391 research outputs found
Phonon switching and combined Fano-Rice effect in optical spectra of bilayer graphene
Recent infrared measurements of phonon peaks in gated bilayer graphene reveal
two striking signatures of electron-phonon interaction: an asymmetric Fano
lineshape and a giant variation of the peak intensity as a function of the
applied gate voltage. In this Letter we provide a unified theoretical framework
which accounts for both these effects and unveils the occurrence of a switching
mechanism between the symmetric () and anti-symmetric () phonon mode
as dominant channel in the optical response. A complete phase diagram of the
optical phonon response is also presented, as a function of both the charge
density and the bandgap.Comment: final versio
Charged-phonon theory and Fano effect in the optical spectroscopy of bilayer graphene
Since their discovery, graphene-based systems represent an exceptional
playground to explore the emergence of peculiar quantum effects. The present
paper focuses on the anomalous appearence of strong infrared phonon resonances
in the optical spectroscopy of bilayer graphene and on their pronounced
Fano-like asymmetry, both tunable in gated devices. By developing a full
microscopic many-body approach for the optical phonon response we explain how
both effects can be quantitatively accounted for by the quantum interference of
electronic and phononic excitations. We show that the phonon modes borrow a
large dipole intensity from the electronic background, the so-called
charged-phonon effect, and at the same time interfer with it, leading to a
typical Fano response. Our approach allows one to disentangle the correct
selection rules that control the relative importance of the two (symmetric and
antisymmetric) relevant phonon modes for different values of the doping and/or
of the gap in bilayer graphene. Finally, we discuss the extension of the same
theoretical scheme to the Raman spectroscopy, to explain the lack of the same
features on the Raman phononic spectra. Besides its remarkable success in
explaining the existing experimental data in graphene-based systems, the
present theoretical approach offers a general scheme for the microscopic
understanding of Fano-like features in a wide variety of other systems.Comment: 16 pages, 11 eps figures, PR
Landau Damping in a 2D Electron Gas with Imposed Quantum Grid
Dielectric properties of semiconductor substrate with imposed two dimensional
(2D) periodic grid of quantum wires or nanotubes (quantum crossbars, QCB) are
studied. It is shown that a capacitive contact between QCB and semiconductor
substrate does not destroy the Luttinger liquid character of the long wave QCB
excitations. However, the dielectric losses of a substrate surface are
drastically modified due to diffraction processes on the QCB superlattice.
QCB-substrate interaction results in additional Landau damping regions of the
substrate plasmons. Their existence, form and the density of losses are
strongly sensitive to the QCB lattice constant.Comment: 9 pages, 12 eps-figure
Infrared phonon activity in pristine graphite
We study experimentally and theoretically the Fano-shaped phonon peak at 1590
cm (0.2 eV) in the in-plane optical conductivity of pristine graphite.
We show that the anomalously large spectral weight and the Fano asymmetry of
the peak can be qualitatively accounted for by a charged-phonon theory. A
crucial role in this context is played by the particle-hole asymmetry of the
electronic -bands.Comment: 5 pages, 4 figures, 1 tabl
Stratified dispersive model for material characterization using terahertz time-domain spectroscopy
We propose a novel THz material analysis approach which provides highly
accurate material parameters and can be used for industrial quality control.
The method treats the inspected material within its environment locally as a
stratified system and describes the light-matter interaction of each layer in a
realistic way. The approach is illustrated in the time-domain and
frequency-domain for two potential fields of implementation of THz technology:
quality control of (coated) paper sheets and car paint multilayers, both
measured in humid air.Comment: 4 pages, 4 figure
Kramers-Kronig constrained variational analysis of optical spectra
A universal method of extraction of the complex dielectric function
from
experimentally accessible optical quantities is developed. The central idea is
that is parameterized independently at each node of a
properly chosen anchor frequency mesh, while is
dynamically coupled to by the Kramers-Kronig (KK)
transformation. This approach can be regarded as a limiting case of the
multi-oscillator fitting of spectra, when the number of oscillators is of the
order of the number of experimental points. In the case of the normal-incidence
reflectivity from a semi-infinite isotropic sample the new method gives
essentially the same result as the conventional KK transformation of
reflectivity. In contrast to the conventional approaches, the proposed
technique is applicable, without readaptation, to virtually all types of
linear-response optical measurements, or arbitrary combinations of
measurements, such as reflectivity, transmission, ellipsometry {\it etc.}, done
on different types of samples, including thin films and anisotropic crystals.Comment: 10 pages, 7 figure
Anisotropy of graphite optical conductivity
The graphite conductivity is evaluated for frequencies between
0.1 eV, the energy of the order of the electron-hole overlap, and 1.5 eV, the
electron nearest hopping energy. The in-plane conductivity per single atomic
sheet is close to the universal graphene conductivity and,
however, contains a singularity conditioned by peculiarities of the electron
dispersion. The conductivity is less in the direction by the factor of the
order of 0.01 governed by electron hopping in this direction.Comment: 3 pages, 3 figure
Optical properties of BiTeBr and BiTeCl
We present a comparative study of the optical properties - reflectance,
transmission and optical conductivity - and Raman spectra of two layered
bismuth-tellurohalides BiTeBr and BiTeCl at 300 K and 5 K, for light polarized
in the a-b planes. Despite different space groups, the optical properties of
the two compounds are very similar. Both materials are doped semiconductors,
with the absorption edge above the optical gap which is lower in BiTeBr (0.62
eV) than in BiTeCl (0.77 eV). The same Rashba splitting is observed in the two
materials. A non-Drude free carrier contribution in the optical conductivity,
as well as three Raman and two infrared phonon modes, are observed in each
compound. There is a dramatic difference in the highest infrared phonon
intensity for the two compounds, and a difference in the doping levels. Aspects
of the strong electron-phonon interaction are identified. Several interband
transitions are assigned, among them the low-lying absorption which has
the same value 0.25 eV in both compounds, and is caused by the Rashba spin
splitting of the conduction band. An additional weak transition is found in
BiTeCl, caused by the lower crystal symmetry.Comment: Accepted in PR
Infrared spectroscopy of hole doped ABA-stacked trilayer graphene
Using infrared spectroscopy, we investigate bottom gated ABA-stacked trilayer
graphene subject to an additional environment-induced p-type doping. We find
that the Slonczewski-Weiss-McClure tight-binding model and the Kubo formula
reproduce the gate voltage-modulated reflectivity spectra very accurately. This
allows us to determine the charge densities and the potentials of the
{\pi}-band electrons on all graphene layers separately and to extract the
interlayer permittivity due to higher energy bands.Comment: 6 pages, 6 figures Corrected sign of fig 3 and visibilty of fig
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