73,871 research outputs found
The phase transition in the anisotropic Heisenberg model with long range dipolar interactions
In this work we have used extensive Monte Carlo calculations to study the
planar to paramagnetic phase transition in the two-dimensional anisotropic
Heisenberg model with dipolar interactions (AHd) considering the true
long-range character of the dipolar interactions by means of the Ewald
summation. Our results are consistent with an order-disorder phase transition
with unusual critical exponents in agreement with our previous results for the
Planar Rotator model with dipolar interactions. Nevertheless, our results
disagrees with the Renormalization Group results of Maier and Schwabl [PRB, 70,
134430 (2004)] and the results of Rapini et. al. [PRB, 75, 014425 (2007)],
where the AHd was studied using a cut-off in the evaluation of the dipolar
interactions. We argue that besides the long-range character of dipolar
interactions their anisotropic character may have a deeper effect in the system
than previously believed. Besides, our results shows that the use of a cut-off
radius in the evaluation of dipolar interactions must be avoided when analyzing
the critical behavior of magnetic systems, since it may lead to erroneous
results.Comment: Accepted for publication in the Journal of Magnetism and Magnetic
Materials. arXiv admin note: substantial text overlap with arXiv:1109.184
Entanglement of Two Impurities through Electron Scattering
We study how two magnetic impurities embedded in a solid can be entangled by
an injected electron scattering between them and by subsequent measurement of
the electron's state. We start by investigating an ideal case where only the
electronic spin interacts successively through the same unitary operation with
the spins of the two impurities. In this case, high (but not maximal)
entanglement can be generated with a significant success probability. We then
consider a more realistic description which includes both the forward and back
scattering amplitudes. In this scenario, we obtain the entanglement between the
impurities as a function of the interaction strength of the electron-impurity
coupling. We find that our scheme allows us to entangle the impurities
maximally with a significant probability
Stability of a two-sublattice spin-glass model
We study the stability of the replica-symmetric solution of a two-sublattice
infinite-range spin-glass model, which can describe the transition from
antiferromagnetic to spin glass state. The eigenvalues associated with
replica-symmetric perturbations are in general complex. The natural
generalization of the usual stability condition is to require the real part of
these eigenvalues to be positive. The necessary and sufficient conditions for
all the roots of the secular equation to have positive real parts is given by
the Hurwitz criterion. The generalized stability condition allows a consistent
analysis of the phase diagram within the replica-symmetric approximation.Comment: 21 pages, 5 figure
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