391 research outputs found
Klein tunneling and electron optics in Dirac-Weyl fermion systems with tilted energy dispersion
The outstanding electronic properties of relativistic-like fermions have been
extensively studied in solid state systems with isotropic linear dispersions
such as graphene. Here, we show that 2D and 3D Dirac-Weyl (DW) materials
exhibiting tilted energy dispersions could induce drastically different
transport phenomena, compared to the non-tilted case. Indeed, the Klein
tunneling of DW fermions of opposite chiralities is predicted to appear along
two separated oblique directions. In addition, valley filtering and beam
splitting effects are easily tailored by dopant engineering techniques while
the refraction of electron waves is dramatically modified by the tilt, thus
paving the way for emerging applications in electron optics and valleytronics.Comment: 5 pages, 5 figures and Supplemental Material, submitted for
publicatio
Stepped Graphene-based Aharonov-Bohm Interferometers
Aharonov-Bohm interferences in the quantum Hall regime are observed when
electrons are transmitted between two edge channels. Such a phenomenon has been
realized in 2D systems such as quantum point contacts, anti-dots and p-n
junctions. Based on a theoretical investigation of the magnetotransport in
stepped graphene, a new kind of Aharonov-Bohm interferometers is proposed
herewith. Indeed, when a strong magnetic field is applied in a proper
direction, oppositely propagating edge states can be achieved in both terrace
and facet zones of the step, leading to the interedge scatterings and hence
strong Aharonov-Bohm oscillations in the conductance in the quantum Hall
regime. Taking place in the unipolar regime, this interference is also
predicted in stepped systems of other 2D layered materials.Comment: 6 pages + 6 figures and a supplemental material, revised and
resubmitte
Optical Hall effect in strained graphene
When passing an optical medium in the presence of a magnetic field, the
polarization of light can be rotated either when reflected at the surface (Kerr
effect) or when transmitted through the material (Faraday rotation). This
phenomenon is a direct consequence of the optical Hall effect arising from the
light-charge carrier interaction in solid state systems subjected to an
external magnetic field, in analogy with the conventional Hall effect. The
optical Hall effect has been explored in many thin films and also more recently
in 2D layered materials. Here, an alternative approach based on strain
engineering is proposed to achieve an optical Hall conductivity in graphene
without magnetic field. Indeed, strain induces lattice symmetry breaking and
hence can result in a finite optical Hall conductivity. First-principles
calculations also predict this strain-induced optical Hall effect in other 2D
materials. Combining with the possibility of tuning the light energy and
polarization, the strain amplitude and direction, and the nature of the optical
medium, large ranges of positive and negative optical Hall conductivities are
predicted, thus opening the way to use these atomistic thin materials in novel
specific opto-electro-mechanical devices.Comment: 20 pages, 9 figures, submitted for publicatio
The Raman fingerprint of rhombohedral graphite
Multi-layer graphene with rhombohedral stacking is a promising carbon phase
possibly displaying correlated states like magnetism or superconductivity due
to the occurrence of a flat surface band at the Fermi level. Recently, flakes
of thickness up to 17 layers were tentatively attributed ABC sequences although
the Raman fingerprint of rhombohedral multilayer graphene is currently unknown
and the 2D resonant Raman spectrum of Bernal graphite not understood. We
provide a first principles description of the 2D Raman peak in three and four
layers graphene (all stackings) as well as in Bernal, rhombohedral and an
alternation of Bernal and rhombohedral graphite. We give practical
prescriptions to identify long range sequences of ABC multi-layer graphene. Our
work is a prerequisite to experimental non-destructive identification and
synthesis of rhombohedral graphite.Comment: 18 pages, 5 pages article + 13 pages supplemental materia
Controllable Spin Current in van der Waals Ferromagnet Fe3GeTe2
The control of spin current is pivotal for spintronic applications,
especially for spin-orbit torque devices. Spin Hall effect (SHE) is a prevalent
method to generate spin current. However, it is difficult to manipulate its
spin polarization in nonmagnet. Recently, the discovery of spin current in
ferromagnet offers opportunity to realize the manipulation. In the present
work, the spin current in van der Waals ferromagnet Fe3GeTe2 (FGT) with varying
magnetization is theoretically investigated. It has been observed that the spin
current in FGT presents the nonlinear behavior with respect to magnetization.
The in-plane and out-of-plane spin polarization emerges simultaneously, and the
bilayer FGT can even exhibit arbitrary spin polarization thanks to the reduced
symmetry. More intriguingly, the correlation between anomalous Hall effect
(AHE) and spin anomalous Hall effect (SAHE) has been interpreted from the
aspect of Berry curvature. This work illustrates that the interplay of symmetry
and magnetism can effectively control the magnitude and spin polarization of
the spin current, providing a practical method to realize exotic spin-orbit
torques
Velocity renormalization and Dirac cone multiplication in graphene superlattices with various barrier edge geometries
The electronic properties of one-dimensional graphene superlattices strongly
depend on the atomic size and orientation of the 1D external periodic
potential. Using a tight-binding approach, we show that the armchair and zigzag
directions in these superlattices have a different impact on the
renormalization of the anisotropic velocity of the charge carriers. For
symmetric potential barriers, the velocity perpendicular to the barrier is
modified for the armchair direction while remaining unchanged in the zigzag
case. For asymmetric barriers, the initial symmetry between the forward and
backward momentum with respect to the Dirac cone symmetry is broken for the
velocity perpendicular (armchair case) or parallel (zigzag case) to the
barriers. At last, Dirac cone multiplication at the charge neutrality point
occurs only for the zigzag geometry. In contrast, band gaps appear in the
electronic structure of the graphene superlattice with barrier in the armchair
direction.Comment: 13 pages, 14 figure
Thermal and electronic transport characteristics of highly stretchable graphene kirigami
For centuries, cutting and folding the papers with special patterns have been
used to build beautiful, flexible and complex three-dimensional structures.
Inspired by the old idea of kirigami (paper cutting), and the outstanding
properties of graphene, recently graphene kirigami structures were fabricated
to enhance the stretchability of graphene. However, the possibility of further
tuning the electronic and thermal transport along the 2D kirigami structures
have remained original to investigate. We therefore performed extensive
atomistic simulations to explore the electronic, heat and load transfer along
various graphene kirigami structures. The mechanical response and thermal
transport were explored using classical molecular dynamics simulations. We then
used a real-space Kubo-Greenwood formalism to investigate the charge transport
characteristics in graphene kirigami. Our results reveal that graphene kirigami
structures present highly anisotropic thermal and electrical transport.
Interestingly, we show the possibility of tuning the thermal conductivity of
graphene by four orders of magnitude. Moreover, we discuss the engineering of
kirigami patterns to further enhance their stretchability by more than 10 times
as compared with pristine graphene. Our study not only provides a general
understanding concerning the engineering of electronic, thermal and mechanical
response of graphene but more importantly can be useful to guide future studies
with respect to the synthesis of other 2D material kirigami structures, to
reach highly flexible and stretchable nanostructures with finely tunable
electronic and thermal properties.Comment: 29 pages, 9 figures, 1 supplementary figur
Magnetoresistance and Magnetic Ordering Fingerprints in Hydrogenated Graphene
Spin-dependent features in the conductivity of graphene, chemically modified
by a random distribution of hydrogen adatoms, are explored theoretically. The
spin effects are taken into account using a mean-field self-consistent Hubbard
model derived from first-principles calculations. A Kubo-Greenwood transport
methodology is used to compute the spin-dependent transport fingerprints of
weakly hydrogenated graphene-based systems with realistic sizes. Conductivity
responses are obtained for paramagnetic, antiferromagnetic, or ferromagnetic
macroscopic states, constructed from the mean-field solutions obtained for
small graphene supercells. Magnetoresistance signals up to are
calculated for hydrogen densities around 0.25%. These theoretical results could
serve as guidance for experimental observation of induced magnetism in
graphene.Comment: 4 pages, 4 figure
Transport properties of 2D graphene containing structural defects
We propose an extensive report on the simulation of electronic transport in
2D graphene in presence of structural defects. Amongst the large variety of
such defects in sp carbon-based materials, we focus on the Stone-Wales
defect and on two divacancy-type reconstructed defects. First, based on ab
initio calculations, a tight-binding model is derived to describe the
electronic structure of these defects. Then, semiclassical transport properties
including the elastic mean free paths, mobilities and conductivities are
computed using an order-N real-space Kubo-Greenwood method. A plateau of
minimum conductivity () is progressively
observed as the density of defects increases. This saturation of the decay of
conductivity to is associated with defect-dependent
resonant energies. Finally, localization phenomena are captured beyond the
semiclassical regime. An Anderson transition is predicted with localization
lengths of the order of tens of nanometers for defect densities around 1%.Comment: 17 pages, 17 figures, submitted to Phys. Rev.
Klein tunneling degradation and enhanced Fabry-P\'erot interference in graphene/h-BN moir\'e-superlattice devices
Hexagonal boron-nitride (h-BN) provides an ideal substrate for supporting
graphene devices to achieve fascinating transport properties, such as Klein
tunneling, electron optics and other novel quantum transport phenomena.
However, depositing graphene on h-BN creates moir\'e superlattices, whose
electronic properties can be significantly manipulated by controlling the
lattice alignment between layers. In this work, the effects of these moir\'e
structures on the transport properties of graphene are investigated using
atomistic simulations. At large misalignment angles (leading to small moir\'e
cells), the transport properties (most remarkably, Klein tunneling) of pristine
graphene devices are conserved. On the other hand, in the nearly aligned cases,
the moir\'e interaction induces stronger effects, significantly affecting
electron transport in graphene. In particular, Klein tunneling is significantly
degraded. In contrast, strong Fabry-P\'erot interference (accordingly, strong
quantum confinement) effects and non-linear I-V characteristics are observed.
P-N interface smoothness engineering is also considered, suggesting as a
potential way to improve these transport features in graphene/h-BN devices.Comment: 21 pages, 8 figures, Supplementary material
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