6,544 research outputs found
Classical Rotons in Cold Atomic Traps
We predict the emergence of a roton minimum in the dispersion relation of
elementary excitations in cold atomic gases in the presence of diffusive light.
In large magneto-topical traps, multiple-scattering of light is responsible for
the collective behavior of the system, which is associated to an effective
Coulomb-like interaction between the atoms. In optically thick clouds, the
re-scattered light undergoes diffusive propagation, which is responsible for a
stochastic short-range force acting on the atoms. We show that the dynamical
competition between these two forces results on a new polariton mode, which
exhibits a roton minimum. Making use of Feynman's formula for the static
structure factor, we show that the roton minimum is related to the appearance
of long-range order in the system.Comment: 5 pages, 3 figure
Structural Properties of the Disordered Spherical and other Mean Field Spin Models
We extend the approach of Aizenman, Sims and Starr for the SK-type models to
their spherical versions. Such an extension has already been performed for
diluted spin glasses. The factorization property of the optimal structures
found by Guerra for the SK model, which holds for diluted models as well, is
verified also in the case of spherical systems, with the due modifications.
Hence we show that there are some common structural features in various mean
field spin models. These similarities seem to be quite paradigmatic, and we
summarize the various techniques typically used to prove the structural
analogies and to tackle the computation of the free energy per spin in the
thermodynamic limit.Comment: 24 page
Path integrals and symmetry breaking for optimal control theory
This paper considers linear-quadratic control of a non-linear dynamical
system subject to arbitrary cost. I show that for this class of stochastic
control problems the non-linear Hamilton-Jacobi-Bellman equation can be
transformed into a linear equation. The transformation is similar to the
transformation used to relate the classical Hamilton-Jacobi equation to the
Schr\"odinger equation. As a result of the linearity, the usual backward
computation can be replaced by a forward diffusion process, that can be
computed by stochastic integration or by the evaluation of a path integral. It
is shown, how in the deterministic limit the PMP formalism is recovered. The
significance of the path integral approach is that it forms the basis for a
number of efficient computational methods, such as MC sampling, the Laplace
approximation and the variational approximation. We show the effectiveness of
the first two methods in number of examples. Examples are given that show the
qualitative difference between stochastic and deterministic control and the
occurrence of symmetry breaking as a function of the noise.Comment: 21 pages, 6 figures, submitted to JSTA
Orbital M1 versus E2 strength in deformed nuclei: A new energy weighted sum rule
Within the unified model of Bohr and Mottelson we derive the following linear
energy weighted sum rule for low energy orbital 1 excitations in even-even
deformed nuclei S_{\rm LE}^{\rm lew} (M_1^{\rm orb}) \cong (6/5) \epsilon
(B(E2; 0^+_1 \rightarrow 2_1^+ K=0)/Z e^2^2) \mu^2_N with B(E2) the E2
strength for the transition from the ground state to the first excited state in
the ground state rotational band, the charge r.m.s. radius squared and
the binding energy per nucleon in the nuclear ground state. It is
shown that this energy weighted sum rule is in good agreement with available
experimental data. The sum rule is derived using a simple ansatz for the
intrinsic ground state wave function that predicts also high energy 1
strength at 2 carrying 50\% of the total moment of the
orbital M1 operator.Comment: REVTEX (3.0), 9 pages, RU924
Description of single and double analog states in the f7/2 shell: The Ti isotopes
The excitation energies of single analog states in even-odd Ti isotopes and
double analog states in even-even Ti isotopes are microscopically described in
a single j-shell formalism. A projection procedure for generalized BCS states
has been used. As an alternative description a particle-core formalism is
presented. The latter picture provides a two-parameter expression for
excitation energies, which describes fairly well the data in four odd and three
even isotopes of Ti.Comment: 14 pages,7 figures, 2 tables. To appear in Phys. Rev.
Effects of Position of Rainfed Rice Field in a Toposequence on Water Availability and Rice Yield in Central Java, Indonesia
The productivity of rainfed rice needs to be increased in order to support the Indonesian Food Security programs, especially rice. Rainfall is one of the main sources of the water availability on the rainfed rice field. This research was conducted from October 2000 to February 2001 at four sites in Central Java Province. The objectives of this research were to study thevariability of water availability that influenced by toposequen's position and to analyze the rice yields due to treatments on toposequence. This experiment was carried out in the farmers' field using “Group Balanced Block in Split-Split Plot Design” and Inceptisols soil order. Mainplots were the four positions in toposequence (top, upper middle, lower middle, and bottom), and the four villages were used as replication. The sub-plot wastreatments group and sub-sub-plot consists of group-1: farmer's practice and without weeding; group-2: farmer's practice without fertilizer and added recommended fertilizer. The research showed that rainfall affected the fluctuation of ground water table and standing water periods. The straw and unhulled rice/grain yields at the bottom position was the highest, and was significantly different from the top position of the toposequence, namely 7.1 and 5.6 t ha-1 for straw yield while 5.2 and 4.0 t ha-1 for grainyield. Without weeding, the straw and grain yields decreased by 1.0 t ha-1 (6.1 to 5.1 t ha-1) and 0.8 t ha-1 (4.6 to 3.8 t ha-1). While, without fertilization, the straw and grain yields were produced only 4.4 and 3.8 t ha-1. Straw and grain yields increased up to 7.0 and 5.1 t ha-1, respectively, when recommended fertilizers were applied
Effects of T=0 two body matrix elements on M1 and Gamow-Teller transitions: isospin decomposition
We perform calculations for M1 transitions and allowed Gamow Teller (GT)
transitions in the even-even Titanium isotopes - Ti, Ti, and
Ti. We first do calculations with the FPD6 interaction. Then to study
the effect of T=0 matrix elements on the M1 and GT rates we introduce a second
interaction in which all the T=0 matrix elements are set equal to zero and a
third in which all the T=0 matrix elements are set to a constant. For the
latter two interactions the T=1 matrix elements are the same as for FPD6. We
are thus able to study the effects of the fluctuating T=0 matrix elements on M1
and GT rates
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