312 research outputs found
A variational approach to the optimized phonon technique for electron-phonon problems
An optimized phonon approach for the numerical diagonalization of interacting
electron-phonon systems is proposed. The variational method is based on an
expansion in coherent states that leads to a dramatic truncation in the phonon
space. The reliability of the approach is demonstrated for the extended
Holstein model showing that different types of lattice distortions are present
at intermediate electron-phonon couplings as observed in strongly correlated
systems. The connection with the density matrix renormalization group is
discussed.Comment: 4 figures; submitted to Phys. Rev.
Coherent Control for a Two-level System Coupled to Phonons
The interband polarizations induced by two phase-locked pulses in a
semiconductor show strong interference effects depending on the time tau_1
separating the pulses. The four-wave mixing signal diffracted from a third
pulse delayed by tau is coherently controlled by tuning tau_1. The four-wave
mixing response is evaluated exactly for a two-level system coupled to a single
LO phonon. In the weak coupling regime it shows oscillations with the phonon
frequency which turn into sharp peaks at multiples of the phonon period for a
larger coupling strength. Destructive interferences between the two
phase-locked pulses produce a splitting of the phonon peaks into a doublet. For
fixed tau but varying tau_1 the signal shows rapid oscillations at the
interband-transition frequency, whose amplitude exhibits bursts at multiples of
the phonon period.Comment: 4 pages, 4 figures, RevTex, content change
Mass Renormalization in the Su-Schrieffer-Heeger Model
This study of the one dimensional Su-Schrieffer-Heeger model in a weak
coupling perturbative regime points out the effective mass behavior as a
function of the adiabatic parameter , is the
zone boundary phonon energy and is the electron band hopping integral.
Computation of low order diagrams shows that two phonons scattering processes
become appreciable in the intermediate regime in which zone boundary phonons
energetically compete with band electrons. Consistently, in the intermediate
(and also moderately antiadiabatic) range the relevant mass renormalization
signals the onset of a polaronic crossover whereas the electrons are
essentially undressed in the fully adiabatic and antiadiabatic systems. The
effective mass is roughly twice as much the bare band value in the intermediate
regime while an abrupt increase (mainly related to the peculiar 1D dispersion
relations) is obtained at .Comment: To be published in Phys.Rev.B - 3 figure
Path integrals approach to resisitivity anomalies in anharmonic systems
Different classes of physical systems with sizeable electron-phonon coupling
and lattice distortions present anomalous resistivity behaviors versus
temperature. We study a molecular lattice Hamiltonian in which polaronic charge
carriers interact with non linear potentials provided by local atomic
fluctuations between two equilibrium sites. We study a molecular lattice
Hamiltonian in which polaronic charge carriers interact with non linear
potentials provided by local atomic fluctuations between two equilibrium sites.
A path integral model is developed to select the class of atomic oscillations
which mainly contributes to the partition function and the electrical
resistivity is computed in a number of representative cases. We argue that the
common origin of the observed resistivity anomalies lies in the time retarded
nature of the polaronic interactions in the local structural instabilities.Comment: 4 figures, to appear in Phys.Rev.B, May 1st (2001
Polaron features of the one-dimensional Holstein Molecular Crystal Model
The polaron features of the one-dimensional Holstein Molecular Crystal Model
are investigated by improving a variational method introduced recently and
based on a linear superposition of Bloch states that describe large and small
polaron wave functions. The mean number of phonons, the polaron kinetic energy,
the electron-phonon local correlation function, and the ground state spectral
weight are calculated and discussed. A crossover regime between large and small
polaron for any value of the adiabatic parameter is found and a
polaron phase diagram is proposed.Comment: 12 pages, 2 figure
Lattice dynamics effects on small polaron properties
This study details the conditions under which strong-coupling perturbation
theory can be applied to the molecular crystal model, a fundamental theoretical
tool for analysis of the polaron properties. I show that lattice dimensionality
and intermolecular forces play a key role in imposing constraints on the
applicability of the perturbative approach. The polaron effective mass has been
computed in different regimes ranging from the fully antiadiabatic to the fully
adiabatic. The polaron masses become essentially dimension independent for
sufficiently strong intermolecular coupling strengths and converge to much
lower values than those tradition-ally obtained in small-polaron theory. I find
evidence for a self-trapping transition in a moderately adiabatic regime at an
electron-phonon coupling value of .3. Our results point to a substantial
independence of the self-trapping event on dimensionality.Comment: 8 pages, 5 figure
The Physics of turbulent and dynamically unstable Herbig-Haro jets
The overall properties of the Herbig-Haro objects such as centerline
velocity, transversal profile of velocity, flow of mass and energy are
explained adopting two models for the turbulent jet. The complex shapes of the
Herbig-Haro objects, such as the arc in HH34 can be explained introducing the
combination of different kinematic effects such as velocity behavior along the
main direction of the jet and the velocity of the star in the interstellar
medium. The behavior of the intensity or brightness of the line of emission is
explored in three different cases : transversal 1D cut, longitudinal 1D cut and
2D map. An analytical explanation for the enhancement in intensity or
brightness such as usually modeled by the bow shock is given by a careful
analysis of the geometrical properties of the torus.Comment: 17 pages, 10 figures. Accepted for publication in Astrophysics &
Spac
Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in √s = 7 TeV pp collisions with the ATLAS detector
A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fb−1 of proton–proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results
Jet size dependence of single jet suppression in lead-lead collisions at sqrt(s(NN)) = 2.76 TeV with the ATLAS detector at the LHC
Measurements of inclusive jet suppression in heavy ion collisions at the LHC
provide direct sensitivity to the physics of jet quenching. In a sample of
lead-lead collisions at sqrt(s) = 2.76 TeV corresponding to an integrated
luminosity of approximately 7 inverse microbarns, ATLAS has measured jets with
a calorimeter over the pseudorapidity interval |eta| < 2.1 and over the
transverse momentum range 38 < pT < 210 GeV. Jets were reconstructed using the
anti-kt algorithm with values for the distance parameter that determines the
nominal jet radius of R = 0.2, 0.3, 0.4 and 0.5. The centrality dependence of
the jet yield is characterized by the jet "central-to-peripheral ratio," Rcp.
Jet production is found to be suppressed by approximately a factor of two in
the 10% most central collisions relative to peripheral collisions. Rcp varies
smoothly with centrality as characterized by the number of participating
nucleons. The observed suppression is only weakly dependent on jet radius and
transverse momentum. These results provide the first direct measurement of
inclusive jet suppression in heavy ion collisions and complement previous
measurements of dijet transverse energy imbalance at the LHC.Comment: 15 pages plus author list (30 pages total), 8 figures, 2 tables,
submitted to Physics Letters B. All figures including auxiliary figures are
available at
http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2011-02
Are there differences in acute phase inflammation markers regarding the type of heart failure?
This study aimed to determine if there are differences in inflammatory markers in the acute phase between systolic heart failure and heart failure with preserved systolic function. One hundred and thirty-one patients with acute heart failure were recruited consecutively. At admission, plasma fibrinogen, C-reactive protein, sialic acid, von Willebrand factor, vascular endothelial growth factor, interleukin-6 and NTproBNP were all evaluated. If the ejection fraction was 45% or over patients were included in the HF-PSF group; the remaining patients were included in the SHF group. The HF-PSF patients were older (72±10 vs 63±12 years, P<0.001), presented a higher rate of atrial fibrillation (56.1 vs 21.3%, P<0.001), and had a lower rate of hemoglobin (12.2±2 vs 13.3±2.1 g/dL, P<0.01). No significant differences were observed in the inflammation markers analyzed among SHF and HF-PSF groups. In the acute phase of heart failure there is a marked elevation of inflammatory markers but there are no differences in the inflammatory markers analyzed between the two different types of heart failure
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