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

Controllable quantum scars in semiconductor quantum dots

Quantum scars are enhancements of quantum probability density along classical periodic orbits. We study the recently discovered phenomenon of strong, perturbation-induced quantum scarring in the two-dimensional harmonic oscillator exposed to a homogeneous magnetic field. We demonstrate that both the geometry and the orientation of the scars are fully controllable with a magnetic field and a focused perturbative potential, respectively. These properties may open a path into an experimental scheme to manipulate electric currents in nanostructures fabricated in a two-dimensional electron gas.Comment: 5 pages, 4 figure

Effect of inhomogeneities on the expansion rate of the Universe

While the expansion rate of a homogeneous isotropic Universe is simply proportional to the square-root of the energy density, the expansion rate of an inhomogeneous Universe also depends on the nature of the density inhomogeneities. In this paper we calculate to second order in perturbation variables the expansion rate of an inhomogeneous Universe and demonstrate corrections to the evolution of the expansion rate. While we find that the mean correction is small, the variance of the correction on the scale of the Hubble radius is sensitive to the physical significance of the unknown spectrum of density perturbations beyond the Hubble radius.Comment: 19 pages, 2 figures Version 2 includes some changes in numerical factors and corrected typos. It is the version accepted for publication in Physical review

On the violation of a local form of the Lieb-Oxford bound

In the framework of density-functional theory, several popular density functionals for exchange and correlation have been constructed to satisfy a local form of the Lieb-Oxford bound. In its original global expression, the bound represents a rigorous lower limit for the indirect Coulomb interaction energy. Here we employ exact-exchange calculations for the G2 test set to show that the local form of the bound is violated in an extensive range of both the dimensionless gradient and the average electron density. Hence, the results demonstrate the severity in the usage of the local form of the bound in functional development. On the other hand, our results suggest alternative ways to construct accurate density functionals for the exchange energy.Comment: (Submitted on 27 April 2012

Semiclassical two-step model for strong-field ionization

We present a semiclassical two-step model for strong-field ionization that accounts for path interferences of tunnel-ionized electrons in the ionic potential beyond perturbation theory. Within the framework of a classical trajectory Monte-Carlo representation of the phase-space dynamics, the model employs the semiclassical approximation to the phase of the full quantum propagator in the exit channel. By comparison with the exact numerical solution of the time-dependent Schr\"odinger equation for strong-field ionization of hydrogen, we show that for suitable choices of the momentum distribution after the first tunneling step, the model yields good quantitative agreement with the full quantum simulation. The two-dimensional photoelectron momentum distributions, the energy spectra, and the angular distributions are found to be in good agreement with the corresponding quantum results. Specifically, the model quantitatively reproduces the fan-like interference patterns in the low-energy part of the two-dimensional momentum distributions as well as the modulations in the photoelectron angular distributions.Comment: 31 pages, 7 figure

Average observational quantities in the timescape cosmology

We examine the properties of a recently proposed observationally viable alternative to homogeneous cosmology with smooth dark energy, the timescape cosmology. In the timescape model cosmic acceleration is realized as an apparent effect related to the calibration of clocks and rods of observers in bound systems relative to volume-average observers in an inhomogeneous geometry in ordinary general relativity. The model is based on an exact solution to a Buchert average of the Einstein equations with backreaction. The present paper examines a number of observational tests which will enable the timescape model to be distinguished from homogeneous cosmologies with a cosmological constant or other smooth dark energy, in current and future generations of dark energy experiments. Predictions are presented for: comoving distance measures; H(z); the equivalent of the dark energy equation of state, w(z); the Om(z) measure of Sahni, Shafieloo and Starobinsky; the Alcock-Paczynski test; the baryon acoustic oscillation measure, D_v; the inhomogeneity test of Clarkson, Bassett and Lu; and the time drift of cosmological redshifts. Where possible, the predictions are compared to recent independent studies of similar measures in homogeneous cosmologies with dark energy. Three separate tests with indications of results in possible tension with the Lambda CDM model are found to be consistent with the expectations of the timescape cosmology.Comment: 22 pages, 12 figures; v2 discussion, references added, matches published versio

Vibrational spectroscopy of trans and cis deuterated formic acid (HCOOD): Anharmonic calculations and experiments in argon and neon matrices

Non Peer reviewe

Landscape evolution and depositional processes in the Miocene Amazonian Pebas lake/wetland system: Evidence from exploratory boreholes in northeastern Peru.

This study of the type and scales of depositional processes and landscape development in western Amazonia during the Miocene is based on the description and interpretation of three boreholes from the Marañon basin (Peru). The Miocene Pebas Formation, and the overlying Marañon Formation and underlying Chambira Formation are lithologically characterised. An age calculation model indicates an Oligocene age for the Chambira Formation, and an Early - early Late Miocene age for the Pebas Formation. The base of the Chambira Formation is placed at a sequence boundary and corresponds to the beginning of a regression. The succession was deposited in floodplains included in a RST and a LST under a seasonal climate with a pronounced dry season. The base of the Pebas Formation is placed at a TS. It represents TST and HST lacustrine and swamp settings at or near sealevel, formed in a tropical monsoon climate alike the present-day climate in the region. At the time, the area was a mosaic of lakes, swamps and fluvial belts, but experienced tidal influence as well. During apparently regularly recurring base level highstands, open aquatic settings (lakes at sea level) were widespread. The depositional system was driven by tectonic subsidence in the area, uplift and erosion in the Andean hinterland and the western rim of the Pebas system (the developing Subandean zone), delta lobe switching and river belt avulsions, as well as presumable Milankovitch scale precipitation/erosion cycles and eustatic sea level variation. The base of the Marañon Formation is placed at a sequence boundary. It represents the end of the Pebas lake/wetland system, and the change to permanent fluvial conditions during the Late Miocene RST and LST

Own or shared silage feeding place for dairy cows?

We studied behaviour, silage eating and milk production of cows when every animal had an own silage feeding place and when the feeding place was shared with two other cows