1,630 research outputs found
Avalanches and Self-Organized Criticality in Superconductors
We review the use of superconductors as a playground for the experimental
study of front roughening and avalanches. Using the magneto-optical technique,
the spatial distribution of the vortex density in the sample is monitored as a
function of time. The roughness and growth exponents corresponding to the
vortex landscape are determined and compared to the exponents that characterize
the avalanches in the framework of Self-Organized Criticality. For those
situations where a thermo-magnetic instability arises, an analytical non-linear
and non-local model is discussed, which is found to be consistent to great
detail with the experimental results. On anisotropic substrates, the anisotropy
regularizes the avalanches
Length Dependence of Ionization Potentials of Trans-Acetylenes: Internally-Consistent DFT/GW Approach
We follow the evolution of the Ionization Potential (IP) for the paradigmatic
quasi-one-dimensional trans-acetylene family of conjugated molecules, from
short to long oligomers and to the infinite polymer trans-poly-acetylene (TPA).
Our results for short oligomers are very close to experimental available data.
We find that the IP varies with oligomer length and converges to the given
value for TPA with a smooth, coupled inverse-length-exponential behavior. Our
prediction is based on an "internally-consistent" scheme to adjust the exchange
mixing parameter of the PBEh hybrid density functional, so as to
obtain a description of the electronic structure consistent with the
quasiparticle approximation for the IP. This is achieved by demanding that the
corresponding quasiparticle correction, in the GW@PBEh approximation, vanishes
for the IP when evaluated at PBEh(). We find that is
also system-dependent and converges with increasing oligomer length, allowing
to capture the dependence of IP and other electronic properties.Comment: 22 pages with 9 figures, submitted to Physical Review
Beyond the Random Phase Approximation for the Electron Correlation Energy: The Importance of Single Excitations
The random phase approximation (RPA) for the electron correlation energy,
combined with the exact-exchange energy, represents the state-of-the-art
exchange-correlation functional within density-functional theory (DFT).
However, the standard RPA practice -- evaluating both the exact-exchange and
the RPA correlation energy using local or semilocal Kohn-Sham (KS) orbitals --
leads to a systematic underbinding of molecules and solids. Here we demonstrate
that this behavior is largely corrected by adding a "single excitation" (SE)
contribution, so far not included in the standard RPA scheme. A similar
improvement can also be achieved by replacing the non-self-consistent
exact-exchange total energy by the corresponding self-consistent Hartree-Fock
total energy, while retaining the RPA correlation energy evaluated using
Kohn-Sham orbitals. Both schemes achieve chemical accuracy for a standard
benchmark set of non-covalent intermolecular interactions.Comment: 5 pages, 4 figures, and an additional supplementary materia
Vertex corrections in localized and extended systems
Within many-body perturbation theory we apply vertex corrections to various
closed-shell atoms and to jellium, using a local approximation for the vertex
consistent with starting the many-body perturbation theory from a DFT-LDA
Green's function. The vertex appears in two places -- in the screened Coulomb
interaction, W, and in the self-energy, \Sigma -- and we obtain a systematic
discrimination of these two effects by turning the vertex in \Sigma on and off.
We also make comparisons to standard GW results within the usual random-phase
approximation (RPA), which omits the vertex from both. When a vertex is
included for closed-shell atoms, both ground-state and excited-state properties
demonstrate only limited improvements over standard GW. For jellium we observe
marked improvement in the quasiparticle band width when the vertex is included
only in W, whereas turning on the vertex in \Sigma leads to an unphysical
quasiparticle dispersion and work function. A simple analysis suggests why
implementation of the vertex only in W is a valid way to improve quasiparticle
energy calculations, while the vertex in \Sigma is unphysical, and points the
way to development of improved vertices for ab initio electronic structure
calculations.Comment: 8 Pages, 6 Figures. Updated with quasiparticle neon results, extended
conclusions and references section. Minor changes: Updated references, minor
improvement
Band gap and band parameters of InN and GaN from quasiparticle energy calculations based on exact-exchange density-functional theory
We have studied the electronic structure of InN and GaN employing G0W0
calculations based on exact-exchange density-functional theory. For InN our
approach predicts a gap of 0.7 eV. Taking the Burnstein-Moss effect into
account, the increase of the apparent quasiparticle gap with increasing
electron concentration is in good agreement with the observed blue shift of the
experimental optical absorption edge. Moreover, the concentration dependence of
the effective mass, which results from the non-parabolicity of the conduction
band, agrees well with recent experimental findings. Based on the quasiparticle
band structure the parameter set for a 4x4 kp Hamiltonian has been derived.Comment: 3 pages including 3 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
GaN/AlN Quantum Dots for Single Qubit Emitters
We study theoretically the electronic properties of -plane GaN/AlN quantum
dots (QDs) with focus on their potential as sources of single polarized photons
for future quantum communication systems. Within the framework of eight-band
k.p theory we calculate the optical interband transitions of the QDs and their
polarization properties. We show that an anisotropy of the QD confinement
potential in the basal plane (e.g. QD elongation or strain anisotropy) leads to
a pronounced linear polarization of the ground state and excited state
transitions. An externally applied uniaxial stress can be used to either induce
a linear polarization of the ground-state transition for emission of single
polarized photons or even to compensate the polarization induced by the
structural elongation.Comment: 6 pages, 9 figures. Accepted at Journal of Physics: Condensed Matte
Exciting prospects for solids: Exact-exchange based functionals meet quasiparticle energy calculations
Focussing on spectroscopic aspects of semiconductors and insulators we will illustrate how quasiparticle energy calculations in the G0W0 approximation can be successfully combined with density-functional theory calculations in the exact-exchange optimised e ective potential approach (OEPx) to achieve a first principles description of the electronic structure that overcomes the limitations of local or gradiant corrected DFT functionals (LDA and GGA)
Determination of two-dimensional current patterns in flat superconductors from magneto-optical measurements: An efficient inversion scheme
How Tribo-Oxidation Alters the Tribological Properties of Copper and Its Oxides
Tribochemical reactions in many applications determine the performance and lifetime of individual parts or entire engineering systems. The underlying processes are however not yet fully understood. Here, the tribological properties of copper and its oxides are investigated under mild tribological loading and for dry sliding. The oxides represent the late stages of a copperâsapphire triboâcontact, once the whole copper surface is covered with an oxide. For this purpose, highâpurity copper, thermallyâoxidized and sintered CuO and CuO samples are tribologically loaded and eventually formed wear particles analyzed. The tribological behavior of the oxides is found to be beneficial for a reduction of the coefficient of friction (COF), mainly due to an increase in hardness. The results reveal tribochemical reactions when copper oxides are present, irrespective of whether they form during sliding or are existent from the beginning. Most strikingly, a reduction of copper oxide to metallic copper is observed in Xâray photoelectron spectroscopy measurements. A more accurate understanding of triboâoxidation will allow for manufacturing wellâdefined surfaces with enhanced tribological properties. This paves the way for extending the lifetime of contacts evincing triboâoxidation
How Tribo-Oxidation Alters the Tribological Properties of Copper and Its Oxides
Tribochemical reactions in many applications determine the performance and lifetime of individual parts or entire engineering systems. The underlying processes are however not yet fully understood. Here, the tribological properties of copper and its oxides are investigated under mild tribological loading and for dry sliding. The oxides represent the late stages of a copperâsapphire triboâcontact, once the whole copper surface is covered with an oxide. For this purpose, highâpurity copper, thermallyâoxidized and sintered CuO and CuO samples are tribologically loaded and eventually formed wear particles analyzed. The tribological behavior of the oxides is found to be beneficial for a reduction of the coefficient of friction (COF), mainly due to an increase in hardness. The results reveal tribochemical reactions when copper oxides are present, irrespective of whether they form during sliding or are existent from the beginning. Most strikingly, a reduction of copper oxide to metallic copper is observed in Xâray photoelectron spectroscopy measurements. A more accurate understanding of triboâoxidation will allow for manufacturing wellâdefined surfaces with enhanced tribological properties. This paves the way for extending the lifetime of contacts evincing triboâoxidation
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