29,511 research outputs found
Some Global Aspects of Duality is String Theory
We explore some of the global aspects of duality transformations in String
Theory and Field Theory. We analyze in some detail the equivalence of dual
models corresponding to different topologies at the level of the partition
function and in terms of the operator correspondence for abelian duality. We
analyze the behavior of the cosmological constant under these transformations.
We also explore several examples of non-abelian duality where the classical
background interpretation can be maintained for the original and the dual
theories. In particular we construct a non-abelian dual of which
turns out to be a three-dimensional black holeComment: 31pp. One figure available upon request. CERN-TH-6991/6
Competing Ferromagnetic and Charge-Ordered States in Models for Manganites: the Origin of the CMR Effect
The one-orbital model for manganites with cooperative phonons and
superexchange coupling has been investigated via large-scale Monte
Carlo (MC) simulations. Results for two-orbitals are also briefly discussed.
Focusing on electronic density =0.75, a regime of competition between
ferromagnetic (FM) metallic and charge-ordered (CO) insulating states was
identified. In the vicinity of the associated bicritical point, colossal
magnetoresistance (CMR) effects were observed. The CMR is associated with the
development of short-distance correlations among polarons, above the spin
ordering temperatures, resembling the charge arrangement of the low-temperature
CO state.Comment: 4 pages, 4 figures, accepted for publication in the Phys. Rev. Let
Reionization Histories of Milky Way Mass Halos
We investigate the connection between the epoch of reionization and the
present day universe, by examining the extended mass reionization histories of
dark matter halos identified at z=0. We combine an N-body dark matter
simulation of a 600 Mpc volume with a three-dimensional, seminumerical
reionization model. This provides reionization redshifts for each particle,
which can then be connected with the properties of their halos at the present
time. We find that the vast majority of present-day halos with masses larger
than ~ few x 10^11 Msun reionize earlier than the rest of the universe. We also
find significant halo-to-halo diversity in mass reionization histories, and
find that in realistic inhomogenous models, the material within a given halo is
not expected to reionize at the same time. In particular, the scatter in
reionization times within individual halos is typically larger than the scatter
among halos. From our fiducial reionization model, we find that the typical 68%
scatter in reionization times within halos is ~ 115 Myr for 10^(12 \pm 0.25)
Msun halos, decreasing slightly to ~ 95 Myr for 10^(15 \pm 0.25) Msun halos. We
find a mild correlation between reionization history and environment: halos
with shorter reionization histories are typically in more clustered
environments, with the strongest trend on a scale of ~ 20 Mpc. Material in
Milky Way mass halos with short reionization histories is preferentially
reionized in relatively large HII regions, implying reionization mostly by
sources external to the progenitors of the present-day halo. We investigate the
impact on our results of varying the reionization model parameters, which span
a range of reionization scenarios with varying timing and morphology.Comment: 11 pages, 10 figures, 1 table. Submitted to Ap
Non-Abelian Duality and Canonical Transformations
We construct explicit canonical transformations producing non-abelian duals
in principal chiral models with arbitrary group G. Some comments concerning the
extension to more general -models, like WZW models, are given.Comment: 9 pags, latex fil
Fragility of the A-type AF and CE Phases of Manganites: An Exotic Insulator-to-Metal Transition Induced by Quenched Disorder
Using Monte Carlo simulations and the two eg-orbital model for manganites,
the stability of the CE and A-type antiferromagnetic insulating states is
analyzed when quenched disorder in the superexchange JAF between the t2g
localized spins and in the on-site energies is introduced. At vanishing or
small values of the electron-(Jahn-Teller)phonon coupling, the previously
hinted "fragility" of these insulating states is studied in detail, focusing on
their charge transport properties. This fragility is here found to induce a
rapid transition from the insulator to a (poor) metallic state upon the
introduction of disorder. A possible qualitative explanation is presented based
on the close proximity in energy of ferromagnetic metallic phases, and also on
percolative ideas valid at large disorder strength. The scenario is compared
with previously discussed insulator-to-metal transitions in other contexts. It
is argued that the effect unveiled here has unique properties that may define a
new class of giant effects in complex oxides. This particularly severe effect
of disorder must be present in other materials as well, in cases involving
phases that arise as a compromise between very different tendencies, as it
occurs with striped states in the cuprates.Comment: 13 pages, 17 figures, RevTex 4, submitted for publicatio
Pseudospectral Model Predictive Control under Partially Learned Dynamics
Trajectory optimization of a controlled dynamical system is an essential part
of autonomy, however many trajectory optimization techniques are limited by the
fidelity of the underlying parametric model. In the field of robotics, a lack
of model knowledge can be overcome with machine learning techniques, utilizing
measurements to build a dynamical model from the data. This paper aims to take
the middle ground between these two approaches by introducing a semi-parametric
representation of the underlying system dynamics. Our goal is to leverage the
considerable information contained in a traditional physics based model and
combine it with a data-driven, non-parametric regression technique known as a
Gaussian Process. Integrating this semi-parametric model with model predictive
pseudospectral control, we demonstrate this technique on both a cart pole and
quadrotor simulation with unmodeled damping and parametric error. In order to
manage parametric uncertainty, we introduce an algorithm that utilizes Sparse
Spectrum Gaussian Processes (SSGP) for online learning after each rollout. We
implement this online learning technique on a cart pole and quadrator, then
demonstrate the use of online learning and obstacle avoidance for the dubin
vehicle dynamics.Comment: Accepted but withdrawn from AIAA Scitech 201
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