671 research outputs found
Transport in graphene antidot barriers and tunneling devices
Periodic arrays of antidots, i.e. nanoscale perforations, in graphene enable
tight confinement of carriers and efficient transport barriers. Such barriers
evade the Klein tunneling mechanism by being of the mass rather than
electrostatic type. While all graphene antidot lattices (GALs) may support
directional barriers, we show, however, that a full transport gap exists only
for certain orientations of the GAL. Moreover, we assess the applicability of
gapped graphene and the Dirac continuum approach as simplified models of
various antidot structures showing that, in particular, the former is an
excellent approximation for transport in GALs supporting a bulk band gap.
Finally, the transport properties of a GAL based resonant tunneling diode is
analyzed indicating that such advanced graphene based devices may, indeed, be
realized using GAL structures.Comment: 12 pages, 9 figures, accepted for publication on Journal of Applied
Physic
Tight-binding study of the magneto-optical properties of gapped graphene
We study the optical properties of gapped graphene in presence of a magnetic
field. We consider a model based on the Dirac equation, with a gap introduced
via a mass term, for which analytical expressions for the diagonal and Hall
optical conductivities can be derived. We discuss the effect of the mass term
on electron-hole symmetry and - symmetry and its implications for
the optical Hall conductivity. We compare these results with those obtained
using a tight-binding model, in which the mass is modeled via a staggered
potential and a magnetic field is included via a Peierls substitution.
Considering antidot lattices as the source of the mass term, we focus on the
limit where the mass term dominates the cyclotron energy. We find that a large
gap quenches the effect of the magnetic field. The role of overlap between
neighboring orbitals is investigated, and we find that the overlap has
pronounced consequences for the optical Hall conductivity that are missed in
the Dirac model.Comment: 10 pages, 9 figures, submitted for Physical Review
Theory of second harmonic generation in few-layered MoS2
Recent experimental results have demonstrated the ability of monolayer
MoS to efficiently generate second harmonic fields with susceptibilities
between 0.1 and 100 nm/V. However, no theoretical calculations exist with which
to interpret these findings. In particular, it is of interest to theoretically
estimate the modulus of the second harmonic response, since experimental
reports on this differ by almost three orders of magnitude. Here, we present
single-particle calculations of the second harmonic response based on a
tight-binding band structure. We compare directly with recent experimental
findings and include in the discussion also spectral features and the effects
of multiple layers
Hypergeometric resummation of self-consistent sunset diagrams for electron-boson quantum many-body systems out of equilibrium
A newly developed hypergeometric resummation technique [H. Mera et al., Phys.
Rev. Lett. 115, 143001 (2015)] provides an easy-to-use recipe to obtain
conserving approximations within the self-consistent nonequilibrium many-body
perturbation theory. We demonstrate the usefulness of this technique by
calculating the phonon-limited electronic current in a model of a
single-molecule junction within the self-consistent Born approximation for the
electron-phonon interacting system, where the perturbation expansion for the
nonequilibrium Green function in powers of the free bosonic propagator
typically consists of a series of non-crossing \sunset" diagrams.
Hypergeometric resummation preserves conservation laws and it is shown to
provide substantial convergence acceleration relative to more standard
approaches to self-consistency. This result strongly suggests that the
convergence of the self-consistent \sunset" series is limited by a branch-cut
singularity, which is accurately described by Gauss hypergeometric functions.
Our results showcase an alternative approach to conservation laws and
self-consistency where expectation values obtained from conserving perturbation
expansions are \summed" to their self-consistent value by analytic continuation
functions able to mimic the convergence-limiting singularity structure.Comment: 13 pages, 6 figure
A Lightweight Multilevel Markup Language for Connecting Software Requirements and Simulations
[Context] Simulation is a powerful tool to validate specified requirements especially for complex systems that constantly monitor and react to characteristics of their environment. The simulators for such systems are complex themselves as they simulate multiple actors with multiple interacting functions in a number of different scenarios. To validate requirements in such simulations, the requirements must be related to the simulation runs. [Problem] In practice, engineers are reluctant to state their requirements in terms of structured languages or models that would allow for a straightforward relation of requirements to simulation runs. Instead, the requirements are expressed as unstructured natural language text that is hard to assess in a set of complex simulation runs. Therefore, the feedback loop between requirements and simulation is very long or non-existent at all. [Principal idea] We aim to close the gap between requirements specifications and simulation by proposing a lightweight markup language for requirements. Our markup language provides a set of annotations on different levels that can be applied to natural language requirements. The annotations are mapped to simulation events. As a result, meaningful information from a set of simulation runs is shown directly in the requirements specification. [Contribution] Instead of forcing the engineer to write requirements in a specific way just for the purpose of relating them to a simulator, the markup language allows annotating the already specified requirements up to a level that is interesting for the engineer. We evaluate our approach by analyzing 8 original requirements of an automotive system in a set of 100 simulation runs
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