13,948 research outputs found
Closed time path approach to the Casimir energy in real media
The closed time path formalism is applied, in the framework of open quantum
systems, to study the time evolution of the expectation value of the
energy-momentum tensor of a scalar field in the presence of real materials. We
analyze quantum fluctuations in a fully non-equilibrium scenario, when the
scalar field is interacting with the polarization degrees of freedom of matter,
described as quantum Brownian particles. A generalized analysis was done for
two types of couplings between the field and the material. On the one hand, we
considered a bilinear coupling, and on the other hand, a (more realistic)
current-type coupling as in the case of the electromagnetic field interacting
with matter. We considered the high temperature limit for the field, keeping
arbitrary temperatures for each part of the volume elements of the material. We
obtained a closed form for the Hadamard propagator, which let us study the
dynamical evolution of the expectations values of the energy-momentum tensor
components from the initial time. We showed that two contributions always take
place in the transient evolution: one of these is associated to the material
and the other one is only associated to the field. Transient features were
studied and the long-time limit was derived in several cases. We proved that in
the steady situation of a field in n + 1 dimensions, the material always
contribute unless is non-dissipative. Conversely, the proper field contribution
vanishes unless the material is non-dissipative or, moreover, at least for the
1 + 1 case, if there are regions without material. We conclude that any steady
quantization scheme in 1 + 1 dimensions must consider both contributions and we
argue why these results are physically expected from a dynamical point of view,
and also could be valid for higher dimensions based on the expected continuity
between the non-dissipative and real material cases.Comment: 28 pages, no figures. Version to appear in Phys. Rev.
Oscillations of Thick Accretion Discs Around Black Holes - II
We present a numerical study of the global modes of oscillation of thick
accretion discs around black holes. We have previously studied the case of
constant distributions of specific angular momentum. In this second paper, we
investigate (i) how the size of the disc affects the oscillation
eigenfrequencies, and (ii) the effect of power-law distributions of angular
momentum on the oscillations. In particular, we compare the oscillations of the
disc with the epicyclic eigenfrequencies of a test particle with different
angular momentum distributions orbiting around the central object. We find that
there is a frequency shift away from the epicyclic eigenfrequency of the test
particle to lower values as the size of the tori is increased. We have also
studied the response of a thick accretion disc to a localized external
perturbation using non constant specific angular momentum distributions within
the disc. We find that in this case it is also possible (as reported previously
for constant angular momentum distributions) to efficiently excite internal
modes of oscillation. In fact we show here that the local perturbations excite
global oscillations (acoustic p modes) closely related to the epicyclic
oscillations of test particles. Our results are particularly relevant in the
context of low mass X-ray binaries and microquasars, and the high frequency
Quasi-Periodic Oscillations (QPOs) observed in them. Our computations make use
of a Smooth Particle Hydrodynamics (SPH) code in azimuthal symmetry, and use a
gravitational potential that mimics the effects of strong gravity.Comment: 10 pages, 8 figures, accepted for publication as a paper in the
Monthly Notices of the Royal Astronomical Societ
Benchmarking Nonequilibrium Green's Functions against Configuration Interaction for time-dependent Auger decay processes
We have recently proposed a Nonequilibrium Green's Function (NEGF) approach
to include Auger decay processes in the ultrafast charge dynamics of
photoionized molecules. Within the so called Generalized Kadanoff-Baym Ansatz
the fundamental unknowns of the NEGF equations are the reduced one-particle
density matrix of bound electrons and the occupations of the continuum states.
Both unknowns are one-time functions like the density in Time-Dependent
Functional Theory (TDDFT). In this work we assess the accuracy of the approach
against Configuration Interaction (CI) calculations in one-dimensional model
systems. Our results show that NEGF correctly captures qualitative and
quantitative features of the relaxation dynamics provided that the energy of
the Auger electron is much larger than the Coulomb repulsion between two holes
in the valence shells. For the accuracy of the results dynamical
electron-electron correlations or, equivalently, memory effects play a pivotal
role. The combination of our NEGF approach with the Sham-Schl\"uter equation
may provide useful insights for the development of TDDFT exchange-correlation
potentials with a history dependence.Comment: 7 pages, 3 figure
Oscillations of Thick Accretion Discs Around Black Holes
We present a numerical study of the response of a thick accretion disc to a
localized, external perturbation with the aim of exciting internal modes of
oscillation. We find that the perturbations efficiently excite global modes
recently identified as acoustic p--modes, and closely related to the epicyclic
oscillations of test particles. The two strongest modes occur at
eigenfrequencies which are in a 3:2 ratio. We have assumed a constant specific
angular momentum distribution within the disc. Our models are in principle
scale--free and can be used to simulate accretion tori around stellar or super
massive black holes.Comment: 4 pages, 4 figures, accepted for publication as a letter in the
Monthly Notices of the Royal Astronomical Societ
Discovering a misaligned CO outflow related to the red MSX source G034.5964-01.0292
The red MSX source G034.5964-01.0292 (MSXG34), catalogued as a massive YSO,
was observed in molecular lines with the aim of discover and study molecular
outflows. We mapped a region of 3'x3' centered at MSXG34 using the Atacama
Submillimeter Telescope Experiment in the 12CO J=3-2 and HCO+ J=4-3 lines with
an angular and spectral resolution of 22" and 0.11 km/s, respectively.
Additionally, public 13CO J=1-0 and near-IR UKIDSS data obtained from the
Galactic Ring Survey and the WFCAM Sciencie Archive, respectively, were
analyzed. We found that the 12CO spectra towards the YSO present a
self-absorption dip, as it is usual in star forming regions, and spectral wings
evidencing outflow activity. The HCO+ was detected only towards the MSXG34
position at v_LSR ~ 14.2 km/s, in coincidence with the 12CO absorption dip and
approximately with the velocity of previous ammonia observations. HCO+ and NH3
are known to be enhanced in molecular outflows. Analyzing the spectral wings of
the 12CO line, we discovered misaligned red- and blue-shifted molecular
outflows associated with MSXG34. The near-IR emission shows a cone-like shape
nebulosity composed by two arc-like features related to the YSO, which can be
due to a cavity cleared in the circumstellar material by a precessing jet. This
can explain the misalignment in the molecular outflows. From the analysis of
the 13CO J=1--0 data we suggest that the YSO is very likely related to a
molecular clump ranging between 10 and 14 km/s. This suggests that MSXG34, with
an associated central velocity of about 14 km/s, may be located in the
background of this clump. Thus, the blue-shifted outflow is probably deflected
by the interaction with dense gas along the line of sight.Comment: Accepted in A&A June 10, 201
Time dependent transport phenomena
The aim of this review is to give a pedagogical introduction to our recently
proposed ab initio theory of quantum transport.Comment: 28 pages, 18 figure
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