5,827 research outputs found
Sharp transition for single polarons in the one-dimensional Su-Schrieffer-Heeger model
We study a single polaron in the Su-Schrieffer-Heeger (SSH) model using four
different techniques (three numerical and one analytical). Polarons show a
smooth crossover from weak to strong coupling, as a function of the
electron-phonon coupling strength , in all models where this coupling
depends only on phonon momentum . In the SSH model the coupling also depends
on the electron momentum ; we find it has a sharp transition, at a critical
coupling strength , between states with zero and nonzero momentum of
the ground state. All other properties of the polaron are also singular at
, except the average number of phonons in the polaronic
cloud. This result is representative of all polarons with coupling depending on
and , and will have important experimental consequences (eg., in ARPES
and conductivity experiments)
Evolution of magnetic phases and orbital occupation in (SrMnO3)n/(LaMnO3)2n superlattices
The magnetic and electronic modifications induced at the interfaces in
(SrMnO)/(LaMnO) superlattices have been investigated
by linear and circular magnetic dichroism in the Mn L x-ray absorption
spectra. Together with theoretical calculations, our data demonstrate that the
charge redistribution across interfaces favors in-plane ferromagnetic (FM)
order and orbital occupation, in agreement with the
average strain. Far from interfaces, inside LaMnO, electron localization
and local strain favor antiferromagnetism (AFM) and
orbital occupation. For the high density of interfacial planes ultimately
leads to dominant FM order forcing the residual AFM phase to be in-plane too,
while for the FM layers are separated by AFM regions having
out-of-plane spin orientation.Comment: accepted for publication as a Rapid Communication in Physical Review
Measuring the Three-Dimensional Structure of Galaxy Clusters. II. Are clusters of galaxies oblate or prolate?
The intrinsic shape of galaxy clusters can be obtained through a combination
of X-ray and Sunyaev-Zeldovich effect observations once cosmological parameters
are assumed to be known. In this paper we discuss the feasibility of modelling
galaxy clusters as either prolate or oblate ellipsoids. We analyze the
intra-cluster medium distribution for a sample of 25 X-ray selected clusters,
with measured Sunyaev-Zeldovich temperature decrements. A mixed population of
prolate and oblate ellipsoids of revolution fits the data well, with prolate
shapes preferred on a 60-76% basis. We observe an excess of clusters nearly
aligned along the line of sight, with respect to what is expected from a
randomly oriented cluster population, which might imply the presence of a
selection bias in our sample. We also find signs that a more general triaxial
morphology might better describe the morphology of galaxy clusters. Additional
constraints from gravitational lensing could disentangle the degeneracy between
an ellipsoidal and a triaxial morphology, and could also allow an unbiased
determination of the Hubble constant.Comment: 9 pages, 8 figures, accepted for publication in Astrophys.
CMS Monte Carlo production in the WLCG computing Grid
Monte Carlo production in CMS has received a major boost in performance and
scale since the past CHEP06 conference. The production system has been re-engineered in order
to incorporate the experience gained in running the previous system and to integrate production
with the new CMS event data model, data management system and data processing framework.
The system is interfaced to the two major computing Grids used by CMS, the LHC Computing
Grid (LCG) and the Open Science Grid (OSG).
Operational experience and integration aspects of the new CMS Monte Carlo production
system is presented together with an analysis of production statistics. The new system
automatically handles job submission, resource monitoring, job queuing, job distribution
according to the available resources, data merging, registration of data into the data
bookkeeping, data location, data transfer and placement systems. Compared to the previous
production system automation, reliability and performance have been considerably improved. A
more efficient use of computing resources and a better handling of the inherent Grid unreliability
have resulted in an increase of production scale by about an order of magnitude, capable of
running in parallel at the order of ten thousand jobs and yielding more than two million events
per day
Effects of SO(10) D-Term on Yukawa Unification and Unstable Minima of the Supersymmetric Scalar Potential
We study the effects of SO(10) D-terms on the allowed parameter space (APS)
in models with and Yukawa unifiction. The former is
allowed only for moderate values of the D-term, if very precise ( 5%)
unification is required. Next we constrain the parameter space by looking for
different dangerous directions where the scalar potential may be unbounded from
below (UFB1 and UFB3). The common trilinear coupling plays a significant
role in constraing the APS. For very precise Yukawa unification,
can be probed at the LHC, where
is the common soft breaking mass for the sfermions. Moreover, an
interesting mass hierarchy with very heavy sfermions but light gauginos, which
is strongly disfavoured in models without D-terms, becomes fairly common in the
presence of the D-terms. The APS exhibits interesting characteristics if
is not the same as the soft breaking mass for the Higgs
sector. In unification models with D-terms, the APS consistent with
Yukawa unification and radiative electroweak symmetry breaking, increases as
the UFB1 constraint becomes weaker. However for , a stronger UFB3
condition still puts, for a given , a stringent upper bound on the
common gaugino mass () and a lower bound on for a given
. The effects of sign of on Yukawa unification and UFB
constraints are also discussed.Comment: Plain Latex, 22 pages, 11 figures. Small changes in the abstract, the
pattern of discussion changed signifiantly, no change in the figures and
results, a few new references added, version published in JP
Free energy of the Fr\"ohlich polaron in two and three dimensions
We present a novel Path Integral Monte Carlo scheme to solve the Fr\"ohlich
polaron model. At intermediate and strong electron-phonon coupling, the polaron
self-trapping is properly taken into account at the level of an effective
action obtained by a preaveraging procedure with a retarded trial action. We
compute the free energy at several couplings and temperatures in three and two
dimensions. Our results show that the accuracy of the Feynman variational upper
bound for the free energy is always better than 5% although the thermodynamics
derived from it is not correct. Our estimates of the ground state energies
demonstrate that the second cumulant correction to the variational upper bound
predicts the self energy to better than 1% at intermediate and strong coupling.Comment: RevTeX 7 pages 3 figures, revised versio
AC 114: a cluster with a soft X-ray tail
We present Chandra observations of the galaxy cluster AC114, which shows a
strongly irregular morphology, with signs of multiple merging activity. We
report the discovery of a soft X-ray filament originating close to the core of
the cluster. We confirm that X-ray emission is associated with two of three
mass concentrations identified in previous gravitational lensing studies of
this object. These two mass concentrations are located at opposite ends of the
soft filament, evidence for interaction between them. In the northern part, the
cluster shows two sharp discontinuities, both in surface brightness and in
temperature, evincing another, more recent merger event which took place in, or
close, to the cluster core. In spite of the merger activity, a combined mass
and lensing analysis shows remarkably good agreement between lensing and X-ray
masses. We therefore advocate for the robustness of the X-ray mass estimates,
and conclude that the assumption of hydrostatic equilibrium can yield accurate
mass estimates even in clusters as dynamically active as AC 114, once the gas
density distribution is properly mapped.Comment: 11 pages; to appear in ApJ 10 August 2004 issu
Charge dynamics of doped holes in high Tc cuprates - A clue from optical conductivity
The charge dynamics in weakly hole doped high temperature superconductors is
studied in terms of the accurate numerical solution to a model of a single hole
interacting with a quantum lattice in an antiferromagnetic background, and
accurate far-infrared ellipsometry measurements. The experimentally observed
two electronic bands in the infrared spectrum can be identified in terms of the
interplay between the electron correlation and electron-phonon interaction
resolving the long standing mystery of the mid-infrared band.Comment: 4 pages, 4 figures. Accepted to Phys. Rev. Letter
Interplay between electron-phonon and Coulomb interactions in cuprates
Evidence for strong electron-phonon coupling in high-Tc cuprates is reviewed,
with emphasis on the electron and phonon spectral functions. Effects due to the
interplay between the Coulomb and electron-phonon interactions are studied. For
weakly doped cuprates, the phonon self-energy is strongly reduced due to
correlation effects, while there is no corresponding strong reduction for the
electron self-energy. Polaron formation is studied, focusing on effects of
Coulomb interaction and antiferromagnetic correlations. It is argued that
experimental indications of polaron formation in undoped cuprates are due to a
strong electron-phonon interaction for these systems.Comment: 43 pages and 22 figure
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