6,309 research outputs found
Formation of correlations and energy-conservation at short time scales
The formation of correlations due to collisions in an interacting nucleonic
system is investigated. Results from one-time kinetic equations are compared
with the Kadanoff and Baym two-time equation with collisions included in Born
approximation. A reasonable agreement is found for a proposed approximation of
the memory effects by a finite duration of collisions. This form of collision
integral is in agreement with intuitive estimates from Fermi's golden rule. The
formation of correlations and the build up time is calculated analytically for
the high temperature and the low temperature limit. Different approximate
expressions are compared with the numerical results. We present analytically
the time dependent interaction energy and the formation time for Gau\ss{}- and
Yukawa type of potentials.Comment: Europ. Lournal Physics A accepte
Correlations in Many-Body Systems with Two-time Green's Functions
The Kadanoff-Baym (KB) equations are solved numerically for infinite nuclear
matter. In particular we calculate correlation energies and correlation times.
Approximating the Green's functions in the KB collision kernel by the free
Green's functions the Levinson equation is obtained. This approximation is
valid for weak interactions and/or low densities. It relates to the extended
quasi-classical approximation for the spectral function. Comparing the
Levinson, Born and KB calculations allows for an estimate of higher order
spectral corrections to the correlations. A decrease in binding energy is
reported due to spectral correlations and off-shell parts in the reduced
density matrix
Magnetic field enhanced structural instability in EuTiO_{3}
EuTiO_{3} undergoes a structural phase transition from cubic to tetragonal at
T_S = 282 K which is not accompanied by any long range magnetic order. However,
it is related to the oxygen ocathedra rotation driven by a zone boundary
acoustic mode softening. Here we show that this displacive second order
structural phase transition can be shifted to higher temperatures by the
application of an external magnetic field (increased by 4 K for mu_{0}H = 9 T).
This observed field dependence is in agreement with theoretical predictions
based on a coupled spin-anharmonic-phonon interaction model.Comment: 4 pages, 4 figure
Encoding databases satisfying a given set of dependencies
Consider a relation schema with a set of dependency constraints. A fundamental question is what is the minimum space where the possible instances of the schema can be "stored". We study the following model. Encode the instances by giving a function which maps the set of possible instances into the set of words of a given length over the binary alphabet in a decodable way. The problem is to find the minimum length needed. This minimum is called the information content of the database. We investigate several cases where the set of dependency constraints consist of relatively simple sets of functional or multivalued dependencies. We also consider the following natural extension. Is it possible to encode the instances such a way that small changes in the instance cause a small change in the code. © 2012 Springer-Verlag
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