2,741 research outputs found
Inverse transition in the two dimensional dipolar frustrated ferromagnet
We show that the mean field phase diagram of the dipolar frustrated
ferromagnet in an external field presents an inverse transition in the
field-temperature plane. The presence of this type of transition has recently
been observed experimentally in ultrathin films of Fe/Cu(001). We study a
coarse-grained model Hamiltonian in two dimensions. The model supports stripe
and bubble equilibrium phases, as well as the paramagnetic phase. At variance
with common expectations, already in a single mode approximation, the model
shows a sequence of paramagnetic-bubbles-stripes-paramagnetic phase transitions
upon lowering the temperature at fixed external field. Going beyond the single
mode approximation leads to the shrinking of the bubbles phase, which is
restricted to a small region near the zero field critical temperature. Monte
Carlo simulations results with a Heisenberg model are consistent with the mean
field results.Comment: 8 pages, 6 figure
Microscopic approach to orientational order of domain walls
We develop a fully microscopic, statistical mechanics approach to study phase
transitions in Ising systems with competing interactions at different scales.
Our aim is to consider orientational and positional order parameters in a
unified framework. In this work we consider two dimensional stripe forming
systems, where nematic, smectic and crystal phases are possible. We introduce a
nematic order parameter in a lattice, which measures orientational order of
interfaces. We develop a mean field approach which leads to a free energy which
is a function of both the magnetization (density) and the orientational
(nematic) order parameters. Self-consistent equations for the order parameters
are obtained and the solutions are described for a particular system, the
Dipolar Frustrated Ising Ferromagnet. We show that this system has an
Ising-nematic phase at low temperatures in the square lattice, where positional
order (staggered magnetization) is zero. At lower temperatures a crystal-stripe
phase may appear. In the continuum limit the present approach connects to a
Ginsburg-Landau theory, which has an isotropic-nematic phase transition with
breaking of a continuous symmetry.Comment: 9 pages, 7 figures, revised and expanded, published versio
Effect of symmetry distortions on photoelectron selection rules and spectra of Bi_2Sr_2CaCu_2O_{8+ delta}
We derive photoelectron selection rules along the glide plane in orthorhombic
Bi_2Sr_2CaCu_2O_{8+\delta} (Bi2212). These selection rules explain the reversed
intensity behavior of the shadow and the main band of the material as a natural
consequence of the variating representation of the final state as a function of
k_\parallel. Our one-step simulations strongly support the structural origin of
the shadow band but we also introduce a scenario for detecting
antiferromagnetic signatures in low doping.Comment: AMS-LaTeX, 5 pages, 4 figure
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
Step-induced unusual magnetic properties of ultrathin Co/Cu films: ab initio study
We have performed ab initio studies to elucidate the unusual magnetic
behavior recently observed in epitaxial Co films upon absorption of
submonolayers of Cu and other materials. We find that a submonolayer amount of
Cu on a stepped Co/Cu (100) film changes dramatically the electronic and
magnetic structure of the system. The effect is mainly due to hybridization of
Co and Cu -electrons when copper forms a ``wire'' next to a Co step at the
surface. As a result, a non-collinear arrangement of magnetic moments
(switching of the easy axis) is promoted. [PACS 75.70.Ak,75.70.-i]Comment: 10 pages, RevTeX 3.0, 4 PostScript figures available on request from
A. Bratkovsky at [email protected]
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