785 research outputs found
Static and dynamic properties of vortices in anisotropic magnetic disks
We investigate the effect of the magnetic anisotropy () on the static
and dynamic properties of magnetic vortices in small disks. Our micromagnetic
calculations reveal that for a range of there is an enlargement of the
vortex core. We analyze the influence of on the dynamics of the vortex
core magnetization reversal under the excitation of a pulsed field. The
presence of , which leads to better resolved vortex structures, allows us
to discuss in more details the role played by the in-plane and perpendicular
components of the gyrotropic field during the vortex-antivortex nucleation and
annihilation.Comment: 4 pages, 4 figure
Kondo Quantum Criticality of Magnetic Adatoms in Graphene
We examine the exchange Hamiltonian for magnetic adatoms in graphene with
localized inner shell states. On symmetry grounds, we predict the existence of
a class of orbitals that lead to a distinct class of quantum critical points in
graphene, where the Kondo temperature scales as
near the critical coupling , and the local spin is effectively screened
by a \emph{super-ohmic} bath. For this class, the RKKY interaction decays
spatially with a fast power law . Away from half filling, we show
that the exchange coupling in graphene can be controlled across the quantum
critical region by gating. We propose that the vicinity of the Kondo quantum
critical point can be directly accessed with scanning tunneling probes and
gating.Comment: 4.1 pages, 3 figures. Added erratum correcting exponent nu=1/3. All
the other results remain vali
Griffiths phases in the strongly disordered Kondo necklace model
The effect of strong disorder on the one-dimensional Kondo necklace model is
studied using a perturbative real-space renormalization group approach which
becomes asymptotically exact in the low energy limit. The phase diagram of the
model presents a random quantum critical point separating two phases; the {\em
random singlet phase} of a quantum disordered XY chain and the random Kondo
phase. We also consider an anisotropic version of the model and show that it
maps on the strongly disordered transverse Ising model. The present results
provide a rigorous microscopic basis for non-Fermi liquid behavior in
disordered heavy fermions due to Griffiths phases.Comment: 4 pages, 4 figure
Adatoms and Anderson localization in graphene
We address the nature of the disordered state that results from the
adsorption of adatoms in graphene. For adatoms that sit at the center of the
honeycomb plaquette, as in the case of most transition metals, we show that the
ones that form a zero-energy resonant state lead to Anderson localization in
the vicinity of the Dirac point. Among those, we show that there is a symmetry
class of adatoms where Anderson localization is suppressed, leading to an
exotic metallic state with large and rare charge droplets, that localizes only
at the Dirac point. We identify the experimental conditions for the observation
of the Anderson transition for adatoms in graphene.Comment: 8 pages, 5 figures, 2 appendixes, Final versio
Phase diagram of the Kondo necklace: a mean-field renormalization group approach
In this paper we investigate the magnetic properties of heavy fermions in the
antiferromagnetic and dense Kondo phases in the framework of the Kondo necklace
model. We use a mean field renormalization group approach to obtain a
temperature versus Kondo coupling phase diagram for this model in
qualitative agreement with Doniach's diagram, proposed on physical grounds. We
further analyze the magnetically disordered phase using a two-sites approach.
We calculate the correlation functions and the magnetic susceptibility that
allow to identify the crossover between the spin-liquid and the local moment
regimes, which occurs at a {\em coherence} temperature.Comment: 5 figure
Quantum Hall Effect in Graphene with Interface-Induced Spin-Orbit Coupling
We consider an effective model for graphene with interface-induced spin-orbit
coupling and calculate the quantum Hall effect in the low-energy limit. We
perform a systematic analysis of the contribution of the different terms of the
effective Hamiltonian to the quantum Hall effect (QHE). By analysing the
spin-splitting of the quantum Hall states as a function of magnetic field and
gate-voltage, we obtain different scaling laws that can be used to characterise
the spin-orbit coupling in experiments. Furthermore, we employ a real-space
quantum transport approach to calculate the quantum Hall conductivity and
investigate the robustness of the QHE to disorder introduced by hydrogen
impurities. For that purpose, we combine first-principles calculations and a
genetic algorithm strategy to obtain a graphene-only Hamiltonian that models
the impurity
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