7,932 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
Inelastic electron transport in polymer nanofibers
In this paper we present theoretical analysis of the electron transport in
conducting polymers. We concentrate on the study of the effects of temperature
on characteristics of the transport. We treat a conducting polymers in a metal
state as a network of metallic-like grains connected by electron quantum
tunneling via intermediate state localized on a polymer chain between the
grains. To analyze the effects of temperature on this kind of electron
intergrain transport we represent the thermal environment as a phonon bath
coupled to the intermediate state. The electron transmission is computed using
the Buttiker model within the scattering matrix formalism. This approach is
further developed, and the dephasing parameter is expessed in terms of relevant
energies including the thermal energy. It is shown that temperature
dependencies of both current and conductance associated with the above
transport mechanism differ from those typical for other conduction mechanisms
in conducting polymers. This could be useful to separate out the contribution
from the intergrain electron tunneling to the net electric current in transport
experiments on various polymer nanofibers. The proposed model could be used to
analyze inelastic electron transport through molecular junctions.Comment: 8 pages, 5 pictures; text added, figures adde
Doubly virtual Compton scattering and the beam normal spin asymmetry
We construct an invariant basis for Compton scattering with two virtual
photons (VVCS). The basis tensors are chosen to be gauge invariant and
orthogonal to each other. The properties of the corresponding 18 invariant
amplitudes are studied in detail. We consider the special case of elastic VVCS
with the virtualities of the initial and final photons equal. The invariant
basis for VVCS in this orthogonal form does not exist in the literatur. We
furthermore use this VVCS tensor for a calculation of the beam normal spin
asymmetry in the forward kinematics. For this, we relate the invariant
amplitudes to the helicity amplitudes of the VVCS reaction. The imaginary parts
of these latter are related to the inclusive cross section by means of the
optical theorem. We use the phenomenological value of the transverse cross
section mbarn and the Callan-Gross relation which relates the
longitudinal cross section to the transverse one. The result of the
calculation agrees with an existing calculation and predicts the negative
values of the asymmetry of the order of 4-6 ppm in the energy range from
6 to 45 ppm and for very forward angles.Comment: 13 pages, 2 figures, revtex, submitted to Phys. Rev. C; new version:
two figures added, typos correcte
Continuous Transition between Antiferromagnetic Insulator and Paramagnetic Metal in the Pyrochlore Iridate Eu2Ir2O7
Our single crystal study of the magneto-thermal and transport properties of
the pyrochlore iridate Eu2Ir2O7 reveals a continuous phase transition from a
paramagnetic metal to an antiferromagnetic insulator for a sample with
stoichiometry within ~1% resolution. The insulating phase has strong proximity
to an antiferromagnetic semimetal, which is stabilized by several % level of
the off-stoichiometry. Our observations suggest that in addition to electronic
correlation and spin-orbit coupling the magnetic order is essential for opening
the charge gap.Comment: 6 pages, 6 figure
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
Spin polaron theory for the photoemission spectra of layered cobaltates
Recently, strong reduction of the quasiparticle peaks and pronounced
incoherent structures have been observed in the photoemission spectra of
layered cobaltates. Surprisingly, these many-body effects are found to increase
near the band insulator regime. We explain these unexpected observations in
terms of a novel spin-polaron model for CoO_2 planes which is based on a fact
of the spin-state quasidegeneracy of Co^{3+} ions in oxides. Scattering of the
photoholes on spin-state fluctuations suppresses their coherent motion. The
observed ``peak-dip-hump'' type lineshapes are well reproduced by the theory.Comment: 4 pages, 4 figure
Theoretical investigation of the magnetic structure in YBa_2Cu_3O_6
As experimentally well established, YBa_2Cu_3O_6 is an antiferromagnet with
the magnetic moments lying on the Cu sites. Starting from this experimental
result and the assumption, that nearest-neighbor Cu atoms within a layer have
exactly antiparallel magnetic moments, the orientation of the magnetic moments
has been determined within a nonadiabatic extension of the Heisenberg model of
magnetism, called nonadiabatic Heisenberg model. Within this group-theoretical
model there exist four stable magnetic structures in YBa_2Cu_3O_6, two of them
are obviously identical with the high- and low-temperature structure
established experimentally. However, not all the magnetic moments which appear
to be antiparallel in neutron-scattering experiments are exactly antiparallel
within this group-theoretical model. Furthermore, within this model the
magnetic moments are not exactly perpendicular to the orthorhombic c axis
Metals get an awkward cousin
A newly predicted state of matter is a simple theoretical example of a phase
that conducts electricity but is not smoothly connected to our conventional
model of metals. A viewpoint on arXiv:1201.5998.Comment: Physics 5, 82 (2012
Electron localization near Mott transition in organic superconductor -(BEDT-TTF)Cu[N(CN)Br
The effect of disorder on the electronic properties near the Mott transition
is studied in an organic superconductor
-(BEDT-TTF)Cu[N(CN)]Br, which is systematically irradiated
by X-ray. We observe that X-ray irradiation causes Anderson-type electron
localization due to molecular disorder. The resistivity at low temperatures
demonstrates variable range hopping conduction with Coulomb interaction. The
experimental results show clearly that the electron localization by disorder is
enhanced by the Coulomb interaction near the Mott transition.Comment: 5 pages, 4 figure
Mutual impedance effects in scanned antenna arrays technical report no. 2
Mutual impedance effects in electronically-scanned antenna array
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