the upwind edges of the snow patches. The effect, however, appears to be active over rather short distances except for very strong wind conditions. Neglecting this effect, the model is able to capture the mean ablation rates for early ablation periods but strongly overestimates snow ablation once the fraction of snow coverage is below a critical value. While radiation dominates snow ablation early in the season, the turbulent flux contribution becomes important late in the season. Simulation results indicate that the air temperatures appear to overestimate the local air temperature above snow patches once the snow coverage is below a critical value. Measured turbulent fluxes support these findings by suggesting a stable internal boundary layer close to the snow surface causing a strong decrease of the sensible heat flux towards the snow cover. Thus, the existence of a stable internal boundary layer above a patchy snow cover exerts a dominant control on the timing and magnitude of snow ablation for patchy snow covers.<br/
Transport in disordered graphene nanoribbons
We study electronic transport in graphene nanoribbons with rough edges. We
first consider a model of weak disorder that corresponds to an armchair ribbon
whose width randomly changes by a single unit cell size. We find that in this
case, the low-temperature conductivity is governed by an effective
one-dimensional hopping between segments of distinct band structure. We then
provide numerical evidence and qualitative arguments that similar behavior also
occurs in the limit of strong uncorrelated boundary disorder.Comment: 5 pages, 3 figures. version as published in PR
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