618 research outputs found
Friedel oscillations at the surfaces of rhombohedral -layer graphene
The low-energy physics of rhombohedral -layer graphene mainly arises on
the external layers, where most of the {\pi} electrons are located. Their Bloch
band structure defines a two-band semimetal; the dispersion relation scales as
with the momentum norm in the vicinity of two nonequivalent
valleys. In this paper, we address the problem of elastic scattering through a
localized impurity located either on the surface of the material or within the
bulk, and focus on the quantum interferences it induces on the two external
layers. It is apprehended in the framework of a -matrix approach, both
numerically and analytically, regardless of the impurity magnitude, which
enables the description of realistic scatters. In rhombohedral multilayer
graphene, the impurity induces Friedel oscillations that always decay as .
As a result, monolayer graphene is the only material of the rhombohedral class
that exhibits -decaying Friedel oscillations. The interference
patterns are subsequently analyzed in momentum space. This analysis enables a
clear distinction between monolayer graphene and multilayer graphene. It also
shows that the interference pattern reveals the whole Bloch band structure, and
highlights the number of layers stacked in the material, as well as the
-quantized Berry phases that characterize the existence of nodal points
in the semimetallic spectrum. Experimentally, these features may be probed from
scanning tunneling microscopy, when imaging the local density of states at the
surfaces of suspended rhombohedral -layer graphene
Dynamical and Reversible Control of Topological Spin Textures
Recent observations of topological spin textures brought spintronics one step
closer to new magnetic memories. Nevertheless, the existence of Skyrmions, as
well as their stabilization, require very specific intrinsic magnetic
properties which are usually fixed in magnets. Here we address the possibility
to dynamically control their intrinsic magnetic interactions by varying the
strength of a high-frequency laser field. It is shown that drastic changes can
be induced in the antiferromagnetic exchange interactions and the latter can
even be reversed to become ferromagnetic, provided the direct exchange is
already non-negligible in equilibrium as predicted, for example, in Si doped
with C, Sn, or Pb adatoms. In the presence of Dzyaloshinskii-Moriya
interactions, this enables us to tune features of ferromagnetic Skyrmions such
as their radius, making them easier to stabilize. Alternatively, such
topological spin textures can occur in frustrated triangular lattices. Then, we
demonstrate that a high-frequency laser field can induce dynamical frustration
in antiferromagnets, where the degree of frustration can subsequently be tuned
suitably to drive the material toward a Skyrmionic phase
Laser-induced topological transitions in phosphorene with inversion symmetry
Recent ab initio calculations and experiments reported
insulating-semimetallic phase transitions in multilayer phosphorene under a
perpendicular dc field, pressure or doping, as a possible route to realize
topological phases. In this work, we show that even a monolayer phosphorene may
undergo Lifshitz transitions toward semimetallic and topological insulating
phases, provided it is rapidly driven by in-plane time-periodic laser fields.
Based on a four-orbital tight-binding description, we give an
inversion-symmetry-based prescription in order to apprehend the topology of the
photon-renormalized band structure, up to the second order in the
high-frequency limit. Apart from the initial band insulating behavior, two
additional phases are thus identified. A semimetallic phase with massless Dirac
electrons may be induced by linear polarized fields, whereas elliptic polarized
fields are likely to drive the material into an anomalous quantum Hall phase.Comment: Includes Supplemental Materia
Majorana Fermions in Honeycomb Lattices
We study the formation of Majorana fermions in honeycomb-lattice structures
in the presence of a Zeeman field, Rashba spin-orbit coupling, and in the
proximity of an s-wave superconductor. We show that an exact mapping exists
between an anisotropic hexagonal-lattice nanoribbon at k = 0 and a
one-dimensional chain, for which the existence of Majorana fermions has been
extensively discussed. Consequently we can predict the conditions for the
emergence of Majorana fermions at the edges of such ribbon, and relate the
existence of Majoranas to a band inversion in the bulk band structure. Moreover
we find that similar situations arise in isotropic lattices and we give some
examples which show the formation of Majorana fermions in these structures.Comment: 7 pages, 9 figure
Friedel oscillations at the Dirac-cone-merging point in anisotropic graphene
We study the Friedel oscillations induced by a localized impurity in
anisotropic graphene. We focus on the limit when the two inequivalent Dirac
points merge. We find that in this limit the Friedel oscillations manifest very
peculiar features, such as a strong asymmetry and an atypical inverse
square-root decay. Our calculations are performed using both a T-matrix
approximation and a tight-binding exact diagonalization technique. They allow
us to obtain numerically the local density of states as a function of energy
and position, as well as an analytical form of the Friedel oscillations in the
continuum limit. The two techniques yield results that are in excellent
agreement, confirming the accuracy of such methods to approach this problem
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