1,885 research outputs found
Hysteresis and re-entrant melting of a self-organized system of classical particles confined in a parabolic trap
A self-organized system composed of classical particles confined in a
two-dimensional parabolic trap and interacting through a potential with a
short-range attractive part and long-range repulsive part is studied as
function of temperature. The influence of the competition between the
short-range attractive part of the inter-particle potential and its long-range
repulsive part on the melting temperature is studied. Different behaviors of
the melting temperature are found depending on the screening length ()
and the strength () of the attractive part of the inter-particle potential.
A re-entrant behavior and a thermal induced phase transition is observed in a
small region of ()-space. A structural hysteresis effect is observed
as a function of temperature and physically understood as due to the presence
of a potential barrier between different configurations of the system.Comment: 8 pages, 6 figure
Confinement of two-dimensional excitons in a non-homogeneous magnetic field
The effective Hamiltonian describing the motion of an exciton in an external
non-homogeneous magnetic field is derived. The magnetic field plays the role of
an effective potential for the exciton motion, results into an increment of the
exciton mass and modifies the exciton kinetic energy operator. In contrast to
the homogeneous field case, the exciton in a non-homogeneous magnetic field can
also be trapped in the low field region and the field gradient increases the
exciton confinement. The trapping energy and wave function of the exciton in a
GaAs two-dimensional electron gas for specific circular magnetic field
configurations are calculated. The results show than excitons can be trapped by
non-homogeneous magnetic fields, and that the trapping energy is strongly
correlated with the shape and strength of the non-homogeneous magnetic field
profile.Comment: 9 pages, 12 figure
Transition from single-file to two-dimensional diffusion of interacting particles in a quasi-one-dimensional channel
Diffusive properties of a monodisperse system of interacting particles
confined to a \textit{quasi}-one-dimensional (Q1D) channel are studied using
molecular dynamics (MD) simulations. We calculate numerically the mean-squared
displacement (MSD) and investigate the influence of the width of the channel
(or the strength of the confinement potential) on diffusion in finite-size
channels of different shapes (i.e., straight and circular). The transition from
single-file diffusion (SFD) to the two-dimensional diffusion regime is
investigated. This transition (regarding the calculation of the scaling
exponent () of the MSD ) as a
function of the width of the channel, is shown to change depending on the
channel's confinement profile. In particular the transition can be either
smooth (i.e., for a parabolic confinement potential) or rather sharp/stepwise
(i.e., for a hard-wall potential), as distinct from infinite channels where
this transition is abrupt. This result can be explained by qualitatively
different distributions of the particle density for the different confinement
potentials.Comment: 13 pages, 11 figure
Exciton trapping in magnetic wire structures
The lateral magnetic confinement of quasi two-dimensional excitons into wire
like structures is studied. Spin effects are take into account and two
different magnetic field profiles are considered, which experimentally can be
created by the deposition of a ferromagnetic stripe on a semiconductor quantum
well with magnetization parallel or perpendicular to the grown direction of the
well. We find that it is possible to confine excitons into one-dimensional (1D)
traps. We show that the dependence of the confinement energy on the exciton
wave vector, which is related to its free direction of motion along the wire
direction, is very small. Through the application of a background magnetic
field it is possible to move the position of the trapping region towards the
edge of the ferromagnetic stripe or even underneath the stripe. The exact
position of this 1D exciton channel depends on the strength of the background
magnetic field and on the magnetic polarisation direction of the ferromagnetic
film.Comment: 10 pages, 7 figures, to be published in J. Phys: Condens. Matte
Conditions for non-monotonic vortex interaction in two-band superconductors
We describe a semi-analytic approach to the two-band Ginzburg-Landau theory,
which predicts the behavior of vortices in two-band superconductors. We show
that the character of the short-range vortex-vortex interaction is determined
by the sign of the normal domain - superconductor interface energy, in analogy
with the conventional differentiation between type-I and type-II
superconductors. However, we also show that the long-range interaction is
determined by a modified Ginzburg-Landau parameter , different from
the standard of a bulk superconductor. This opens the possibility for
non-monotonic vortex-vortex interaction, which is temperature-dependent, and
can be further tuned by alterations of the material on the microscopic scale
Approximate Dynamic Programming via a Smoothed Linear Program
We present a novel linear program for the approximation of the dynamic programming cost-to-go function in high-dimensional stochastic control problems. LP approaches to approximate DP have typically relied on a natural “projection” of a well-studied linear program for exact dynamic programming. Such programs restrict attention to approximations that are lower bounds to the optimal cost-to-go function. Our program—the “smoothed approximate linear program”—is distinct from such approaches and relaxes the restriction to lower bounding approximations in an appropriate fashion while remaining computationally tractable. Doing so appears to have several advantages: First, we demonstrate bounds on the quality of approximation to the optimal cost-to-go function afforded by our approach. These bounds are, in general, no worse than those available for extant LP approaches and for specific problem instances can be shown to be arbitrarily stronger. Second, experiments with our approach on a pair of challenging problems (the game of Tetris and a queueing network control problem) show that the approach outperforms the existing LP approach (which has previously been shown to be competitive with several ADP algorithms) by a substantial margin
Bounds for Markov Decision Processes
We consider the problem of producing lower bounds on the optimal cost-to-go function of a Markov decision problem. We present two approaches to this problem: one based on the methodology of approximate linear programming (ALP) and another based on the so-called martingale duality approach. We show that these two approaches are intimately connected. Exploring this connection leads us to the problem of finding "optimal" martingale penalties within the martingale duality approach which we dub the pathwise optimization (PO) problem. We show interesting cases where the PO problem admits a tractable solution and establish that these solutions produce tighter approximations than the ALP approach. © 2013 The Institute of Electrical and Electronics Engineers, Inc
Parametrização de um modelo para estimativa da produtividade de soja para as regiões sul e centro-oeste do Brasil.
A produtividade de qualquer cultivo agrícola é resultado da interação genótipo e ambiente. No Brasil, cerca de 95% das indenizações pagas pelos instrumentos públicos de seguridade agrícola do Brasil estão relacionados à seca ou aos eventos de à chuva em excesso, mostrando sua importância. Na cultura de soja há dois períodos críticos no caso de déficit hídrico: da semeadura à emergência e no enchimento dos grãos. Em se tratando de um país como o Brasil cuja dimensão é continental e o cultivo de soja é nacional, a utilização de modelos que permitam obter estimativas confiáveis, rápidas e que necessitem de poucos dados operacionais são essenciais para seu gerenciamento. Usando o Modelo da Zona Agroecológica (MZA) foi feita a estimativa da produtividade de soja, a fim de se utilizar dados de fácil obtenção e que permitissem sua utilização em diversas localidades. O modelo foi avaliado para as condições das regiões Sul e Centro-Oeste entre as safras entre 2002 a 2006. Comparou-se as estimativas com os dados experimentais, verificando-se o bom desempenho do método para as regiões estudadas, com R²=0,54 numa análise global, mas com R²=0,97 para Dourados (MS) e R²=0,70 para Londrina (PR).CBA 2009
(Giant) Vortex - (anti) vortex interaction in bulk superconductors: The Ginzburg-Landau theory
The vortex-vortex interaction potential in bulk superconductors is calculated
within the Ginzburg-Landau (GL) theory and is obtained from a numerical
solution of a set of two coupled non-linear GL differential equations for the
vector potential and the superconducting order parameter, where the merger of
vortices into a giant vortex is allowed. Further, the interaction potentials
between a vortex and a giant vortex and between a vortex and an antivortex are
obtained for both type-I and type-II superconductors. Our numerical results
agree asymptotically with the analytical expressions for large inter-vortex
separations which are available in the literature. We propose new empirical
expressions valid over the full interaction range, which are fitted to our
numerical data for different values of the GL parameter
Two-component mixture of charged particles confined in a channel: melting
The melting of a binary system of charged particles confined in a {\it
quasi}-one-dimensional parabolic channel is studied through Monte Carlo
simulations. At zero temperature the particles are ordered in parallel chains.
The melting is anisotropic and different melting temperatures are obtained
according to the spatial direction, and the different types of particles
present in the system. Melting is very different for the single-, two- and
four-chain configurations. A temperature induced structural phase transition is
found between two different four chain ordered states which is absent in the
mono-disperse system. In the mixed regime, where the two types of particles are
only slightly different, melting is almost isotropic and a thermally induced
homogeneous distribution of the distinct types of charges is observed.Comment: To appear in Journal of Physics: condensed matter ; (13 pages, 12
figures
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