7,426 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
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&
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
Low-energy excitations of the one-dimensional half-filled SU(4) Hubbard model with an attractive on-site interaction: Density-matrix renormalization-group calculations and perturbation theory
We investigate low-energy excitations of the one-dimensional half-filled
SU(4) Hubbard model with an attractive on-site interaction U < 0 using the
density matrix renormalization group method as well as a perturbation theory.
We find that the ground state is a charge density wave state with a long range
order. The ground state is completely incompressible since all the excitations
are gapful. The charge gap which is the same as the four-particle excitation
gap is a non-monotonic function of U, while the spin gap and others increase
with increasing |U| and have linear asymptotic behaviors.Comment: 4 pages, 3 figures, submitte
Towards first-principles understanding of the metal-insulator transition in fluid alkali metals
By treating the electron-ion interaction as perturbation in the
first-principles Hamiltonian, we have calculated the density response functions
of a fluid alkali metal to find an interesting charge instability due to
anomalous electronic density fluctuations occurring at some finite wave vector
{\bi Q} in a dilute fluid phase above the liquid-gas critical point. Since
|{\bi Q}| is smaller than the diameter of the Fermi surface, this instability
necessarily impedes the electric conduction, implying its close relevance to
the metal-insulator transition in fluid alkali metals.Comment: 11 pages, 5 figure
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
Quantum glass phases in the disordered Bose-Hubbard model
The phase diagram of the Bose-Hubbard model in the presence of off-diagonal
disorder is determined using Quantum Monte Carlo simulations. A sequence of
quantum glass phases intervene at the interface between the Mott insulating and
the Superfluid phases of the clean system. In addition to the standard Bose
glass phase, the coexistence of gapless and gapped regions close to the Mott
insulating phase leads to a novel Mott glass regime which is incompressible yet
gapless. Numerical evidence for the properties of these phases is given in
terms of global (compressibility, superfluid stiffness) and local
(compressibility, momentum distribution) observables
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