2,940 research outputs found
Einstein-Yang-Mills-Chern-Simons solutions in D=2n+1 dimensions
We investigate finite energy solutions of the
Einstein--Yang-Mills--Chern-Simons system in odd spacetime dimensions, D=2n+1,
with n>1. Our configurations are static and spherically symmetric, approaching
at infinity a Minkowski spacetime background. In contrast with the Abelian
case, the contribution of the Chern-Simons term is nontrivial already in the
static, spherically symmetric limit. Both globally regular, particle-like
solutions and black holes are constructed numerically for several values of D.
These solutions carry a nonzero electric charge and have finite mass. For
globally regular solutions, the value of the electric charge is fixed by the
Chern-Simons coupling constant. The black holes can be thought as non-linear
superpositions of Reissner-Nordstrom and non-Abelian configurations. A
systematic discussion of the solutions is given for D=5, in which case the
Reissner-Nordstrom black hole becomes unstable and develops non-Abelian hair.
We show that some of these non-Abelian configurations are stable under linear,
spherically symmetric perturbations. A detailed discussion of an exact D=5
solution describing extremal black holes and solitons is also provided.Comment: 34 pages, 14 figures; v2: misprints corrected and references adde
Training Induced Positive Exchange Bias in NiFe/IrMn Bilayers
Positive exchange bias has been observed in the
NiFe/IrMn bilayer system via soft x-ray resonant
magnetic scattering. After field cooling of the system through the blocking
temperature of the antiferromagnet, an initial conventional negative exchange
bias is removed after training i. e. successive magnetization reversals,
resulting in a positive exchange bias for a temperature range down to 30 K
below the blocking temperature (450 K). This new manifestation of magnetic
training is discussed in terms of metastable magnetic disorder at the
magnetically frustrated interface during magnetization reversal.Comment: 4 pages, 3 figure
Neutron resonances in planar waveguides
Results of experimental investigations of a neutron resonances width in
planar waveguides using the time-of-flight reflectometer REMUR of the IBR-2
pulsed reactor are reported and comparison with theoretical calculations is
presented. The intensity of the neutron microbeam emitted from the waveguide
edge was registered as a function of the neutron wavelength and the incident
beam angular divergence. The possible applications of this method for the
investigations of layered nanostructures are discussed
The exchange bias phenomenon in uncompensated interfaces: Theory and Monte Carlo simulations
We performed Monte Carlo simulations in a bilayer system composed by two thin
films, one ferromagnetic (FM) and the other antiferromagnetic (AFM). Two
lattice structures for the films were considered: simple cubic (sc) and a body
center cubic (bcc). In both lattices structures we imposed an uncompensated
interfacial spin structure, in particular we emulated a FeF2-FM system in the
case of the (bcc) lattice. Our analysis focused on the incidence of the
interfacial strength interactions between the films J_eb and the effect of
thermal fluctuations on the bias field H_EB. We first performed Monte Carlo
simulations on a microscopic model based on classical Heisenberg spin
variables. To analyze the simulation results we also introduced a simplified
model that assumes coherent rotation of spins located on the same layer
parallel to the interface. We found that, depending on the AFM film anisotropy
to exchange ratio, the bias field is either controlled by the intrinsic pinning
of a domain wall parallel to the interface or by the stability of the first AFM
layer (quasi domain wall) near the interface.Comment: 18 pages, 11 figure
Thermodynamic behavior of the XXZ Heisenberg s=1/2 chain around the factorizing magnetic field
We have investigated the zero and finite temperature behaviors of the
anisotropic antiferromagnetic Heisenberg XXZ spin-1/2 chain in the presence of
a transverse magnetic field (h). The attention is concentrated on an interval
of magnetic field between the factorizing field (h_f) and the critical one
(h_c). The model presents a spin-flop phase for 0<h<h_f with an energy scale
which is defined by the long range antiferromagnetic order while it undergoes
an entanglement phase transition at h=h_f. The entanglement estimators clearly
show that the entanglement is lost exactly at h=h_f which justifies different
quantum correlations on both sides of the factorizing field. As a consequence
of zero entanglement (at h=h_f) the ground state is known exactly as a product
of single particle states which is the starting point for initiating a spin
wave theory. The linear spin wave theory is implemented to obtain the specific
heat and thermal entanglement of the model in the interested region. A double
peak structure is found in the specific heat around h=h_f which manifests the
existence of two energy scales in the system as a result of two competing
orders before the critical point. These results are confirmed by the low
temperature Lanczos data which we have computed.Comment: Will be published in JPCM (2010), 7 figure
Stable black hole solutions with non-Abelian fields
We construct finite mass, asymptotically flat black hole solutions in d=4
Einstein-Yang-Mills theory augmented with higher order curvature terms of the
gauge field. They possess non-Abelian hair in addition to Coulomb electric
charge, and, below some non-zero critical temperature, they are
thermodynamically preferred over the Reissner-Nordstrom solution. Our results
indicate the existence of hairy non-Abelian black holes which are stable under
linear, spherically symmetric perturbations.Comment: 8 pages, 3 figure
Asymptotically flat, stable black hole solutions in Einstein--Yang-Mills--Chern-Simons theory
We construct finite mass, asymptotically flat black hole solutions in d=5
Einstein--Yang-Mills--Chern-Simons theory. Our results indicate the existence
of a second order phase transition between Reissner-Nordstrom solutions and the
non-Abelian black holes which generically are thermodynamically preferred. Some
of the non-Abelian configurations are also stable under linear, spherically
symmetric perturbations. In addition a solution in closed form describing an
extremal black hole with non-Abelian hair is found for a special value of the
Chern-Simons coupling constant.Comment: 9 pages, 3 figure
Nonabelian solutions in N=4, D=5 gauged supergravity
We consider static, nonabelian solutions in N=4, D=5 Romans' gauged
supergravity model. Numerical arguments are presented for the existence of
asymptotically anti-de Sitter configurations in the version of the
theory, with a dilaton potential presenting a stationary point. Considering the
version of the theory with a Liouville dilaton potential, we look for
configurations with unusual topology. A new exact solution is presented, and a
counterterm method is proposed to compute the mass and action.Comment: 15 pages, 4 figure
Dual Behavior of Antiferromagnetic Uncompensated Spins in NiFe/IrMn Exchange Biased Bilayers
We present a comprehensive study of the exchange bias effect in a model
system. Through numerical analysis of the exchange bias and coercive fields as
a function of the antiferromagnetic layer thickness we deduce the absolute
value of the averaged anisotropy constant of the antiferromagnet. We show that
the anisotropy of IrMn exhibits a finite size effect as a function of
thickness. The interfacial spin disorder involved in the data analysis is
further supported by the observation of the dual behavior of the interfacial
uncompensated spins. Utilizing soft x-ray resonant magnetic reflectometry we
have observed that the antiferromagnetic uncompensated spins are dominantly
frozen with nearly no rotating spins due to the chemical intermixing, which
correlates to the inferred mechanism for the exchange bias.Comment: 4 pages, 3 figure
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