22,961 research outputs found
Optical third harmonic generation in black phosphorus
We present a calculation of Third Harmonic Generation (THG) for two-band
systems using the length gauge that avoids unphysical divergences otherwise
present in the evaluation of the third order current density response. The
calculation is applied to bulk and monolayer black Phosphorus (bP) using a
non-orthogonal tight-binding model. Results show that the low energy response
is dominated by mixed inter-intraband processes and estimates of the magnitude
of THG susceptibility are comparable to recent experimental reports for bulk bP
samples.Comment: 9 pages, 5 figure
Iterative approach to arbitrary nonlinear optical response functions of graphene
Two-dimensional materials constitute an exciting platform for nonlinear
optics with large nonlinearities that are tunable by gating. Hence,
gate-tunable harmonic generation and intensity-dependent refraction have been
observed in e.g. graphene and transition-metal dichalcogenides, whose
electronic structures are accurately modelled by the (massive) Dirac equation.
We exploit on the simplicity of this model and demonstrate here that arbitrary
nonlinear response functions follow from a simple iterative approach. The power
of this approach is illustrated by analytical expressions for harmonic
generation and intensity-dependent refraction, both computed up to ninth order
in the pump field. Moreover, the results allow for arbitrary band gaps and
gating potentials. As illustrative applications, we consider (i)
gate-dependence of third- and fifth-harmonic generation in gapped and gapless
graphene, (ii) intensity-dependent refractive index of graphene up to ninth
order, and (iii) intensity-dependence of high-harmonic generation.Comment: 6 pages, 5 figures. Supplemental material: 6 pages, 2 figure
Linear and nonlinear optical response of crystals using length and velocity gauges: Effect of basis truncation
We study the effects of a truncated band structure on the linear and
nonlinear optical response of crystals using four methods. These are
constructed by (i) choosing either length or velocity gauge for the
perturbation and (ii) computing the current density either directly or via the
time-derivative of the polarization density. In the infinite band limit, the
results of all four methods are identical, but basis truncation breaks their
equivalence. In particular, certain response functions vanish identically and
unphysical low-frequency divergences are observed for few-band models in the
velocity gauge. Using hexagonal boron nitride (hBN) monolayer as a case study,
we analyze the problems associated with all methods and identify the optimal
one. Our results show that the length gauge calculations provide the fastest
convergence rates as well as the most accurate spectra for any basis size and,
moreover, that low-frequency divergences are eliminated.Comment: 11 pages, 7 figure
The Transformation Process of Fathers of Children with Disabilities: An Exploratory Case Study
While the vital role that fathers play in the development of their children is emphasized in recent literature, the majority of research relative to child development focuses on mothers. This imbalance is even more evident relative to research with parents of children with disabilities, leaving human service providers with few evidence based practices for appropriately addressing the needs of fathers raising children with disabilities. Research suggests that having a child with a disability, while challenging, can also have a significant positive impact on the family system and potentially offer a transformational experience for the parent. Guided by a theoretical model of transformational outcomes, the purpose of this qualitative case study was to investigate how two veteran fathers of children with disabilities describe their transformative process. Using qualitative inquiry methods, the fathers’ were interviewed and their narratives were transcribed and analyzed to discover emerging themes. Findings indicated that laughter was a prominent emotion throughout the narrative and that the fathers used both positive and negative descriptors to define their experiences. Implications of these findings for human service professionals supporting families of children with disabilities are discussed. Attending to the unique needs of fathers can improve the overall functionality of the family system
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
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}
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