16 research outputs found
Inelastic Quantum Transport and Peierls-like Mechanism in Carbon Nanotubes
We report on a theoretical study of inelastic quantum transport in
carbon nanotubes. By using a many-body description of the electron-phonon
interaction in Fock space, a novel mechanism involving optical phonon emission
(absorption) is shown to induce an unprecedented energy gap opening at half the
phonon energy, , above (below) the charge neutrality point.
This mechanism, which is prevented by Pauli blocking at low bias voltages, is
activated at bias voltages in the order of .Comment: 4 pages, 4 figure
Hierarchy of Floquet gaps and edge states for driven honeycomb lattices
Electromagnetic driving in a honeycomb lattice can induce gaps and
topological edge states with a structure of increasing complexity as the
frequency of the driving lowers. While the high frequency case is the most
simple to analyze we focus on the multiple photon processes allowed in the low
frequency regime to unveil the hierarchy of Floquet edge-states. In the case of
low intensities an analytical approach allows us to derive effective
Hamiltonians and address the topological character of each gap in a
constructive manner. At high intensities we obtain the net number of edge
states, given by the winding number, with a numerical calculation of the Chern
numbers of each Floquet band. Using these methods, we find a hierarchy that
resembles that of a Russian nesting doll. This hierarchy classifies the gaps
and the associated edge states in different orders according to the
electron-photon coupling strength. For large driving intensities, we rely on
the numerical calculation of the winding number, illustrated in a map of
topological phase transitions. The hierarchy unveiled with the low energy
effective Hamiltonians, alongside with the map of topological phase transitions
discloses the complexity of the Floquet band structure in the low frequency
regime. The proposed method for obtaining the effective Hamiltonian can be
easily adapted to other Dirac Hamiltonians of two dimensional materials and
even the surface of a 3D topological insulator.Comment: Phys. Rev. A 91, 04362
Laser-induced effects on the electronic features of graphene nanoribbons
We study the interplay between lateral confinement and photon-induced
processes on the electronic properties of illuminated graphene nanoribbons. We
find that by tuning the device setup (edges geometries, ribbon width and
polarization direction), a laser with frequency {\Omega} may either not affect
the electronic structure, or induce bandgaps or depletions at \hbar {\Omega}/2,
and/or at other energies not commensurate with half the photon energy. Similar
features are also observed in the dc conductance, suggesting the use of the
polarization direction to switch on and off the graphene device. Our results
could guide the design of novel types of optoelectronic nano-devices.Comment: 4 pages, 3 figure
Floquet topological phase transitions in a periodically quenched dimer
We report on the theoretical investigation of the topological properties of a
periodically quenched one-dimensional dimerized lattice where a piece-wise
constant Hamiltonian switches from to at a partition time
within each driving period . We examine different dimerization patterns for
and and the interplay with the driving parameters that lead to the
emergence of topological states both at zero energy and at the edge of the
Brillouin-Floquet quasi-energy zone. We illustrate different phenomena,
including the occurrence of both edge states in a semimetal spectrum, the
topological transitions, and the generation of zero-energy topological states
from trivial snapshots. The role of the different symmetries in our results is
also discussed.Comment: 13 pages, 10 figure
Tuning laser-induced bandgaps in graphene
Could a laser field lead to the much sought-after tunable bandgaps in
graphene? By using Floquet theory combined with Green's functions techniques,
we predict that a laser field in the mid-infrared range can produce observable
bandgaps in the electronic structure of graphene. Furthermore, we show how they
can be tuned by using the laser polarization. Our results could serve as a
guidance to design opto-electronic nano-devices.Comment: 4 pages, 3 figures, to appear in Applied Physics Letter
AC transport in graphene-based Fabry-Perot devices
We report on a theoretical study of the effects of time-dependent fields on
electronic transport through graphene nanoribbon devices. The Fabry-P\'{e}rot
interference pattern is modified by an ac gating in a way that depends strongly
on the shape of the graphene edges. While for armchair edges the patterns are
found to be regular and can be controlled very efficiently by tuning the ac
field, samples with zigzag edges exhibit a much more complex interference
pattern due to their peculiar electronic structure. These studies highlight the
main role played by geometric details of graphene nanoribbons within the
coherent transport regime. We also extend our analysis to noise power response,
identifying under which conditions it is possible to minimize the current
fluctuations as well as exploring scaling properties of noise with length and
width of the systems
Mono-parametric quantum charge pumping: interplay between spatial interference and photon-assisted tunneling
We analyze quantum charge pumping in an open ring with a dot embedded in one
of its arms. We show that cyclic driving of the dot levels by a \textit{single}
parameter leads to a pumped current when a static magnetic flux is
simultaneously applied to the ring. Based on the computation of the
Floquet-Green's functions, we show that for low driving frequencies ,
the interplay between the spatial interference through the ring plus
photon-assisted tunneling gives an average direct current (dc) which is
proportional to . The direction of the pumped current can be
reversed by changing the applied magnetic field.Comment: 7 pages, 4 figures. To appear in Phys. Rev.
Non-perturbative laser effects on the electrical properties of graphene nanoribbons
The use of Floquet theory combined with a realistic description of the
electronic structure of illuminated graphene and graphene nanoribbons is
developed to assess the emergence of non-adiabatic and non-perturbative effects
on the electronic properties. Here, we introduce an efficient computational
scheme and illustrate its use by applying it to graphene nanoribbons in the
presence of both linear and circular polarization. The interplay between
confinement due to the finite sample size and laser-induced transitions is
shown to lead to sharp features on the average conductance and density of
states. Particular emphasis is given to the emergence of the bulk limit
response.Comment: 14 pages, 8 figures, to appear in J. Phys.: Condens. Matter, special
issue on "Ultrafast and nonlinear optics in carbon nanomaterials
A valley of opportunities
After nearly two decades of graphene research, condensed-matter physicists Luis Foá Torres and Sergio O Valenzuela delve into the ongoing mystery of the material's perplexing non-local response and the "valley Hall effect