581 research outputs found
Exactly solvable path integral for open cavities in terms of quasinormal modes
We evaluate the finite-temperature Euclidean phase-space path integral for
the generating functional of a scalar field inside a leaky cavity. Provided the
source is confined to the cavity, one can first of all integrate out the fields
on the outside to obtain an effective action for the cavity alone.
Subsequently, one uses an expansion of the cavity field in terms of its
quasinormal modes (QNMs)-the exact, exponentially damped eigenstates of the
classical evolution operator, which previously have been shown to be complete
for a large class of models. Dissipation causes the effective cavity action to
be nondiagonal in the QNM basis. The inversion of this action matrix inherent
in the Gaussian path integral to obtain the generating functional is therefore
nontrivial, but can be accomplished by invoking a novel QNM sum rule. The
results are consistent with those obtained previously using canonical
quantization.Comment: REVTeX, 26 pages, submitted to Phys. Rev.
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
X-Ray-Diffraction Study of Charge-Density-Waves and Oxygen-Ordering in YBa2Cu3O6+x Superconductor
We report a temperature-dependent increase below 300 K of diffuse
superlattice peaks corresponding to q_0 =(~2/5,0,0) in an under-doped
YBa_2Cu_3O_6+x superconductor (x~0.63). These peaks reveal strong c-axis
correlations involving the CuO_2 bilayers, show a non-uniform increase below
\~220 K with a plateau for ~100-160 K, and appear to saturate in the
superconducting phase. We interpret this unconventional T-dependence of the
``oxygen-ordering'' peaks as a manifestation of a charge density wave in the
CuO_2 planes coupled to the oxygen-vacancy ordering.Comment: 4 pages, 4 figure
Quantum Monte Carlo and variational approaches to the Holstein model
Based on the canonical Lang-Firsov transformation of the Hamiltonian we
develop a very efficient quantum Monte Carlo algorithm for the Holstein model
with one electron. Separation of the fermionic degrees of freedom by a
reweighting of the probability distribution leads to a dramatic reduction in
computational effort. A principal component representation of the phonon
degrees of freedom allows to sample completely uncorrelated phonon
configurations. The combination of these elements enables us to perform
efficient simulations for a wide range of temperature, phonon frequency and
electron-phonon coupling on clusters large enough to avoid finite-size effects.
The algorithm is tested in one dimension and the data are compared with
exact-diagonalization results and with existing work. Moreover, the ideas
presented here can also be applied to the many-electron case. In the
one-electron case considered here, the physics of the Holstein model can be
described by a simple variational approach.Comment: 18 pages, 11 Figures, v2: one typo correcte
Bohmian Mechanics at Space-Time Singularities. II. Spacelike Singularities
We develop an extension of Bohmian mechanics by defining Bohm-like
trajectories for quantum particles in a curved background space-time containing
a spacelike singularity. As an example of such a metric we use the
Schwarzschild metric, which contains two spacelike singularities, one in the
past and one in the future. Since the particle world lines are everywhere
timelike or lightlike, particles can be annihilated but not created at a future
spacelike singularity, and created but not annihilated at a past spacelike
singularity. It is argued that in the presence of future (past) spacelike
singularities, there is a unique natural Bohm-like evolution law directed to
the future (past). This law differs from the one in non-singular space-times
mainly in two ways: it involves Fock space since the particle number is not
conserved, and the wave function is replaced by a density matrix. In
particular, we determine the evolution equation for the density matrix, a
pure-to-mixed evolution equation of a quasi-Lindblad form. We have to leave
open whether a curvature cut-off needs to be introduced for this equation to be
well defined.Comment: 42 pages LaTeX, no figures; v2 shortened and minor change
Berry phases and pairing symmetry in Holstein-Hubbard polaron systems
We study the tunneling dynamics of dopant-induced hole polarons which are
self-localized by electron-phonon coupling in a two-dimensional antiferro-
magnet. Our treatment is based on a path integral formulation of the adia-
batic approximation, combined with many-body tight-binding, instanton, con-
strained lattice dynamics, and many-body exact diagonalization techniques. Our
results are mainly based on the Holstein- and, for comparison, on the
Holstein-Hubbard model. We also study effects of 2nd neighbor hopping and
long-range electron-electron Coulomb repulsion. The polaron tunneling dynamics
is mapped onto an effective low-energy Hamiltonian which takes the form of a
fermion tight-binding model with occupancy dependent, predominant- ly 2nd and
3rd neighbor tunneling matrix elements, excluded double occupan- cy, and an
effective intersite charge interactions. Antiferromagnetic spin correlations in
the original many-electron Hamiltonian are reflected by an attractive
contribution to the 1st neighbor charge interaction and by Berry phase factors
which determine the signs of effective polaron tunneling ma- trix elements. In
the two-polaron case, these phase factors lead to polaron pair wave functions
of either -wave symmetry or p-wave symme- try with zero and
nonzero total pair momentum, respectively. Implications for the doping
dependent isotope effect, pseudo-gap and Tc of a superconduc- ting polaron pair
condensate are discussed/compared to observed in cuprates.Comment: 23 pages, revtex, 13 ps figure
Pulsar-wind nebulae and magnetar outflows: observations at radio, X-ray, and gamma-ray wavelengths
We review observations of several classes of neutron-star-powered outflows:
pulsar-wind nebulae (PWNe) inside shell supernova remnants (SNRs), PWNe
interacting directly with interstellar medium (ISM), and magnetar-powered
outflows. We describe radio, X-ray, and gamma-ray observations of PWNe,
focusing first on integrated spectral-energy distributions (SEDs) and global
spectral properties. High-resolution X-ray imaging of PWNe shows a bewildering
array of morphologies, with jets, trails, and other structures. Several of the
23 so far identified magnetars show evidence for continuous or sporadic
emission of material, sometimes associated with giant flares, and a few
possible "magnetar-wind nebulae" have been recently identified.Comment: 61 pages, 44 figures (reduced in quality for size reasons). Published
in Space Science Reviews, "Jets and Winds in Pulsar Wind Nebulae, Gamma-ray
Bursts and Blazars: Physics of Extreme Energy Release
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
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