14,255 research outputs found
Prospects for Measuring from Jets at HERA with Polarized Protons
The measurement of the polarized gluon distribution function
from photon gluon fusion processes in electron proton deep inelastic scattering
producing two jets has been investigated. The study is based on the MEPJET
simulation program. The size of the expected spin asymmetry and corresponding
statistical uncertainties for a possible measurement with polarized beams of
electrons and protons at HERA have been estimated. The results show that the
asymmetry can reach a few percents.Comment: 8 pages (Latex) plus 3 figures enclosed as a uuencoded postscript
file. The complete paper, including figures, is also available via anonymous
ftp at ftp://www-ttp.physik.uni-karlsruhe.de/ , or via www at
http://www-ttp.physik.uni-karlsruhe.de/cgi-bin/preprints
Quantum phases in mixtures of fermionic atoms
A mixture of spin-polarized light and heavy fermionic atoms on a finite size
2D optical lattice is considered at various temperatures and values of the
coupling between the two atomic species. In the case, where the heavy atoms are
immobile in comparison to the light atoms, this system can be seen as a
correlated binary alloy related to the Falicov-Kimball model. The heavy atoms
represent a scattering environment for the light atoms. The distributions of
the binary alloy are discussed in terms of strong- and weak-coupling
expansions. We further present numerical results for the intermediate
interaction regime and for the density of states of the light particles. The
numerical approach is based on a combination of a Monte-Carlo simulation and an
exact diagonalization method. We find that the scattering by the correlated
heavy atoms can open a gap in the spectrum of the light atoms, either for
strong interaction or small temperatures.Comment: 15 pages, 8 figure
Influence of ion implantation on the magnetic and transport properties of manganite films
We have used oxygen ions irradiation to generate controlled structural
disorder in thin manganite films. Conductive atomic force microscopy CAFM),
transport and magnetic measurements were performed to analyze the influence of
the implantation process in the physical properties of the films. CAFM images
show regions with different conductivity values, probably due to the random
distribution of point defect or inhomogeneous changes of the local Mn3+/4+
ratio to reduce lattice strains of the irradiated areas. The transport and
magnetic properties of these systems are interpreted in this context.
Metal-insulator transition can be described in the frame of a percolative
model. Disorder increases the distance between conducting regions, lowering the
observed TMI. Point defect disorder increases localization of the carriers due
to increased disorder and locally enhanced strain field. Remarkably, even with
the inhomogeneous nature of the samples, no sign of low field magnetoresistance
was found. Point defect disorder decreases the system magnetization but doesn t
seem to change the magnetic transition temperature. As a consequence, an
important decoupling between the magnetic and the metal-insulator transition is
found for ion irradiated films as opposed to the classical double exchange
model scenario.Comment: 27 pages, 11 Figure
Frequency splitting of intervalley phonons in graphene
We study the thermal distribution of intervalley phonons in a graphene sheet.
These phonons have two components with the same frequency. The degeneracy of
the two modes is preserved by weak electron-phonon coupling. A sufficiently
strong electron-phonon coupling, however, can result in a splitting into an
optical and an acoustic phonon branch, which creates a fluctuating gap in the
electronic spectrum. We describe these effects by treating the phonon
distribution within a saddle-point approximation. Fluctuations around the
saddle point indicate a Berezinskii-Kosterlitz-Thouless transition of the
acoustic branch. This transition might be observable in the polarization of
Raman scattered light.Comment: 5 pages, 1 figur
The reaction 13C(alpha,n)16O: a background for the observation of geo-neutrinos
The absolute cross section of the C(,n)O reaction has
been measured at E = 0.8 to 8.0 MeV with an overall accuracy of 4%.
The precision is needed to subtract reliably a background in the observation of
geo-neutrinos, e.g. in the KamLAND detector.Comment: LaTex file, 13 pages including 3 ps figures. Any request to
[email protected]. Phys. Rev . C, to appea
Polytopality and Cartesian products of graphs
We study the question of polytopality of graphs: when is a given graph the
graph of a polytope? We first review the known necessary conditions for a graph
to be polytopal, and we provide several families of graphs which satisfy all
these conditions, but which nonetheless are not graphs of polytopes. Our main
contribution concerns the polytopality of Cartesian products of non-polytopal
graphs. On the one hand, we show that products of simple polytopes are the only
simple polytopes whose graph is a product. On the other hand, we provide a
general method to construct (non-simple) polytopal products whose factors are
not polytopal.Comment: 21 pages, 10 figure
Functionalizing self-assembled GaN quantum dot superlattices by Eu-implantation
Self-assembled GaN quantum dots (QDs) stacked in superlattices (SL) with AlN spacer layers were implanted with Europium ions to fluences of 1013, 1014, and 1015 cm−2. The damage level introduced in the QDs by the implantation stays well below that of thick GaN epilayers. For the lowest fluence, the structural properties remain unchanged after implantation and annealing while for higher fluences the implantation damage causes an expansion of the SL in the [0001] direction which increases with implantation fluence and is only partly reversed after thermal annealing at 1000 °C. Nevertheless, in all cases, the SL quality remains very good after implantation and annealing with Eu ions incorporated preferentially into near-substitutional cation sites. Eu3+ optical activation is achieved after annealing in all samples. In the sample implanted with the lowest fluence, the Eu3+ emission arises mainly from Eu incorporated inside the QDs while for the higher fluences only the emission from Eu inside the AlN-buffer, capping, and spacer layers is observed.
© 2010 American Institute of PhysicsFCT-PTDC/CTM/100756/2008program PESSOA EGIDE/GRICESFCT-SFRH/BD/45774/2008FCT-SFRH/BD/44635/200
Density of states "width parity" effect in d-wave superconducting quantum wires
We calculate the density of states (DOS) in a clean mesoscopic d-wave
superconducting quantum wire, i.e. a sample of infinite length but finite width
. For open boundary conditions, the DOS at zero energy is found to be zero
if is even, and nonzero if is odd. At finite chemical potential, all
chains are gapped but the qualtitative differences between even and odd
remain.Comment: 7 pages, 8 figures, new figures and extended discussio
Origin of magnetic moments in defective TiO2 single crystals
In this paper we show that ferromagnetism can be induced in pure TiO2 single
crystals by oxygen ion irradiation. By combining x-ray diffraction,
Raman-scattering, and electron spin resonance spectroscopy, a defect complex,
\emph{i.e.} Ti ions on the substitutional sites accompanied by oxygen
vacancies, has been identified in irradiated TiO2. This kind of defect complex
results in a local (TiO) stretching Raman mode. We elucidate that
Ti ions with one unpaired 3d electron provide the local magnetic
moments.Comment: 4 pages, 4 figures, to be published at Phys. Rev.
Breakdown of anomalous channeling with ion energy for accurate strain determination in gan-based heterostructures
The influence of the beam energy on the determination of strain state with ion channeling in GaN-based heterostructures (HSs) is addressed. Experimental results show that anomalous channeling may hinder an accurate analysis due to the steering effects at the HS interface, which are more intense at lower ion energies. The experimental angular scans have been well reproduced by Monte Carlo simulations, correlating the steering effects with the close encounter probability at the interface. Consequently, limitations in the determination of the strain state by ion channeling can be overcome by selecting the adequate beam energy
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