3,061 research outputs found
Coarse grained models of stripe forming systems: phase diagrams, anomalies and scaling hypothesis
Two coarse-grained models which capture some universal characteristics of
stripe forming systems are stud- ied. At high temperatures, the structure
factors of both models attain their maxima on a circle in reciprocal space, as
a consequence of generic isotropic competing interactions. Although this is
known to lead to some universal properties, we show that the phase diagrams
have important differences, which are a consequence of the particular k
dependence of the fluctuation spectrum in each model. The phase diagrams are
computed in a mean field approximation and also after inclusion of small
fluctuations, which are shown to modify drastically the mean field behavior.
Observables like the modulation length and magnetization profiles are computed
for the whole temperature range accessible to both models and some important
differences in behavior are observed. A stripe compression modulus is computed,
showing an anomalous behavior with temperature as recently reported in related
models. Also, a recently proposed scaling hypothesis for modulated systems is
tested and found to be valid for both models studied.Comment: 9 pages, 13 figure
The nematic phase in stripe forming systems within the self consistent screening approximation
We show that in order to describe the isotropic-nematic transition in stripe
forming systems with isotropic competing interactions of the Brazovskii class
it is necessary to consider the next to leading order in a 1/N approximation
for the effective Hamiltonian. This can be conveniently accomplished within the
self-consistent screening approximation. We solve the relevant equations and
show that the self-energy in this approximation is able to generate the
essential wave vector dependence to account for the anisotropic character of
two-point correlation function characteristic of a nematic phase.Comment: 8 pages, 4 figure
Non Linear Moving-Average Conditional Heteroskedasticity
Ever since the appearance of the ARCH model (Engle 1982a), an impressive array of variance specifications belonging to the same class of models has emerged. Despite numerous successful developments, several empirical studies seem to show that their performance is not always appropriate. In this paper a new conditional heteroskedastic variance model is proposed: the Non-Linear Moving Average Conditional Heteroskedasticity (NLMACH). Its properties are similar to those of the ARCH-class specifications although it does not belong to this class and represents an alternative for modeling conditional volatility through a non-linear moving average specification. Pseudo Maximum likelihood allows for ease of estimation.Conditional Heteroskedastic Models, NLMACH(q), Volatility.
Shifted Landau levels in curved graphene sheets
We study the Landau levels in curved graphene sheets by measuring the
discrete energy spectrum in the presence of a magnetic field. We observe that
in rippled graphene sheets, the Landau energy levels satisfy the same square
root dependence on the energy quantum number as in flat sheets, . Though, we find that the Landau levels in curved sheets are shifted
towards lower energies by an amount proportional to the average spatial
deformation of the sheet. Our findings are relevant for the quantum Hall effect
in curved graphene sheets, which is directly related to Landau quantization.
For the purpose of this study, we develop a new numerical method, based on the
quantum lattice Boltzmann method, to solve the Dirac equation on curved
manifolds, describing the low-energetic states in strained graphene sheets
Nature of Long-Range Order in Stripe-Forming Systems with Long-Range Repulsive Interactions
We study two dimensional stripe forming systems with competing repulsive
interactions decaying as . We derive an effective Hamiltonian with
a short range part and a generalized dipolar interaction which depends on the
exponent . An approximate map of this model to a known XY model with
dipolar interactions allows us to conclude that, for long range
orientational order of stripes can exist in two dimensions, and establish the
universality class of the models. When no long-range order is
possible, but a phase transition in the KT universality class is still present.
These two different critical scenarios should be observed in experimentally
relevant two dimensional systems like electronic liquids () and
dipolar magnetic films (). Results from Langevin simulations of
Coulomb and dipolar systems give support to the theoretical results.Comment: 5 pages, 2 figures. Supplemental Material include
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