12,674 research outputs found
Abrasion of flat rotating shapes
We report on the erosion of flat linoleum "pebbles" under steady rotation in
a slurry of abrasive grit. To quantify shape as a function of time, we develop
a general method in which the pebble is photographed from multiple angles with
respect to the grid of pixels in a digital camera. This reduces digitization
noise, and allows the local curvature of the contour to be computed with a
controllable degree of uncertainty. Several shape descriptors are then employed
to follow the evolution of different initial shapes toward a circle, where
abrasion halts. The results are in good quantitative agreement with a simple
model, where we propose that points along the contour move radially inward in
proportion to the product of the radius and the derivative of radius with
respect to angle
Local correlation functional for electrons in two dimensions
We derive a local approximation for the correlation energy in two-dimensional
electronic systems. In the derivation we follow the scheme originally developed
by Colle and Salvetti for three dimensions, and consider a Gaussian
approximation for the pair density. Then, we introduce an ad-hoc modification
which better accounts for both the long-range correlation, and the
kinetic-energy contribution to the correlation energy. The resulting functional
is local, and depends parametrically on the number of electrons in the system.
We apply this functional to the homogeneous electron gas and to a set of
two-dimensional quantum dots covering a wide range of electron densities and
thus various amounts of correlation. In all test cases we find an excellent
agreement between our results and the exact correlation energies. Our
correlation functional has a form that is simple and straightforward to
implement, but broadly outperforms the commonly used local-density
approximation
Construction of the B88 exchange-energy functional in two dimensions
We construct a generalized-gradient approximation for the exchange-energy
density of finite two-dimensional systems. Guided by non-empirical principles,
we include the proper small-gradient limit and the proper tail for the
exchange-hole potential. The observed performance is superior to that of the
two-dimensional local-density approximation, which underlines the usefulness of
the approach in practical applications
Ab-initio angle and energy resolved photoelectron spectroscopy with time-dependent density-functional theory
We present a time-dependent density-functional method able to describe the
photoelectron spectrum of atoms and molecules when excited by laser pulses.
This computationally feasible scheme is based on a geometrical partitioning
that efficiently gives access to photoelectron spectroscopy in time-dependent
density-functional calculations. By using a geometrical approach, we provide a
simple description of momentum-resolved photoe- mission including multi-photon
effects. The approach is validated by comparison with results in the literature
and exact calculations. Furthermore, we present numerical photoelectron angular
distributions for randomly oriented nitrogen molecules in a short near infrared
intense laser pulse and helium-(I) angular spectra for aligned carbon monoxide
and benzene.Comment: Accepted for publication on Phys. Rev.
Antibonding Ground state of Adatom Molecules in Bulk Dirac Semimetals
The ground state of the diatomic molecules in nature is inevitably bonding,
and its first excited state is antibonding. We demonstrate theoretically that,
for a pair of distant adatoms placed buried in three-dimensional-Dirac
semimetals, this natural order of the states can be reversed and an antibonding
ground state occurs at the lowest energy of the so-called bound states in the
continuum. We propose an experimental protocol with the use of a scanning
tunneling microscope tip to visualize the topographic map of the local density
of states on the surface of the system to reveal the emerging physics
Alloying effects on the optical properties of GeSi nanocrystals from TDDFT and comparison with effective-medium theory
We present the optical spectra of GeSi alloy nanocrystals
calculated with time-dependent density-functional theory in the adiabatic
local-density ap proximation (TDLDA). The spectra change smoothly as a function
of the compositio n . On the Ge side of the composition range, the lowest
excitations at the ab sorption edge are almost pure Kohn-Sham
independent-particle HOMO-LUMO transitio ns, while for higher Si contents
strong mixing of transitions is found. Within T DLDA the first peak is slightly
higher in energy than in earlier independent-par ticle calculations. However,
the absorption onset and in particular its composit ion dependence is similar
to independent-particle results. Moreover, classical depolarization effects are
responsible for a very strong suppression of the abs orption intensity. We show
that they can be taken into account in a simpler way using Maxwell-Garnett
classical effective-medium theory. Emission spectra are in vestigated by
calculating the absorption of excited nanocrystals at their relaxe d geometry.
The structural contribution to the Stokes shift is about 0.5 eV. Th e
decomposition of the emission spectra in terms of independent-particle transit
ions is similar to what is found for absorption. For the emission, very weak
tra nsitions are found in Ge-rich clusters well below the strong absorption
onset.Comment: submitted to Phys. Rev.
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