26,423 research outputs found
Innovation and National Security: The Loss of Economic and Military Strength through the Theft of Ideas
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
Minimal surface singularities are Lipschitz normally embedded
Any germ of a complex analytic space is equipped with two natural metrics:
the {\it outer metric} induced by the hermitian metric of the ambient space and
the {\it inner metric}, which is the associated riemannian metric on the germ.
We show that minimal surface singularities are Lipschitz normally embedded
(LNE), i.e., the identity map is a bilipschitz homeomorphism between outer and
inner metrics, and that they are the only rational surface singularities with
this property.Comment: This paper is a major revision of the 2015 version. It now builds on
the paper arXiv:1806.11240 by the same authors which gives a general
characterization of Lipschitz normally embedded surface singularitie
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Possible immunoenhancement of persistent viremia by feline leukemia virus envelope glycoprotein vaccines in challenge-exposure situations where whole inactivated virus vaccines were protective.
Kittens immunized with purified native FeLV-gp70 or -gp85 envelope proteins developed ELISA, but not virus neutralizing, antibodies in their serum to both whole FeLV and FeLV-gp70. Kittens vaccinated with envelope proteins and infected with feline sarcoma virus (FeSV) developed smaller tumors than nonvaccinates, but a greater incidence of persistent retroviremia. Similarly, FeLV-gp70 and -gp85 vaccinated kittens were more apt to become persistently retroviremic following virulent FeLV challenge exposure than nonvaccinates. Kittens vaccinated with inactivated whole FeLV developed smaller tumors after FeSV inoculation and had a lower incidence of persistent retroviremia than nonvaccinates. The protective effect of inactivated whole FeLV vaccine against persistent retroviremia was also seen with FeLV challenge-exposed cats. Protection afforded by inactivated whole FeLV vaccine was not associated with virus neutralizing antibodies, although ELISA antibodies to both whole FeLV and FeLV-gp70 were induced by vaccination
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
Optical second harmonic generation from Wannier excitons
Excitonic effects in the linear optical response of semiconductors are
well-known and the subject of countless experimental and theoretical studies.
For the technologically important second order nonlinear response, however,
description of excitonic effects has proved to be difficult. In this work, a
simplified three-band Wannier exciton model of cubic semiconductors is applied
and a closed form expression for the complex second harmonic response function
including broadening is derived. Our calculated spectra are found to be in
excellent agreement with the measured response near the band edge. In addition,
a very substantial enhancement of the nonlinear response is predicted for the
transparency region
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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