5,009 research outputs found
Fixing the shadows while moving the gnomon
It is a common practice to fix a vertical gnomon and study the moving shadow
cast by it. This shows our local solar time and gives us a hint regarding the
season in which we perform the observation. The moving shadow can also tell us
our latitude with high precision. In this paper we propose to exchange the
roles and while keeping the shadows fixed on the ground we will move the
gnomon. This lets us understand in a simple way the relevance of the tropical
lines of latitude and the behavior of shadows in different locations. We then
put these ideas into practice using sticks and threads during a solstice on two
sites located on opposite sides of the Tropic of Capricorn.Comment: Published version available at
http://cms.iafe.uba.ar/gangui/didaastro/#Publication
Combining Density Functional Theory and Density Matrix Functional Theory
We combine density-functional theory with density-matrix functional theory to
get the best of both worlds. This is achieved by range separation of the
electronic interaction which permits to rigorously combine a short-range
density functional with a long-range density-matrix functional. The short-range
density functional is approximated by the short-range version of the
Perdew-Burke-Ernzerhof functional (srPBE). The long-range density-matrix
functional is approximated by the long-range version of the Buijse-Baerends
functional (lrBB). The obtained srPBE+lrBB method accurately describes both
static and dynamic electron correlation at a computational cost similar to that
of standard density-functional approximations. This is shown for the
dissociation curves of the H, LiH, BH and HF molecules.Comment: 4 pages, 5 figure
Full Configuration Interaction wave function as a formal solution to the Optimized Effective Potential and Kohn-Sham models in finite basis sets
Using finite basis sets, it is shown how to construct a local Hamiltonian,
such that one of its infinitely many degenerate eigenfunctions is the ground
state full configuration interaction (FCI) wave function in that basis set.
Formally, the local potential of this Hamiltonian is the optimized effective
potential and the exact Kohn-Sham potential at the same time, because the FCI
wave function yields the exact ground-state density and energy. It is not the
aim of this paper to provide a new algorithm for obtaining FCI wave functions.
A new potential is the goal
The emergence of superconductivity in BaNi2(Ge1-xPx)2 at a structural instability
The physical properties and structural evolution of the 122-type solid
solution BaNi2(Ge1-xPx)2 are reported. The in-plane X-X (X = Ge1-xPx) dimer
formation present in the end member BaNi2Ge2, which results in a structural
transition to orthorhombic symmetry, is completely suppressed to zero
temperature on P substitution near x = 0.7, and a dome-shape superconducting
phase with a maximum Tc = 2.9 K emerges. Clear indications of phonon softening
and enhanced electron-phonon coupling are observed at the composition of the
structural instability. Our findings show that dimer breaking offers new
possibilities as a tuning parameter of superconductivity.Comment: 14 pages, 4 figure
Time Optimal Control of Coupled Qubits Under Non-Stationary Interactions
In this article, we give a complete characterization of all the unitary
transformations that can be synthesized in a given time for a system of coupled
spin-1/2 in presence of general time varying coupling tensor. Our treatment is
quite general and our results help to characterize the reachable set at all
times for a class of bilinear control systems with time varying drift and
unbounded control amplitude. These results are of fundamental interest in
geometric control theory and have applications to control of coupled spins in
solid state NMR spectroscopy.Comment: 4 page
Kerr effect as a tool for the investigation of dynamic heterogeneities
We propose a dynamic Kerr effect experiment for the distinction between
dynamic heterogeneous and homogeneous relaxation in glassy systems. The
possibility of this distinction is due to the inherent nonlinearity of the Kerr
effect signal. We model the slow reorientational molecular motion in
supercooled liquids in terms of non-inertial rotational diffusion. The Kerr
effect response, consisting of two terms, is calculated for heterogeneous and
for homogeneous variants of the stochastic model. It turns out that the
experiment is able to distinguish between the two scenarios. We furthermore
show that exchange between relatively 'slow' and 'fast' environments does not
affect the possibility of frequency-selective modifications. It is demonstrated
how information about changes in the width of the relaxation time distribution
can be obtained from experimental results.Comment: 23 pages incl. 6 figures accepted for publication in The Journal of
Chemical Physic
A Computational Fluid Dynamics (CFD) Analysis of the Aerodynamic Effects of the Seams on a Two-Dimensional Representation of a Soccer Ball
Most major sports today use a dedicated ball or projectile with specific shape, size, and surface geometry, except for soccer. Over the history of the sport, the surface geometry and design stayed relatively unchanged, sewn together using 32 pentagonal and hexagonal panels. However, recent innovations in panel designs differ substantially from the traditional 32 panel ball. The effects these new designs have on the aerodynamic characteristics of the ball have remained largely unknown, even with the influx of experimental research completed in the past decade. Experimental studies have been broad in scope, analyzing an entire ball in wind tunnels or full flow paths in trajectory analyses. Computational efforts have been too assumptive in flow conditions, such as a fully turbulent flow field, which has not yielded accurate representations of the flow phenomenon. This study investigates the aerodynamic effects of the seam on a two-dimensional representation of a non-rotating soccer ball using Computational Fluid Dynamics (CFD). By applying a transitional solver to the narrowed scope of a two-dimensional flow domain, with a single seam in cross-flow, the effects of the seam on the boundary layer and overall transient flow structure can be more accurately modeled. Data analysis suggests the seam produces a local effect on skin friction, however, that effect does not materialize into a premature boundary layer transition or delayed separation point, as predicted by literature. A detailed flow visualization is consistent with this result, displaying expected symmetric vortex shedding similar to a smooth cylinder, but not fully capturing the effects of the seam, reinforcing the need for expanding computational research efforts in this field
Correlation potentials for molecular bond dissociation within the self-consistent random phase approximation
Self-consistent correlation potentials for H and LiH for various
inter-atomic separations are obtained within the random phase approximation
(RPA) of density functional theory. The RPA correlation potential shows a peak
at the bond midpoint, which is an exact feature of the true correlation
potential, but lacks another exact feature: the step important to preserve
integer charge on the atomic fragments in the dissociation limit. An analysis
of the RPA energy functional in terms of fractional charge is given which
confirms these observations. We find that the RPA misses the derivative
discontinuity at odd integer particle numbers but explicitly eliminates the
fractional spin error in the exact-exchange functional. The latter finding
explains the accurate total energy in the dissociation limit.Comment: 9 pages, 10 figure
Bond Breaking and Bond Formation: How Electron Correlation is Captured in Many-Body Perturbation Theory and Density-Functional Theory
For the paradigmatic case of H2-dissociation we compare state-of-the-art
many-body perturbation theory (MBPT) in the GW approximation and
density-functional theory (DFT) in the exact-exchange plus random-phase
approximation for the correlation energy (EX+cRPA). For an unbiased comparison
and to prevent spurious starting point effects both approaches are iterated to
full self-consistency (i.e. sc-RPA and sc-GW). The exchange-correlation
diagrams in both approaches are topologically identical, but in sc-RPA they are
evaluated with non-interacting and in sc-GW with interacting Green functions.
This has a profound consequence for the dissociation region, where sc-RPA is
superior to sc-GW. We argue that for a given diagrammatic expansion, the DFT
framework outperforms the many-body framework when it comes to bond-breaking.
We attribute this to the difference in the correlation energy rather than the
treatment of the kinetic energy.Comment: 6 pages, 4 figure
Orthodontists’ and Orthodontic Residents’ Education in Treating Underserved Patients: Effects on Professional Attitudes and Behavior
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/153711/1/jddj002203372009735tb04730x.pd
- …