10 research outputs found
Spectral features due to inter-Landau-level transitions in the Raman spectrum of bilayer graphene
We investigate the contribution of the low-energy electronic excitations
towards the Raman spectrum of bilayer graphene for the incoming photon energy
Omega >> 1eV. Starting with the four-band tight-binding model, we derive an
effective scattering amplitude that can be incorporated into the commonly used
two-band approximation. Due to the influence of the high-energy bands, this
effective scattering amplitude is different from the contact interaction
amplitude obtained within the two-band model alone. We then calculate the
spectral density of the inelastic light scattering accompanied by the
excitation of electron-hole pairs in bilayer graphene. In the absence of a
magnetic field, due to the parabolic dispersion of the low-energy bands in a
bilayer crystal, this contribution is constant and in doped structures has a
threshold at twice the Fermi energy. In an external magnetic field, the
dominant Raman-active modes are the n_{-} to n_{+} inter-Landau-level
transitions with crossed polarisation of in/out photons. We estimate the
quantum efficiency of a single n_{-} to n_{+} transition in the magnetic field
of 10T as I_{n_{-} to n_{+}}~10^{-12}.Comment: 7 pages, 3 figures, expanded version published in PR
Tunable Fermi surface topology and Lifshitz transition in bilayer graphene
Bilayer graphene is a highly tunable material: not only can one tune the
Fermi energy using standard gates, as in single-layer graphene, but the band
structure can also be modified by external perturbations such as transverse
electric fields or strain. We review the theoretical basics of the band
structure of bilayer graphene and study the evolution of the band structure
under the influence of these two external parameters. We highlight their key
role concerning the ease to experimentally probe the presence of a Lifshitz
transition, which consists in a change of Fermi contour topology as a function
of energy close to the edges of the conduction and valence bands. Using a
device geometry that allows the application of exceptionally high displacement
fields, we then illustrate in detail the way to probe the topology changes
experimentally using quantum Hall effect measurements in a gapped bilayer
graphene system.Comment: To be published in Synthetic Metals, special issue "Advances in
Graphene
Transport signatures of pseudomagnetic Landau levels in strained graphene ribbons
In inhomogeneously strained graphene, low-energy electrons experience a valley-antisymmetric pseudomagnetic field which leads to the formation of localized states at the edge between the valence and conduction bands, understood in terms of peculiar n=0 pseudomagnetic Landau levels. Here we show that such states can manifest themselves as an isolated quadruplet of low-energy conductance resonances in a suspended stretched graphene ribbon, where clamping by the metallic contacts results in a strong inhomogeneity of strain near the ribbon ends
Moiré Superlattice Effects and Band Structure Evolution in Near-30-Degree Twisted Bilayer Graphene
In stacks of two-dimensional crystals, mismatch of their lattice constants
and misalignment of crystallographic axes lead to formation of moir\'{e}
patterns. We show that moir\'{e} superlattice effects persist in twisted
bilayer graphene (tBLG) with large twists and short moir\'{e} periods. Using
angle-resolved photoemission, we observe dramatic changes in valence band
topology across large regions of the Brillouin zone, including the vicinity of
the saddle point at and across 3 eV from the Dirac points. In this energy
range, we resolve several moir\'{e} minibands and detect signatures of
secondary Dirac points in the reconstructed dispersions. For twists
, the low-energy minigaps are not due to cone
anti-crossing as is the case at smaller twist angles but rather due to
moir\'{e} scattering of electrons in one graphene layer on the potential of the
other which generates intervalley coupling. Our work demonstrates robustness of
mechanisms which enable engineering of electronic dispersions of stacks of
two-dimensional crystals by tuning the interface twist angles. It also shows
that large-angle tBLG hosts electronic minigaps and van Hove singularities of
different origin which, given recent progress in extreme doping of graphene,
could be explored experimentally.Comment: main text: 25 pages, 5 figures; supplement: 22 pages, 7 figure
Large local lattice expansion in graphene adlayers grown on copper
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