339 research outputs found
Role of longitudinal fluctuations in L10 FePt
L10 FePt is a technologically important material for a range of novel data storage applications. In the ordered FePt structure the normally nonmagnetic Pt ion acquires a magnetic moment, which depends on the local field originating from the neighboring Fe atoms. In this work a model of FePt is constructed in which the induced Pt moment is simulated by using combined longitudinal and rotational spin dynamics. The model is parameterized to include a linear variation of the moment with the exchange field, so that at the Pt site the magnetic moment depends on the Fe ordering. The Curie temperature of FePt is calculated and agrees well with similar models that incorporate the Pt dynamics through an effective Fe-only Hamiltonian. By computing the dynamic correlation function the anisotropy field and the Gilbert damping are extracted over a range of temperatures. The anisotropy exhibits a power-law dependence on the magnetization with exponent n≈2.1. This agrees well with what was observed experimentally, and it is obtained without including a two-ion anisotropy term as in other approaches. Our work shows that incorporating longitudinal fluctuations into spin dynamics calculations is crucial for understanding the properties of materials with induced moments
Perfect magnetohydrodynamics as a field theory
We propose the generally covariant action for the theory of a self-coupled
complex scalar field and electromagnetism which by virtue of constraints is
equivalent, in the regime of long wavelengths, to perfect magnetohydrodynamics
(MHD). We recover from it the Euler equation with Lorentz force, and the
thermodynamic relations for a prefect fluid. The equation of state of the
latter is related to the scalar field's self potential. We introduce 1+3
notation to elucidate the relation between MHD and field variables. In our
approach the requirement that the scalar field be single valued leads to the
quantization of a certain circulation in steps of ; this feature leads,
in the classical limit, to the conservation of that circulation. The
circulation is identical to that in Oron's generalization of Kelvin's
circulation theorem to perfect MHD; we here characterize the new conserved
helicity associated with it. We also demonstrate the existence for MHD of two
Bernoulli-like theorems for each spacetime symmetry of the flow and geometry;
one of these is pertinent to suitably defined potential flow. We exhibit the
conserved quantities explicitly in the case that two symmetries are
simultaneously present, and give examples. Also in this case we exhibit a new
conserved MHD circulation distinct from Oron's, and provide an example.Comment: RevTeX, 16 pages, no figures; clarifications added and typos
corrected; version to be published in Phys. Rev.
Random Hamiltonian in thermal equilibrium
A framework for the investigation of disordered quantum systems in thermal
equilibrium is proposed. The approach is based on a dynamical model--which
consists of a combination of a double-bracket gradient flow and a uniform
Brownian fluctuation--that `equilibrates' the Hamiltonian into a canonical
distribution. The resulting equilibrium state is used to calculate quenched and
annealed averages of quantum observables.Comment: 8 pages, 4 figures. To appear in DICE 2008 conference proceeding
Reality and causality in quantum gravity modified electrodynamics
We present a general description of the propagation properties of quantum
gravity modified electrodynamics characterized by constitutive relations up to
second order in the correction parameter. The effective description corresponds
to an electrodynamics in a dispersive and absorptive non-local medium, where
the Green functions and the refraction indices can be explicitly calculated.
The reality of the electromagnetic field together with the requirement of
causal propagation in a given referrence frame leads to restrictions in the
form of such refraction indices. In particular, absorption must be present in
all cases and, contrary to the usual assumption, it is the dominant aspect in
those effective models which exhibit linear effects in the correction parameter
not related to birefringence. In such a situation absorption is linear while
propagation is quadratical in the correction parameter.Comment: 15 pages, LaTex, minor changes to clarify some points, version
accepted for publicatio
Cosmo-dynamics and dark energy with a quadratic EoS: anisotropic models, large-scale perturbations and cosmological singularities
In general relativity, for fluids with a linear equation of state (EoS) or
scalar fields, the high isotropy of the universe requires special initial
conditions, and singularities are anisotropic in general. In the brane world
scenario anisotropy at the singularity is suppressed by an effective quadratic
equation of state. There is no reason why the effective EoS of matter should be
linear at the highest energies, and a non-linear EoS may describe dark energy
or unified dark matter (Paper I, astro-ph/0512224). In view of this, here we
study the effects of a quadratic EoS in homogenous and inhomogeneous
cosmological models in general relativity, in order to understand if in this
context the quadratic EoS can isotropize the universe at early times. With
respect to Paper I, here we use the simplified EoS P=alpha rho + rho^2/rho_c,
which still allows for an effective cosmological constant and phantom behavior,
and is general enough to analyze the dynamics at high energies. We first study
anisotropic Bianchi I and V models, focusing on singularities. Using dynamical
systems methods, we find the fixed points of the system and study their
stability. We find that models with standard non-phantom behavior are in
general asymptotic in the past to an isotropic fixed point IS, i.e. in these
models even an arbitrarily large anisotropy is suppressed in the past: the
singularity is matter dominated. Using covariant and gauge invariant variables,
we then study linear perturbations about the homogenous and isotropic spatially
flat models with a quadratic EoS. We find that, in the large scale limit, all
perturbations decay asymptotically in the past, indicating that the isotropic
fixed point IS is the general asymptotic past attractor for non phantom
inhomogeneous models with a quadratic EoS. (Abridged)Comment: 16 pages, 6 figure
Magnetized Tolman-Bondi Collapse
We investigate the gravitational implosion of magnetized matter by studying
the inhomogeneous collapse of a weakly magnetized Tolman-Bondi spacetime. The
role of the field is analyzed by looking at the convergence of neighboring
particle worldlines. In particular, we identify the magnetically related
stresses in the Raychaudhuri equation and use the Tolman-Bondi metric to
evaluate their impact on the collapsing dust. We find that, despite the low
energy level of the field, the Lorentz force dominates the advanced stages of
the collapse, leading to a strongly anisotropic contraction. In addition, of
all the magnetic stresses, those that resist the collapse are found to grow
faster.Comment: 6 pages, RevTex; v2: physical interpretation of the results slightly
changed, references added, version accepted in Phys. Rev. D (2006
Incomplete equilibrium in long-range interacting systems
We use a Hamiltonian dynamics to discuss the statistical mechanics of
long-lasting quasi-stationary states particularly relevant for long-range
interacting systems. Despite the presence of an anomalous single-particle
velocity distribution, we find that the Central Limit Theorem implies the
Boltzmann expression in Gibbs' -space. We identify the nonequilibrium
sub-manifold of -space characterizing the anomalous behavior and show
that by restricting the Boltzmann-Gibbs approach to this sub-manifold we obtain
the statistical mechanics of the quasi-stationary states.Comment: Title changed, throughout revision of the tex
Accelerated expansion of a universe containing a self-interacting Bose-Einstein gas
Acceleration of the universe is obtained from a model of non-relativistic
particles with a short-range attractive interaction, at low enough temperature
to produce a Bose-Einstein condensate. Conditions are derived for
negative-pressure behavior. In particular, we show that a phantom-accelerated
regime at the beginning of the universe solves the horizon problem,
consistently with nucleosynthesis.Comment: 18 pages, 4 figure
Photon-graviton pair conversion
We consider the conversion of gravitons and photons as a four-wave mixing
process. A nonlinear coupled systems of equations involving two gravitons and
two photons is obtained, and the energy exchange between the different degrees
of freedom is found. The scattering amplitudes are obtained, from which a
crossection for incoherent processes can be found. An analytical example is
given, and applications to the early Universe are discussed.Comment: 5 pages, slightly modified as compared to v1, to appear in Class.
Quantum Grav. as a Letter to the Edito
Averaging anisotropic cosmologies
We examine the effects of spatial inhomogeneities on irrotational anisotropic
cosmologies by looking at the average properties of anisotropic pressure-free
models. Adopting the Buchert scheme, we recast the averaged scalar equations in
Bianchi-type form and close the standard system by introducing a propagation
formula for the average shear magnitude. We then investigate the evolution of
anisotropic average vacuum models and those filled with pressureless matter. In
the latter case we show that the backreaction effects can modify the familiar
Kasner-like singularity and potentially remove Mixmaster-type oscillations. The
presence of nonzero average shear in our equations also allows us to examine
the constraints that a phase of backreaction-driven accelerated expansion might
put on the anisotropy of the averaged domain. We close by assessing the status
of these and other attempts to define and calculate `average' spacetime
behaviour in general relativity.Comment: revised version, to appear in CQ
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