6,481 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
$\pi N$ 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

### Electronic origin of the incommensurate modulation in the structure of phosphorus IV

An incommensurate modulated structure was found recently in a light group V
element phosphorous in the phase P-IV stable in the pressure range 107-137 GPa.
We consider configurations of the Brillouin zone and Fermi sphere within a
nearly-free-electron model in order to analyze the importance of these
configurations for the crystal structure energy. For the phase P-IV with the
base-centered orthorhombic structure, oC2, we consider a commensurate
approximant with an 11-fold supercell along the c-axis and a modulation wave
vector equal 3/11 which is close to the experimentally observed value of 0.267.
Atomic shifts due to the modulation result in appearance of satellite
reflections and hence in a formation of additional Brillouin zone planes. The
stability of this structure is attributed to the lowering of the electronic
band structure energy due to Brillouin zone - Fermi surface interactions

### 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 $\sigma_T\sim0.1$ mbarn and the Callan-Gross relation which relates the
longitudinal cross section $\sigma_L$ to the transverse one. The result of the
calculation agrees with an existing calculation and predicts the negative
values of the asymmetry $B_n$ 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

### Temperature-dependent Cross Sections for Charmonium Dissociation in Collisions with Pions and Rhos in Hadronic Matter

Meson-charmonium dissociation reactions governed by the quark interchange are
studied with temperature-dependent quark potentials. Quark-antiquark
relative-motion wave functions and masses of charmonia and charmed mesons are
determined by the central spin-independent part of the potentials or by the
central spin-independent part and a smeared spin-spin interaction. The
prominent temperature dependence of the masses is found. Based on the
potentials, the wave functions, and the meson masses, we obtain
temperature-dependent cross sections for fifteen pion-charmonium and
rho-charmonium dissociation reactions. The numerical cross sections are
parametrized for future applications in hadronic matter. The particular
temperature dependence of the J/psi bound state leads to unusual behavior of
the cross sections for endothermic J/psi dissociation reactions. The quantum
numbers of psi' and chi_c can not make their difference in mass in the
temperature region 0.6T_c < T < T_c, but can make the psi' dissociation
different from the chi_c dissociation.Comment: 52 pages, 23 figures, 6 table

### Conductivity in a disordered one-dimensional system of interacting fermions

Dynamical conductivity in a disordered one-dimensional model of interacting
fermions is studied numerically at high temperatures and in the
weak-interaction regime in order to find a signature of many-body localization
and vanishing d.c. transport coefficients. On the contrary, we find in the
regime of moderately strong local disorder that the d.c. conductivity sigma0
scales linearly with the interaction strength while being exponentially
dependent on the disorder. According to the behavior of the charge stiffness
evaluated at the fixed number of particles, the absence of the many-body
localization seems related to an increase of the effective localization length
with the interaction.Comment: 4 pages, 5 figures, submitted to PR

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