32 research outputs found
Landau levels in deformed bilayer graphene at low magnetic fields
We review the effect of uniaxial strain on the low-energy electronic
dispersion and Landau level structure of bilayer graphene. Based on the
tight-binding approach, we derive a strain-induced term in the low-energy
Hamiltonian and show how strain affects the low-energy electronic band
structure. Depending on the magnitude and direction of applied strain, we
identify three regimes of qualitatively different electronic dispersions. We
also show that in a weak magnetic field, sufficient strain results in the
filling factor ff=+-4 being the most stable in the quantum Hall effect
measurement, instead of ff=+-8 in unperturbed bilayer at a weak magnetic field.
To mention, in one of the strain regimes, the activation gap at ff=+-4 is, down
to very low fields, weakly dependent on the strength of the magnetic field.Comment: 14 single-column pages, 5 figures, more details on material presented
in arXiv:1104.502
Spectroscopic Signatures of Electronic Excitations in Raman Scattering in Thin Films of Rhombohedral Graphite
Rhombohedral graphite features peculiar electronic properties, including
persistence of low-energy surface bands of a topological nature. Here, we study
the contribution of electron-hole excitations towards inelastic light
scattering in thin films of rhombohedral graphite. We show that, in contrast to
the featureless electron-hole contribution towards Raman spectrum of graphitic
films with Bernal stacking, the inelastic light scattering accompanied by
electron-hole excitations in crystals with rhombohedral stacking produces
distinct features in the Raman signal which can be used both to identify the
stacking and to determine the number of layers in the film.Comment: 15 pages in preprint format, 4 figures, accepted versio
Electronic Raman Scattering in Twistronic Few-Layer Graphene
We study electronic contribution to the Raman scattering signals of two-,
three- and four-layer graphene with layers at one of the interfaces twisted by
a small angle with respect to each other. We find that the Raman spectra of
these systems feature two peaks produced by van Hove singularities in moir\'{e}
minibands of twistronic graphene, one related to direct hybridization of Dirac
states, and the other resulting from band folding caused by moir\'{e}
superlattice. The positions of both peaks strongly depend on the twist angle,
so that their detection can be used for non-invasive characterization of the
twist, even in hBN-encapsulated structures.Comment: 7 pages (including 4 figures) + 10 pages (3 figures) supplemen
Moiré band model and band gaps of graphene on hexagonal boron nitride
Nearly aligned graphene on hexagonal boron nitride (G/BN) can be accurately
modeled by a Dirac Hamiltonian perturbed by smoothly varying moir\'e pattern
pseudospin fields. Here, we present the moir\'e-band model of G/BN for
arbitrary small twist angles under a framework that combines symmetry
considerations with input from ab-initio calculations. Our analysis of the band
gaps at the primary and secondary Dirac points highlights the role of inversion
symmetry breaking contributions of the moir\'e patterns, leading to primary
Dirac point gaps when the moir\'e strains give rise to a finite average mass,
and to secondary gaps when the moir\'e pseudospin components are mixed
appropriately. The pseudomagnetic strain fields which can reach values of up to
40 Tesla near symmetry points in the moir\'e cell stem almost entirely
from virtual hopping and dominate over the contributions arising from bond
length distortions due to the moir\'e strains.Comment: 14 pages, 8 figures, 3 table
ARPES signatures of few-layer twistronic graphenes
Diverse emergent correlated electron phenomena have been observed in twisted
graphene layers due to electronic interactions with the moir\'e superlattice
potential. Many electronic structure predictions have been reported exploring
this new field, but with few momentum-resolved electronic structure
measurements to test them. Here we use angle-resolved photoemission
spectroscopy (ARPES) to study the twist-dependent () electronic band structure of few-layer graphenes, including twisted
bilayer, monolayer-on-bilayer, and double-bilayer graphene (tDBG). Direct
comparison is made between experiment and theory, using a hybrid
model for interlayer coupling and implementing
photon-energy-dependent phase shifts for photo-electrons from consecutive
layers to simulate ARPES spectra. Quantitative agreement between experiment and
theory is found across twist angles, stacking geometries, and back-gate
voltages, validating the models and revealing displacement field induced gap
openings in twisted graphenes. However, for tDBG at ,
close to the predicted magic-angle of , a flat band is found
near the Fermi-level with measured bandwidth of meV. Analysis of
the gap between the flat band and the next valence band shows significant
deviations between experiment (meV) and the theoretical model
(meV), indicative of the importance of lattice relaxation in this
regime
Band dispersion in the deep 1s core level of graphene
Chemical bonding in molecules and solids arises from the overlap of valence
electron wave functions, forming extended molecular orbitals and dispersing
Bloch states, respectively. Core electrons with high binding energies, on the
other hand, are localized to their respective atoms and their wave functions do
not overlap significantly. Here we report the observation of band formation and
considerable dispersion (up to 60 meV) in the core level of the carbon
atoms forming graphene, despite the high C binding energy of 284
eV. Due to a Young's double slit-like interference effect, a situation arises
in which only the bonding or only the anti-bonding states is observed for a
given photoemission geometry.Comment: 12 pages, 3 figures, including supplementary materia
Visualizing Orbital Content of Electronic Bands in Anisotropic 2D Semiconducting ReSe2
Many properties of layered materials change as they are thinned from their
bulk forms down to single layers, with examples including indirect-to-direct
band gap transition in 2H semiconducting transition metal dichalcogenides as
well as thickness-dependent changes in the valence band structure in
post-transition metal monochalcogenides and black phosphorus. Here, we use
angle-resolved photoemission spectroscopy to study the electronic band
structure of monolayer ReSe, a semiconductor with a distorted 1T
structure and in-plane anisotropy. By changing the polarization of incoming
photons, we demonstrate that for ReSe, in contrast to the 2H materials,
the out-of-plane transition metal and chalcogen orbitals do
not contribute significantly to the top of the valence band which explains the
reported weak changes in the electronic structure of this compound as a
function of layer number. We estimate a band gap of 1.7 eV in pristine
ReSe using scanning tunneling spectroscopy and explore the implications
on the gap following surface-doping with potassium. A lower bound of 1.4 eV is
estimated for the gap in the fully doped case, suggesting that doping-dependent
many-body effects significantly affect the electronic properties of ReSe.
Our results, supported by density functional theory calculations, provide
insight into the mechanisms behind polarization-dependent optical properties of
rhenium dichalcogenides and highlight their place amongst two-dimensional
crystals.Comment: 37 pages (including Supporting Information), 7 figures in the main
tex
Transport Spectroscopy of Symmetry-Broken Insulating States in Bilayer Graphene
The flat bands in bilayer graphene(BLG) are sensitive to electric fields
E\bot directed between the layers, and magnify the electron-electron
interaction effects, thus making BLG an attractive platform for new
two-dimensional (2D) electron physics[1-5]. Theories[6-16] have suggested the
possibility of a variety of interesting broken symmetry states, some
characterized by spontaneous mass gaps, when the electron-density is at the
carrier neutrality point (CNP). The theoretically proposed gaps[6,7,10] in
bilayer graphene are analogous[17,18] to the masses generated by broken
symmetries in particle physics and give rise to large momentum-space Berry
curvatures[8,19] accompanied by spontaneous quantum Hall effects[7-9]. Though
recent experiments[20-23] have provided convincing evidence of strong
electronic correlations near the CNP in BLG, the presence of gaps is difficult
to establish because of the lack of direct spectroscopic measurements. Here we
present transport measurements in ultra-clean double-gated BLG, using
source-drain bias as a spectroscopic tool to resolve a gap of ~2 meV at the
CNP. The gap can be closed by an electric field E\bot \sim13 mV/nm but
increases monotonically with a magnetic field B, with an apparent particle-hole
asymmetry above the gap, thus providing the first mapping of the ground states
in BLG.Comment: 4 figure
Lorentz violation, Gravity, Dissipation and Holography
We reconsider Lorentz Violation (LV) at the fundamental level. We show that
Lorentz Violation is intimately connected with gravity and that LV couplings in
QFT must always be fields in a gravitational sector. Diffeomorphism invariance
must be intact and the LV couplings transform as tensors under coordinate/frame
changes. Therefore searching for LV is one of the most sensitive ways of
looking for new physics, either new interactions or modifications of known
ones. Energy dissipation/Cerenkov radiation is shown to be a generic feature of
LV in QFT. A general computation is done in strongly coupled theories with
gravity duals. It is shown that in scale invariant regimes, the energy
dissipation rate depends non-triviallly on two characteristic exponents, the
Lifshitz exponent and the hyperscaling violation exponent.Comment: LateX, 51 pages, 9 figures. (v2) References and comments added.
Misprints correcte