940 research outputs found
Can we always get the entanglement entropy from the Kadanoff-Baym equations? The case of the T-matrix approximation
We study the time-dependent transmission of entanglement entropy through an
out-of-equilibrium model interacting device in a quantum transport set-up. The
dynamics is performed via the Kadanoff-Baym equations within many-body
perturbation theory. The double occupancy , needed to determine the entanglement entropy, is obtained from
the equations of motion of the single-particle Green's function. A remarkable
result of our calculations is that can become negative, thus not permitting to evaluate the
entanglement entropy. This is a shortcoming of approximate, and yet conserving,
many-body self-energies. Among the tested perturbation schemes, the -matrix
approximation stands out for two reasons: it compares well to exact results in
the low density regime and it always provides a non-negative . For the second part of this statement, we
give an analytical proof. Finally, the transmission of entanglement across the
device is diminished by interactions but can be amplified by a current flowing
through the system.Comment: 6 pages, 6 figure
A multichannel reflectometer for edge density profile measurements at the ICRF antenna in ASDEX upgrade
A multichannel reflectometer will be built for the new three-straps ICRF antenna of ASDEX Upgrade (AUG), to study the density behavior in front of it. Ten different accesses to the plasma are available for the three reflectometer channels that can be interchanged without breaking the machine vacuum. Frequency is scanned from 40 GHz to 68 GHz, in 10 mu s, which corresponds to a cut-off density ranging from 10(18) divided by 10(19)m(-3) in the Right cut-off of the X-mode propagation, for standard toroidal magnetic field values of AUG
Analyzing the success of T-matrix diagrammatic theories in representing a modified Hubbard model
We present a systematic study of various forms of renormalization that can be
applied in the calculation of the self-energy of the Hubbard model within the
T-matrix approximation. We compare the exact solutions of the attractive and
repulsive Hubbard models, for linear chains of lengths up to eight sites, with
all possible taxonomies of the T-matrix approximation. For the attractive
Hubbard model, the success of a minimally self-consistent theory found earlier
in the atomic limit (Phys. Rev. B 71, 155111 (2005)) is not maintained for
finite clusters unless one is in the very strong correlation limit. For the
repulsive model, in the weak correlation limit at low electronic densities --
that is, where one would expect a self-consistent T-matrix theory to be
adequate -- we find the fully renormalized theory to be most successful. In our
studies we employ a modified Hubbard interaction that eliminates all Hartree
diagrams, an idea which was proposed earlier (Phys. Rev. B 63, 035104 (2000)).Comment: Includes modified discussion of 1st-order phase transition. Accepted
for publication in J. Phys.: Condensed Matte
Physical properties of dense cores in Orion B9
We aim to determine the physical and chemical properties of dense cores in
Orion B9. We observed the NH3(1,1) and (2,2), and the N2H+(3-2) lines towards
the submm peak positions. These data are used in conjunction with our LABOCA
870 micron dust continuum data. The gas kinetic temperature in the cores is
between ~9.4-13.9 K. The non-thermal velocity dispersion is subsonic in most of
the cores. The non-thermal linewidth in protostellar cores appears to increase
with increasing bolometric luminosity. The core masses are very likely drawn
from the same parent distribution as the core masses in Orion B North. Starless
cores in the region are likely to be gravitationally bound, and thus
prestellar. Some of the cores have a lower radial velocity than the systemic
velocity of the region, suggesting that they are members of the "low-velocity
part" of Orion B. The observed core-separation distances deviate from the
corresponding random-like model distributions. The distances between the
nearest-neighbours are comparable to the thermal Jeans length. The fractional
abundances of NH3 and N2H+ in the cores are ~1.5-9.8x10^{-8} and
~0.2-5.9x10^{-10}, respectively. The NH3 abundance appears to decrease with
increasing H2 column and number densities. The NH3/N2H+ column density ratio is
larger in starless cores than in cores with embedded protostars. The core
population in Orion B9 is comparable in physical properties to those in nearby
low-mass star-forming regions. It is unclear if the origin of cores could be
explained by turbulent fragmentation. On the other hand, many of the core
properties conform with the picture of dynamic core evolution. The Orion B9
region has probably been influenced by the feedback from the nearby Ori OB 1b
group, and the fragmentation of the parental cloud into cores could be caused
by gravitational instability.Comment: 17 pages, 11 figures, 7 tables. Accepted for publication in Astronomy
and Astrophysics. Version 2: minor language corrections adde
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