4,594 research outputs found
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
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 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
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
Nonlinear photocurrents in two-dimensional systems based on graphene and boron nitride
DC photoelectrical currents can be generated purely as a non-linear effect in
uniform media lacking inversion symmetry without the need for a material
junction or bias voltages to drive it, in what is termed photogalvanic effect.
These currents are strongly dependent on the polarization state of the
radiation, as well as on topological properties of the underlying Fermi surface
such as its Berry curvature. In order to study the intrinsic photogalvanic
response of gapped graphene (GG), biased bilayer graphene (BBG), and hexagonal
boron nitride (hBN), we compute the non-linear current using a perturbative
expansion of the density matrix. This allows a microscopic description of the
quadratic response to an electromagnetic field in these materials, which we
analyze as a function of temperature and electron density. We find that the
intrinsic response is robust across these systems and allows for currents in
the range of pA cm/W to nA cm/W. At the independent-particle level, the
response of hBN-based structures is significant only in the ultra-violet due to
their sizeable band-gap. However, when Coulomb interactions are accounted for
by explicit solution of the Bethe-Salpeter equation, we find that the
photoconductivity is strongly modified by transitions involving exciton levels
in the gap region, whose spectral weight dominates in the overall frequency
range. Biased bilayers and gapped monolayers of graphene have a strong
photoconductivity in the visible and infrared window, allowing for photocurrent
densities of several nA cm/W. We further show that the richer electronic
dispersion of BBG at low energies and the ability to change its band-gap on
demand allows a higher tunability of the photocurrent, including not only its
magnitude but also, and significantly, its polarity.Comment: Updating with published version and respective references; 14 pages,
11 figure
Rigorous perturbation theory versus variational methods in the spectral study of carbon nanotubes
Recent two-photon photo-luminescence experiments give accurate data for the
ground and first excited excitonic energies at different nanotube radii. In
this paper we compare the analytic approximations proved in \cite{CDR}, with a
standard variational approach. We show an excellent agreement at sufficiently
small radii.Comment: Accepted for publication in Contemporary Mathematic
Correlation and dimensional effects of trions in carbon nanotubes
We study the binding energies of singlet trions, i.e. charged excitons, in
carbon nanotubes. The problem is modeled, through the effective-mass model, as
a three-particle complex on the surface of a cylinder, which we investigate
using both one- and two-dimensional expansions of the wave function. The
effects of dimensionality and correlation are studied in detail. We find that
the Hartree-Fock approximation significantly underestimates the trion binding
energy. Combined with band structures calculated using a non-orthogonal nearest
neighbour tight binding model, the results from the cylinder model are used to
compute physical binding energies for a wide selection of carbon nanotubes. In
addition, the dependence on dielectric screening is examined. Our findings
indicate that trions are detectable at room temperature in carbon nanotubes
with radius below 8{\AA}
Boron and nitrogen doping in graphene antidot lattices
Bottom-up fabrication of graphene antidot lattices (GALs) has previously
yielded atomically precise structures with sub-nanometer periodicity. Focusing
on this type of experimentally realized GAL, we perform density functional
theory calculations on the pristine structure as well as GALs with edge carbon
atoms substituted with boron or nitrogen. We show that p- and n-type doping
levels emerge with activation energies that depend on the level of
hydrogenation at the impurity. Furthermore, a tight-binding parameterization
together with a Green's function method are used to describe more dilute
doping.Comment: 8 pages, 7 figure
Optical properties of graphene antidot lattices
Undoped graphene is semi-metallic and thus not suitable for many electronic
and optoelectronic applications requiring gapped semiconductor materials.
However, a periodic array of holes (antidot lattice) renders graphene
semiconducting with a controllable band gap. Using atomistic modelling, we
demonstrate that this artificial nanomaterial is a dipole-allowed direct gap
semiconductor with a very pronounced optical absorption edge. Hence, optical
infrared spectroscopy should be an ideal probe of the electronic structure. To
address realistic experimental situations, we include effects due to disorder
and the presence of a substrate in the analysis.Comment: 11 pages, 9 figures, accepted for publication in Phys. Rev.
The Faraday effect revisited: General theory
This paper is the first in a series revisiting the Faraday effect, or more
generally, the theory of electronic quantum transport/optical response in bulk
media in the presence of a constant magnetic field. The independent electron
approximation is assumed. At zero temperature and zero frequency, if the Fermi
energy lies in a spectral gap, we rigorously prove the Widom-Streda formula.
For free electrons, the transverse conductivity can be explicitly computed and
coincides with the classical result. In the general case, using magnetic
perturbation theory, the conductivity tensor is expanded in powers of the
strength of the magnetic field . Then the linear term in of this
expansion is written down in terms of the zero magnetic field Green function
and the zero field current operator. In the periodic case, the linear term in
of the conductivity tensor is expressed in terms of zero magnetic field
Bloch functions and energies. No derivatives with respect to the quasi-momentum
appear and thereby all ambiguities are removed, in contrast to earlier work.Comment: Final version, accepted for publication in J. Math. Phy
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