1,320 research outputs found
On the origin of magnetic anisotropy in two dimensional CrI
The observation of ferromagnetic order in a monolayer of CrI has been
recently reported, with a Curie temperature of 45 Kelvin and off-plane easy
axis. Here we study the origin of magnetic anisotropy, a necessary ingredient
to have magnetic order in two dimensions, combining two levels of modeling,
density functional calculations and spin model Hamiltonians. We find two
different contributions to the magnetic anisotropy of the material, both
favoring off-plane magnetization and contributing to open a gap in the spin
wave spectrum. First, ferromagnetic super-exchange across the 90
degree Cr-I-Cr bonds, are anisotropic, due to the spin orbit interaction of the
ligand I atoms. Second, a much smaller contribution that comes from the single
ion anisotropy of the Cr atom. Our results permit to establish the XXZ
Hamiltonian, with a very small single ion anisotropy, as the adequate spin
model for this system. Using spin wave theory we estimate the Curie temperature
and we highlight the essential role played by the gap that magnetic anisotropy
induces on the magnon spectrum.Comment: 8 pages, 5 figure
Noncollinear magnetic phases and edge states in graphene quantum Hall bars
Application of a perpendicular magnetic field to charge neutral graphene is
expected to result in a variety of broken symmetry phases, including
antiferromagnetic, canted and ferromagnetic. All these phases open a gap in
bulk but have very different edge states and non-collinear spin order, recently
confirmed experimentally. Here we provide an integrated description of both
edge and bulk for the various magnetic phases of graphene Hall bars making use
of a non-collinear mean field Hubbard model. Our calculations show that, at the
edges, the three types of magnetic order are either enhanced (zigzag) or
suppressed (armchair). Interestingly, we find that preformed local moments in
zigzag edges interact with the quantum Spin Hall like edge states of the
ferromagnetic phase and can induce back-scattering.Comment: 5 pages, 4 figure
Quantum spin Hall phase in multilayer graphene
The so called quantum spin Hall phase is a topologically non trivial
insulating phase that is predicted to appear in graphene and graphene-like
systems. In this work we address the question of whether this topological
property persists in multilayered systems. We consider two situations: purely
multilayer graphene and heterostructures where graphene is encapsulated by
trivial insulators with a strong spin-orbit coupling. We use a four orbital
tight-binding model that includes the full atomic spin-orbit coupling and we
calculate the topological invariant of the bulk states as well as the
edge states of semi-infinite crystals with armchair termination. For
homogeneous multilayers we find that even when the spin-orbit interaction opens
a gap for all the possible stackings, only those with odd number of layers host
gapless edge states while those with even number of layers are trivial
insulators. For the heterostructures where graphene is encapsulated by trivial
insulators, it turns out that the interlayer coupling is able to induce a
topological gap whose size is controlled by the spin-orbit coupling of the
encapsulating materials, indicating that the quantum spin Hall phase can be
induced by proximity to trivial insulators.Comment: 7 pages, 6 figure
Topological features of hydrogenated graphene
Hydrogen adatoms are one of the most the promising proposals for the
functionalization of graphene. Hydrogen induces narrow resonances near the
Dirac energy, which lead to the formation of magnetic moments. Furthermore,
they also create local lattice distortions which enhance the spin-orbit
coupling. The combination of magnetism and spin-orbit coupling allows for a
rich variety of phases, some of which have non trivial topological features. We
analyze the interplay between magnetism and spin-orbit coupling in ordered
arrays of hydrogen on graphene monolayers, and classify the different phases
that may arise. We extend our model to consider arrays of adsorbates in
graphene-like crystals with stronger intrinsic spin-orbit couplings.Comment: 6 pages, 4 figure
Real space mapping of topological invariants using artificial neural networks
Topological invariants allow to characterize Hamiltonians, predicting the
existence of topologically protected in-gap modes. Those invariants can be
computed by tracing the evolution of the occupied wavefunctions under twisted
boundary conditions. However, those procedures do not allow to calculate a
topological invariant by evaluating the system locally, and thus require
information about the wavefunctions in the whole system. Here we show that
artificial neural networks can be trained to identify the topological order by
evaluating a local projection of the density matrix. We demonstrate this for
two different models, a 1-D topological superconductor and a 2-D quantum
anomalous Hall state, both with spatially modulated parameters. Our neural
network correctly identifies the different topological domains in real space,
predicting the location of in-gap states. By combining a neural network with a
calculation of the electronic states that uses the Kernel Polynomial Method, we
show that the local evaluation of the invariant can be carried out by
evaluating a local quantity, in particular for systems without translational
symmetry consisting of tens of thousands of atoms. Our results show that
supervised learning is an efficient methodology to characterize the local
topology of a system.Comment: 9 pages, 6 figure
Majorana Zero Modes in Graphene
A clear demonstration of topological superconductivity (TS) and Majorana zero
modes remains one of the major pending goal in the field of topological
materials. One common strategy to generate TS is through the coupling of an
s-wave superconductor to a helical half-metallic system. Numerous proposals for
the latter have been put forward in the literature, most of them based on
semiconductors or topological insulators with strong spin-orbit coupling. Here
we demonstrate an alternative approach for the creation of TS in
graphene/superconductor junctions without the need of spin-orbit coupling. Our
prediction stems from the helicity of graphene's zero Landau level edge states
in the presence of interactions, and on the possibility, experimentally
demonstrated, to tune their magnetic properties with in-plane magnetic fields.
We show how canted antiferromagnetic ordering in the graphene bulk close to
neutrality induces TS along the junction, and gives rise to isolated,
topologically protected Majorana bound states at either end. We also discuss
possible strategies to detect their presence in graphene Josephson junctions
through Fraunhofer pattern anomalies and Andreev spectroscopy. The latter in
particular exhibits strong unambiguous signatures of the presence of the
Majorana states in the form of universal zero bias anomalies. Remarkable
progress has recently been reported in the fabrication of the proposed type of
junctions, which offers a promising outlook for Majorana physics in graphene
systems.Comment: 14 pages, 8 figures. Included simulations of Andreev spectroscopy and
mor
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