3,824 research outputs found
On the validity of power functionals for the homogeneous electron gas in reduced.density-matrix-functional theory
Physically valid and numerically efficient approximations for the exchange
and correlation energy are critical for reduced density-matrix functional
theory to become a widely used method in electronic structure calculations.
Here we examine the physical limits of power functionals of the form
for the scaling function in the exchange-correlation
energy. To this end we obtain numerically the minimizing momentum distributions
for the three- and two-dimensional homogeneous electron gas, respectively. In
particular, we examine the limiting values for the power to yield
physically sound solutions that satisfy the Lieb-Oxford lower bound for the
exchange-correlation energy and exclude pinned states with the condition
for all wave vectors . The results refine the
constraints previously obtained from trial momentum distributions. We also
compute the values for that yield the exact correlation energy and its
kinetic part for both the three- and two-dimensional electron gas. In both
systems, narrow regimes of validity and accuracy are found at and at for the density parameter, corresponding to
relatively low densities.Comment: Phys. Rev. A (in print, 2016
Non-collinear spin-spiral phase for the uniform electron gas within Reduced-Density-Matrix-Functional Theory
The non-collinear spin-spiral density wave of the uniform electron gas is
studied in the framework of Reduced-Density-Matrix-Functional Theory. For the
Hartree-Fock approximation, which can be obtained as a limiting case of
Reduced-Density-Matrix-Functional Theory, Overhauser showed a long time ago
that the paramagnetic state of the electron gas is unstable with respect to the
formation of charge or spin density waves. Here we not only present a detailed
numerical investigation of the spin-spiral density wave in the Hartree-Fock
approximation but also investigate the effects of correlations on the
spin-spiral density wave instability by means of a recently proposed
density-matrix functional.Comment: 9 pages, 10 figure
On the role of invariants for the parameter estimation problem in Hamiltonian systems
Baake M, Baake E, Eich E. On the role of invariants for the parameter estimation problem in Hamiltonian systems. Physics letters. 1993;180(1-2):74-82.The parameter estimation problem is discussed for differential equations that describe a Hamiltonian system. Since the conserved total energy is an invariant which contains all parameters of the system, we can achieve parameter estimation without any numerical integration. This is demonstrated for data in the chaotic region of the Hénon-Heiles system and of the planar double pendulum. We show that the method works well for ideal as well as noisy data. In this context, an appropriate method for the generation of reliable time series in the presence of an invariant is discussed. Finally, it is shown that our method also provides a simple approach to global fitting in discrete dynamical systems with invariants
Magnon frequency renormalization by the electronic geometrical spin torque in itinerant magnets
We investigate non-adiabatic effects on the magnon frequency in an interacting system of localized spins and itinerant electrons. Including the lowest order corrections to the adiabatic dynamics in an analytically solvable model, applicable to simple ferromagnets like Fe, Co and Ni, we find that the magnon frequency is renormalized by a geometrical torque arising from the electronic spin Berry curvature. Comparison to exact numerical simulations reveals that our analytical solution captures essential low-energy features, and provides a mechanism for the magnon frequency hardening observed in recent first principles calculations for Fe, provided the geometrical torque is taken into account
Exchange-correlation approximations for reduced-density-matrix-functional theory at finite temperature: Capturing magnetic phase transitions in the homogeneous electron gas
We derive an intrinsically temperature-dependent approximation to the correlation grand potential for many-electron systems in thermodynamical equilibrium in the context of finite-temperature reduced-density-matrix-functional theory (FT-RDMFT). We demonstrate its accuracy by calculating the magnetic phase diagram of the homogeneous electron gas. We compare it to known limits from highly accurate quantum Monte Carlo calculations as well as to phase diagrams obtained within existing exchange-correlation approximations from density functional theory and zero-temperature RDMFT
10 dB emission suppression in a structured low index medium
Significant suppression of radiation in 3D structured media with small
refractive index 1.4-1.6, such as of glass or polymers, is a desirable feature
yet to be obtained. For periodical structures this is realised at frequencies
of the complete photonic band gap (CPBG), which up to now was demonstrated to
open for materials with refractive index of at least 1.9. We present here a
quasiperiodic 3D structure consisting of multiple overlapping gratings with a
homogeneous distribution of Bragg peaks on a sphere in reciprocal space, which
allows efficient suppression of emission. Recently we have presented the
theoretical model, considering interaction with the neighbouring gratings only,
that estimates a finite CPBG for arbitrarily small refractive indices and thus
complete emission suppression in infinite structures. However, numerical
simulations demonstrate a finite leakage of power from emitter not predicted by
the model. Still the simulations show -10 dB suppression in 3D structures with
optimised number of gratings. Astonishingly, as we show here, this limit is
almost independent of the refractive index contrast. Also, the structures with
a defined number of gratings show maximal suppression at certain refractive
indices, losing the suppression even at higher refractive indices. The -10 dB
suppression is demonstrated for refractive index contrast as low as 1.30
Crystalline and electronic structure of single-layer TaS
Single-layer TaS is epitaxially grown on Au(111) substrates. The
resulting two-dimensional crystals adopt the 1H polymorph. The electronic
structure is determined by angle-resolved photoemission spectroscopy and found
to be in excellent agreement with density functional theory calculations. The
single layer TaS is found to be strongly n-doped, with a carrier
concentration of 0.3(1) extra electrons per unit cell. No superconducting or
charge density wave state is observed by scanning tunneling microscopy at
temperatures down to 4.7 K.Comment: 6 pages, 4 figure
Simultaneous sampling of phosphate, arsenate, and selenate in water by Diffusive Gradients in Thin Films (DGT)
Ã…rsliste 2004The DGT sampler was studied for simultaneous collection of phosphate, arsenate, and selenate in water. The DGT-based diffusion coefficients found were somewhat lower than the theoretical ones for free diffusion in water, but similar to what has been found earlier for phosphate and arsenate in diffusion cell experiments with the DGT membrane. Our results indicate that D sampler with the ferrihydrite adsorbent functions adequately as an adsorbent for phosphate, arsenate, and selenate in water, and may thus be used for simultaneous sampling of these three ions in water. The good performance for arsenate fills a gap in the DGT toolbox for passive sampling of high-priority trace elements, while it for selenate may prove very useful for assessing the potential for Se deficiency or toxicity in soil water.Agricultural University of Norway, Ã…
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