4,438 research outputs found
Pseudo-magnetic field distribution and pseudo-Landau levels in suspended graphene flakes
Combining the tight-binding approximation and linear elasticity theory for a
planar membrane, we investigate stretching of a graphene flake assuming that
two opposite edges of the sample are clamped by the contacts. We show that,
depending on the aspect ratio of the flake and its orientation, gapped states
may form in the membrane in the vicinity of the contacts. This gap in the
pre-contact region should be biggest for the armchair orientation of the flake
and width to length ratio of around 1.Comment: 7 pages + 3 figure
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
Influence of Zeeman splitting and thermally excited polaron states on magneto-electrical and magneto-thermal properties of magnetoresistive polycrystalline manganite La_{0.8}Sr_{0.2}MnO_3
Some possible connection between spin and charge degrees of freedom in
magneto-resistive manganites is investigated through a thorough experimental
study of the magnetic (AC susceptibility and DC magnetization) and transport
(resistivity and thermal conductivity) properties. Measurements are reported in
the case of well characterized polycrystalline La_{0.8}Sr_{0.2}MnO_3 samples.
The experimental results suggest rather strong field-induced polarization
effects in our material, clearly indicating the presence of ordered FM regions
inside the semiconducting phase. Using an analytical expression which fits the
spontaneous DC magnetization, the temperature and magnetic field dependences of
both electrical resistivity and thermal conductivity data are found to be well
reproduced through a universal scenario based on two mechanisms: (i) a
magnetization dependent spin polaron hopping influenced by a Zeeman splitting
effect, and (ii) properly defined thermally excited polaron states which have
to be taken into account in order to correctly describe the behavior of the
less conducting region. Using the experimentally found values of the magnetic
and electron localization temperatures, we obtain L=0.5nm and m_p=3.2m_e for
estimates of the localization length (size of the spin polaron) and effective
polaron mass, respectively.Comment: Accepted for publication in Journal of Applied Physic
Percolation-induced exponential scaling in the large current tails of random resistor networks
There is a renewed surge in percolation-induced transport properties of
diverse nano-particle composites (cf. RSC Nanoscience & Nanotechnology Series,
Paul O'Brien Editor-in-Chief). We note in particular a broad interest in
nano-composites exhibiting sharp electrical property gains at and above
percolation threshold, which motivated us to revisit the classical setting of
percolation in random resistor networks but from a multiscale perspective. For
each realization of random resistor networks above threshold, we use network
graph representations and associated algorithms to identify and restrict to the
percolating component, thereby preconditioning the network both in size and
accuracy by filtering {\it a priori} zero current-carrying bonds. We then
simulate many realizations per bond density and analyze scaling behavior of the
complete current distribution supported on the percolating component. We first
confirm the celebrated power-law distribution of small currents at the
percolation threshold, and second we confirm results on scaling of the maximum
current in the network that is associated with the backbone of the percolating
cluster. These properties are then placed in context with global features of
the current distribution, and in particular the dominant role of the large
current tail that is most relevant for material science applications. We
identify a robust, exponential large current tail that: 1. persists above
threshold; 2. expands broadly over and dominates the current distribution at
the expense of the vanishing power law scaling in the small current tail; and
3. by taking second moments, reproduces the experimentally observed power law
scaling of bulk conductivity above threshold
Energy solutions to one-dimensional singular parabolic problems with data are viscosity solutions
We study one-dimensional very singular parabolic equations with periodic
boundary conditions and initial data in , which is the energy space. We
show existence of solutions in this energy space and then we prove that they
are viscosity solutions in the sense of Giga-Giga.Comment: 15 page
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
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
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
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