11,391 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
Properties of derivations on some convolution algebras
For all the convolution algebras and
, the derivations are of the form for suitable measures , where . We describe the
(weakly) compact as well as the (weakly) Montel derivations on these algebras
in terms of properties of the measure . Moreover, for all these algebras
we show that the extension of to a natural dual space is weak-star
continuous.Comment: 12 page
Exact polarizability and plasmon resonances of partly buried nanowires
The electrostatic polarizability for both vertical and horizontal
polarization of two conjoined half-cylinders partly buried in a substrate is
derived in an analytical closed-form expression. Using the derived analytical
polarizabilities we analyze the localized surface plasmon resonances of three
important metal nanowire configurations: (1) a half-cylinder, (2) a
half-cylinder on a substrate, and (3) a cylinder partly buried in a substrate.
Among other results we show that the substrate plays an important role for
spectral location of the plasmon resonances. Our analytical results enable an
easy, fast, and exact analysis of many complicated plasmonic nanowire
configurations including nanowires on substrates. This is important both for
comparison with experimental data, for applications, and as benchmarks for
numerical methods
Symplectic integration and physical interpretation of time-dependent coupled-cluster theory
The formulation of the time-dependent Schrodinger equation in terms of
coupled-cluster theory is outlined, with emphasis on the bivariational
framework and its classical Hamiltonian structure. An indefinite inner product
is introduced, inducing physical interpretation of coupled-cluster states in
the form of transition probabilities, autocorrelation functions, and explicitly
real values for observables, solving interpretation issues which are present in
time-dependent coupled-cluster theory and in ground-state calculations of
molecular systems under influence of external magnetic fields. The problem of
the numerical integration of the equations of motion is considered, and a
critial evaluation of the standard fourth-order Runge--Kutta scheme and the
symplectic Gauss integrator of variable order is given, including several
illustrative numerical experiments. While the Gauss integrator is stable even
for laser pulses well above the perturbation limit, our experiments indicate
that a system-dependent upper limit exists for the external field strengths.
Above this limit, time-dependent coupled-cluster calculations become very
challenging numerically, even in the full configuration interaction limit. The
source of these numerical instabilities is shown to be rapid increases of the
amplitudes as ultrashort high-intensity laser pulses pump the system out of the
ground state into states that are virtually orthogonal to the static
Hartree-Fock reference determinant.Comment: 14 pages, 13 figure
Quantum spill out in few-nanometer metal gaps: Effect on gap plasmons and reflectance from ultrasharp groove arrays
Plasmons in ultranarrow metal gaps are highly sensitive to the electron
density profile at the metal surfaces. Using a fully quantum mechanical
approach, we study the effects of electron spill-out on gap plasmons and
reflectance from ultrasharp metal grooves. We demonstrate that the mode index
of ultranarrow gap plasmons converges to the bulk refractive index in the limit
of vanishing gap and, thereby, rectify the unphysical divergence found in
classical models. Surprisingly, spill-out also significantly increases the
plasmonic absorption for few-nanometer gaps and lowers the reflectance from
arrays of ultrasharp metal grooves. These findings are explained in terms of
enhanced gap plasmon absorption taking place inside the gap 1-2 {\AA} from the
walls and delocalization near the groove bottom. Reflectance calculations
taking spill-out into account are shown to be in much better agreement with
measurements compared with classical models
Field-induced dissociation of two-dimensional excitons in transition-metal dichalcogenides
Generation of photocurrents in semiconducting materials requires dissociation
of excitons into free charge carriers. While thermal agitation is sufficient to
induce dissociation in most bulk materials, an additional push is required to
induce efficient dissociation of the strongly bound excitons in monolayer
transition-metal dichalcogenides (TMDs). Recently, static in-plane electric
fields have proven to be a promising candidate. In the present paper, we
introduce a numerical procedure, based on exterior complex scaling, capable of
computing field-induced exciton dissociation rates for a wider range of field
strengths than previously reported in literature. We present both Stark shifts
and dissociation rates for excitons in various TMDs calculated within the
Mott-Wannier model. Here, we find that the field induced dissociation rate is
strongly dependent on the dielectric screening environment. Furthermore,
applying weak-field asymptotic theory (WFAT) to the Keldysh potential, we are
able to derive an analytical expression for exciton dissociation rates in the
weak-field region
Quantum spill-out in nanometer-thin gold slabs: Effect on plasmon mode index and plasmonic absorption
A quantum mechanical approach and local response theory are applied to study
plasmons propagating in nanometer-thin gold slabs sandwiched between different
dielectrics. The metal slab supports two different kinds of modes, classified
as long-range and short-range plasmons. Quantum spill-out is found to
significantly increase the imaginary part of their mode indices, and,
surprisingly, even for slabs wide enough to approach bulk the increase is 20%.
This is explained in terms of enhanced plasmonic absorption, which mainly takes
place at narrow peaks located near the slab surface
Optical Hall conductivity in bulk and nanostructured graphene beyond the Dirac approximation
We present a perturbative method for calculating the optical Hall
conductivity in a tight-binding framework based on the Kubo formalism. The
method involves diagonalization only of the Hamiltonian in absence of the
magnetic field, and thus avoids the computational problems usually arising due
to the huge magnetic unit cells required to maintain translational invariance
in presence of a Peierls phase. A recipe for applying the method to numerical
calculations of the magneto-optical response is presented. We apply the
formalism to the case of ordinary and gapped graphene in a next-nearest
neighbour tight-binding model as well as graphene antidot lattices. In both
case, we find unique signatures in the Hall response, that are not captured in
continuum (Dirac) approximations. These include a non-zero optical Hall
conductivity even when the chemical potential is at the Dirac point energy.
Numerical results suggest that this effect should be measurable in experiments.Comment: 7 pages, 4 figures, accepted in Physical Review
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
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