293 research outputs found
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
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
Transport Length Scales in Disordered Graphene-based Materials: Strong Localization Regimes and Dimensionality Effects
We report on a numerical study of quantum transport in disordered two
dimensional graphene and graphene nanoribbons. By using the Kubo and the
Landauer approaches, transport length scales in the diffusive (mean free path,
charge mobilities) and localized regimes (localization lengths) are computed,
assuming a short range disorder (Anderson-type). In agreement with localization
scaling theory, the electronic systems are found to undergo a conventional
Anderson localization in the zero temperature limit. Localization lengths in
weakly disordered ribbons are found to differ by two orders of magnitude
depending on their edge symmetry, but always remain several orders of magnitude
smaller than those computed for 2D graphene for the same disorder strength.
This pinpoints the role of transport dimensionality and edge effects.Comment: 4 pages, Phys. rev. Lett. (in press
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
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.
Electrons scattering in the monolayer graphene with the short-range impurities
Scattering problem for electrons in monolayer graphene with short-range
perturbations of the types "local chemical potential" and "local gap" has been
solved. Zero gap and non-zero gap kinds of graphene are considered. The
determined S-matrix can be used for calculation of such observables as
conductance and optical absorption
Bound electron states in the monolayer graphene with short-range impurities
Bound electron states in impure graphene are considered. Short-range
perturbations for defect and impurities of the types "local chemical potential"
and "local gap" are taken into account.Comment: 3 figure
Band widths and gaps from the Tran-Blaha functional : Comparison with many-body perturbation theory
For a set of ten crystalline materials (oxides and semiconductors), we
compute the electronic band structures using the Tran-Blaha [Phys. Rev. Lett.
102, 226401 (2009)] (TB09) functional. The band widths and gaps are compared
with those from the local-density approximation (LDA) functional, many-body
perturbation theory (MBPT), and experiments. At the density-functional theory
(DFT) level, TB09 leads to band gaps in much better agreement with experiments
than LDA. However, we observe that it globally underestimates, often strongly,
the valence (and conduction) band widths (more than LDA). MBPT corrections are
calculated starting from both LDA and TB09 eigenenergies and wavefunctions.
They lead to a much better agreement with experimental data for band widths.
The band gaps obtained starting from TB09 are close to those from
quasi-particle self-consistent GW calculations, at a much reduced cost.
Finally, we explore the possibility to tune one of the semi-empirical
parameters of the TB09 functional in order to obtain simultaneously better band
gaps and widths. We find that these requirements are conflicting.Comment: 18 pages, 16 figure
Strain Modulated Superlattices in Graphene
Strain engineering of graphene takes advantage of one of the most dramatic
responses of Dirac electrons enabling their manipulation via strain-induced
pseudo-magnetic fields. Numerous theoretically proposed devices, such as
resonant cavities and valley filters, as well as novel phenomena, such as snake
states, could potentially be enabled via this effect. These proposals, however,
require strong, spatially oscillating magnetic fields while to date only the
generation and effects of pseudo-gauge fields which vary at a length scale much
larger than the magnetic length have been reported. Here we create a periodic
pseudo-gauge field profile using periodic strain that varies at the length
scale comparable to the magnetic length and study its effects on Dirac
electrons. A periodic strain profile is achieved by pulling on graphene with
extreme (>10%) strain and forming nanoscale ripples, akin to a plastic wrap
pulled taut at its edges. Combining scanning tunneling microscopy and atomistic
calculations, we find that spatially oscillating strain results in a new
quantization different from the familiar Landau quantization observed in
previous studies. We also find that graphene ripples are characterized by large
variations in carbon-carbon bond length, directly impacting the electronic
coupling between atoms, which within a single ripple can be as different as in
two different materials. The result is a single graphene sheet that effectively
acts as an electronic superlattice. Our results thus also establish a novel
approach to synthesize an effective 2D lateral heterostructure - by periodic
modulation of lattice strain.Comment: 18 pages, 5 figures and supplementary informatio
Large phosphorene in-plane contraction induced by interlayer interactions in graphene-phosphorene heterostructures
Intralayer deformation in van der Waals (vdW) heterostructures is generally
assumed to be negligible due to the weak nature of the interactions between the
layers, especially when the interfaces are found incoherent. In the present
work, graphene-phosphorene vdW-heterostructures are investigated with the
Density Functional Theory (DFT). The challenge of treating nearly
incommensurate (very large) supercell in DFT is bypassed by considering
different energetic quantities in the grand canonical ensemble, alternative to
the formation energy, in order to take into account the mismatch elastic
contribution of the different layers. In the investigated heterostructures, it
is found that phosphorene contracts by ~4% in the armchair direction when
compared to its free-standing form. This large contraction leads to important
changes in term of electronic properties, with the direct electronic optical
transition of phosphorene becoming indirect in specific vdW-heterostructures.
More generally, such a contraction indicates strong substrate effects in
supported or encapsulated phosphorene -neglected hitherto- and paves the way to
substrate-controlled stress- tronic in such 2D crystal. In addition, the
stability of these vdW-heterostructures are investigated as a function of the
rotation angle between the layers and as a function of the stacking
composition. The alignment of the specific crystalline directions of graphene
and phosphorene is found energetically favored. In parallel, several several
models based on DFT-estimated quantities are presented; they allow notably a
better understanding of the global mutual accommodation of 2D materials in
their corresponding interfaces, that is predicted to be non-negligible even in
the case of incommensurate interfaces.Comment: 33 pages, 6 figure
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