502 research outputs found
Self-consistency over the charge-density in dynamical mean-field theory: a linear muffin-tin implementation and some physical implications
We present a simple implementation of the dynamical mean-field theory
approach to the electronic structure of strongly correlated materials. This
implementation achieves full self-consistency over the charge density, taking
into account correlation-induced changes to the total charge density and
effective Kohn-Sham Hamiltonian. A linear muffin-tin orbital basis-set is used,
and the charge density is computed from moments of the many body
momentum-distribution matrix. The calculation of the total energy is also
considered, with a proper treatment of high-frequency tails of the Green's
function and self-energy. The method is illustrated on two materials with
well-localized 4f electrons, insulating cerium sesquioxide Ce2O3 and the
gamma-phase of metallic cerium, using the Hubbard-I approximation to the
dynamical mean-field self-energy. The momentum-integrated spectral function and
momentum-resolved dispersion of the Hubbard bands are calculated, as well as
the volume-dependence of the total energy. We show that full self-consistency
over the charge density, taking into account its modification by strong
correlations, can be important for the computation of both thermodynamical and
spectral properties, particularly in the case of the oxide material.Comment: 20 pages, 6 figures (submitted in The Physical Review B
Electronic structure and the glass transition in pnictide and chalcogenide semiconductor alloys. Part I: The formation of the -network
Semiconductor glasses exhibit many unique optical and electronic anomalies.
We have put forth a semi-phenomenological scenario (J. Chem. Phys. 132, 044508
(2010)) in which several of these anomalies arise from deep midgap electronic
states residing on high-strain regions intrinsic to the activated transport
above the glass transition. Here we demonstrate at the molecular level how this
scenario is realized in an important class of semiconductor glasses, namely
chalcogen and pnictogen containing alloys. Both the glass itself and the
intrinsic electronic midgap states emerge as a result of the formation of a
network composed of -bonded atomic -orbitals that are only weakly
hybridized. Despite a large number of weak bonds, these -networks are
stable with respect to competing types of bonding, while exhibiting a high
degree of structural degeneracy. The stability is rationalized with the help of
a hereby proposed structural model, by which -networks are
symmetry-broken and distorted versions of a high symmetry structure. The latter
structure exhibits exact octahedral coordination and is fully
covalently-bonded. The present approach provides a microscopic route to a fully
consistent description of the electronic and structural excitations in vitreous
semiconductors.Comment: 22 pages, 17 figures, revised version, final version to appear in J.
Chem. Phy
Structure stability in the simple element sodium under pressure
The simple alkali metal Na, that crystallizes in a body-centred cubic
structure at ambient pressure, exhibits a wealth of complex phases at extreme
conditions as found by experimental studies. The analysis of the mechanism of
stabilization of some of these phases, namely, the low-temperature Sm-type
phase and the high-pressure cI16 and oP8 phases, shows that they satisfy the
criteria for the Hume-Rothery mechanism. These phases appear to be stabilized
due to a formation of numerous planes in a Brillouin-Jones zone in the vicinity
of the Fermi sphere of Na, which leads to the reduction of the overall
electronic energy. For the oP8 phase, this mechanism seems to be working if one
assumes that Na becomes divalent metal at this density. The oP8 phase of Na is
analysed in comparison with the MnP-type oP8 phases known in binary compounds,
as well as in relation to the hP4 structure of the NiAs-type
Electronic Structure of Transition Metals Fe, Ni and Cu in the GW Approximation
The quasiparticle band structures of 3d transition metals, ferromagnetic Fe,
Ni and paramagnetic Cu, are calculated by the GW approximation. The width of
occupied 3d valence band, which is overestimated in the LSDA, is in good
agreement with experimental observation. However the exchange splitting and
satellite in spectra are not reproduced and it is required to go beyond the GW
approximation. The effects of static screening and dynamical correlation are
discussed in detail in comparison with the results of the static COHSEX
approximation. The dynamical screening effects are important for band width
narrowing.Comment: 4 pages, 3 figure
Theoretical investigation of carbon defects and diffusion in α-quartz
The geometries, formation energies, and diffusion barriers of carbon point defects in silica (α-quartz) have been calculated using a charge-self-consistent density-functional based nonorthogonal tight-binding method. It is found that bonded interstitial carbon configurations have significantly lower formation energies (on the order of 5 eV) than substitutionals. The activation energy of atomic C diffusion via trapping and detrapping in interstitial positions is about 2.7 eV. Extraction of a CO molecule requires an activation energy <3.1 eV but the CO molecule can diffuse with an activation energy <0.4 eV. Retrapping in oxygen vacancies is hindered—unlike for O2—by a barrier of about 2 eV
A Precise Proper Motion for the Crab Pulsar, and the Difficulty of Testing Spin-Kick Alignment for Young Neutron Stars
We present a detailed measurement of the proper motion of the Crab pulsar,
with the primary goal of comparing the direction of its proper motion with the
projected axis of its pulsar wind nebula (the projected spin axis of the
pulsar). We demonstrate that our measurement is robust and has an uncertainty
of only +/-0.4 mas/yr on each component of the proper motion. We find mu_alpha
= -11.7+/-0.4+/-0.5 mas/yr and mu_delta = +4.2+/-0.4+/-0.5 mas/yr relative to
the pulsar's standard of rest, where the two uncertainties are from the
measurement and the ncertainties in correcting the proper motion reference
frame, respectively. Comparing this proper motion to the symmetry axis of the
pulsar wind nebula, we must also consider the unknown velocity of the pulsar's
progenitor (assumed to be ~10 km/s), and hence add an additional uncertainty of
+/-2 mas/yr to each component of the proper motion, although this could be
significantly larger. This implies a projected misalignment with the nebular
axis of 14+/-2+/-9 degrees. We conclude that the precision of individual
measurements which compare the direction of motion of a neutron star to a fixed
axis will often be limited by fundamental uncertainties regarding reference
frames and progenitor properties. The question of spin-kick (mis)alignment, and
its implications for asymmetries and other processes during supernova
core-collapse, is best approached by considering a statistical ensemble of such
measurements, rather than detailed studies of individual sources. [abriged]Comment: 15 pages, 6 figures. Accepted for publication in ApJ. Figure 3 fixed,
included at low resolutio
Atomistic origins of high-performance in hybrid halide perovskite solar cells
The performance of organometallic perovskite solar cells has rapidly
surpassed that of both conventional dye-sensitised and organic photovoltaics.
High power conversion efficiency can be realised in both mesoporous and
thin-film device architectures. We address the origin of this success in the
context of the materials chemistry and physics of the bulk perovskite as
described by electronic structure calculations. In addition to the basic
optoelectronic properties essential for an efficient photovoltaic device
(spectrally suitable band gap, high optical absorption, low carrier effective
masses), the materials are structurally and compositionally flexible. As we
show, hybrid perovskites exhibit spontaneous electric polarisation; we also
suggest ways in which this can be tuned through judicious choice of the organic
cation. The presence of ferroelectric domains will result in internal junctions
that may aid separation of photoexcited electron and hole pairs, and reduction
of recombination through segregation of charge carriers. The combination of
high dielectric constant and low effective mass promotes both Wannier-Mott
exciton separation and effective ionisation of donor and acceptor defects. The
photoferroic effect could be exploited in nanostructured films to generate a
higher open circuit voltage and may contribute to the current-voltage
hysteresis observed in perovskite solar cells.Comment: 6 pages, 5 figure
The composition of the protosolar disk and the formation conditions for comets
Conditions in the protosolar nebula have left their mark in the composition
of cometary volatiles, thought to be some of the most pristine material in the
solar system. Cometary compositions represent the end point of processing that
began in the parent molecular cloud core and continued through the collapse of
that core to form the protosun and the solar nebula, and finally during the
evolution of the solar nebula itself as the cometary bodies were accreting.
Disentangling the effects of the various epochs on the final composition of a
comet is complicated. But comets are not the only source of information about
the solar nebula. Protostellar disks around young stars similar to the protosun
provide a way of investigating the evolution of disks similar to the solar
nebula while they are in the process of evolving to form their own solar
systems. In this way we can learn about the physical and chemical conditions
under which comets formed, and about the types of dynamical processing that
shaped the solar system we see today.
This paper summarizes some recent contributions to our understanding of both
cometary volatiles and the composition, structure and evolution of protostellar
disks.Comment: To appear in Space Science Reviews. The final publication is
available at Springer via http://dx.doi.org/10.1007/s11214-015-0167-
Human Impacts on Forest Biodiversity in Protected Walnut-Fruit Forests in Kyrgyzstan
We used a spatially explicit model of forest dynamics, supported by empirical field data and socioeconomic data, to examine the impacts of human disturbances on a protected forest landscape in Kyrgyzstan. Local use of 27 fruit and nut species was recorded and modeled. Results indicated that in the presence of fuelwood cutting with or without grazing, species of high socioeconomic impor- tance such as Juglans regia, Malus spp., and Armeniaca vulgaris were largely eliminated from the landscape after 50–150 yr. In the absence of disturbance or in the presence of grazing only, decline of these species occurred at a much lower rate, owing to competi- tive interactions between tree species. This suggests that the current intensity of fuelwood harvesting is not sustainable. Conversely, cur- rent grazing intensities were found to have relatively little impact on forest structure and composition, and could potentially play a positive role in supporting regeneration of tree species. These results indicate that both positive and negative impacts on biodiversity can arise from human populations living within a protected area. Potentially, these could be reconciled through the development of participatory approaches to conservation management within this reserve, to ensure the maintenance of its high conservation value while meeting human needs
Melanoma cells break down LPA to establish local gradients that drive chemotactic dispersal.
The high mortality of melanoma is caused by rapid spread of cancer cells, which occurs unusually early in tumour evolution. Unlike most solid tumours, thickness rather than cytological markers or differentiation is the best guide to metastatic potential. Multiple stimuli that drive melanoma cell migration have been described, but it is not clear which are responsible for invasion, nor if chemotactic gradients exist in real tumours. In a chamber-based assay for melanoma dispersal, we find that cells migrate efficiently away from one another, even in initially homogeneous medium. This dispersal is driven by positive chemotaxis rather than chemorepulsion or contact inhibition. The principal chemoattractant, unexpectedly active across all tumour stages, is the lipid agonist lysophosphatidic acid (LPA) acting through the LPA receptor LPAR1. LPA induces chemotaxis of remarkable accuracy, and is both necessary and sufficient for chemotaxis and invasion in 2-D and 3-D assays. Growth factors, often described as tumour attractants, cause negligible chemotaxis themselves, but potentiate chemotaxis to LPA. Cells rapidly break down LPA present at substantial levels in culture medium and normal skin to generate outward-facing gradients. We measure LPA gradients across the margins of melanomas in vivo, confirming the physiological importance of our results. We conclude that LPA chemotaxis provides a strong drive for melanoma cells to invade outwards. Cells create their own gradients by acting as a sink, breaking down locally present LPA, and thus forming a gradient that is low in the tumour and high in the surrounding areas. The key step is not acquisition of sensitivity to the chemoattractant, but rather the tumour growing to break down enough LPA to form a gradient. Thus the stimulus that drives cell dispersal is not the presence of LPA itself, but the self-generated, outward-directed gradient
- …