5,675 research outputs found
Room temperature dynamic correlation between methylammonium molecules in lead-iodine based perovskites: An ab-initio molecular dynamics perspective
The high efficiency of lead organo-metal-halide perovskite solar cells has
raised many questions about the role of the methylammonium (MA) molecules in
the Pb-I framework. Experiments indicate that the MA molecules are able to
'freely' spin around at room temperature even though they carry an intrinsic
dipole moment. We have performed large supercell (2592 atoms) finite
temperature ab-initio molecular dynamics calculations to study the correlation
between the molecules in the framework. An underlying long range
anti-ferroelectric ordering of the molecular dipoles is observed. The dynamical
correlation between neighboring molecules shows a maximum around room
temperature in the mid-temperature phase. In this phase, the rotations are slow
enough to (partially) couple to neighbors via the Pb-I cage. This results in a
collective motion of neighboring molecules in which the cage acts as the
mediator. At lower and higher temperatures the motions are less correlated
Relativistic +BSE study of the optical properties of Ruddlesden-Popper iridates
We study the optical properties of the Ruddlesden-Popper series of iridates
SrIrO (=1, 2 and ) by solving the
Bethe-Salpeter equation (BSE), where the quasiparticle (QP) energies and
screened interactions are obtained by the approximation including
spin-orbit coupling. The computed optical conductivity spectra show strong
excitonic effects and reproduce very well the experimentally observed
double-peak structure, in particular for the spin-orbital Mott insulators
SrIrO and SrIrO. However, does not account well for
the correlated metallic state of SrIrO owing to a much too small band
renormalization, and this affects the overall quality of the optical
conductivity. Our analysis describes well the progressive redshift of the main
optical peaks as a function of dimensionality (), which is correlated with
the gradual decrease of the electronic correlation (quantified by the
constrained random phase approximation) towards the metallic limit.
We have also assessed the quality of a computationally cheaper BSE approach
that is based on a model dielectric function and conducted on top of DFT+
one-electron energies. Unfortunately, this model BSE approach does not
accurately reproduce the outcome of the full +BSE method and leads to
larger deviations to the measured spectra.Comment: 13 pages, 8 figure
The random phase approximation applied to ice
Standard density functionals without van der Waals interactions yield an
unsatisfactory description of ice phases, specifically, high density phases
occurring under pressure are too unstable compared to the common low density
phase I observed at ambient conditions. Although the description is
improved by using functionals that include van der Waals interactions, the
errors in relative volumes remain sizable. Here we assess the random phase
approximation (RPA) for the correlation energy and compare our results to
experimental data as well as diffusion Monte Carlo data for ice. The RPA yields
a very balanced description for all considered phases, approaching the accuracy
of diffusion Monte Carlo in relative energies and volumes. This opens a route
towards a concise description of molecular water phases on surfaces and in
cavities
Anisotropic Magnetic Couplings and Structure-Driven Canted to Collinear Transitions in Spin-orbit Coupled Sr2IrO4
We put forward a scheme to study the anisotropic magnetic couplings in
Sr2IrO4 by mapping fully relativistic constrained noncollinear density
functional theory including an on-site Hubbard U correction onto a general spin
model Hamiltonian. This procedure allows for the simultaneous account and
direct control of the lattice, spin and orbital interactions within a fully ab
initio scheme. We compute the isotropic, single site anisotropy and
Dzyaloshinskii-Moriya (DM) coupling parameters, and clarify that the origin of
the canted magnetic state in Sr2IrO4 arises from the interplay between
structural distortions and the competition between isotropic exchange and DM
interactions. A complete magnetic phase diagram with respect to the tetragonal
distortion and the rotation of IrO6 octahedra is constructed, revealing the
presence of two types of canted to collinear magnetic transitions: a spin-flop
transition with increasing tetragonal distortion and a complete quenching of
the basal weak ferromagnetic moment below a critical octahedral rotation
Modified Hemagglutination Tests for COVID-19 Serology in Resource-Poor Settings: Ready for Prime-Time?
During the ongoing COVID-19 pandemic, serology has suffered several manufacturing and budget bottlenecks. Kode technology exposes exogenous antigens on the surface of cells; in the case of red blood cells, modified cells are called kodecytes, making antibody\u2013antigen reactions detectable by the old-fashioned hemagglutination test. In this commentary, we review evidence supporting the utility of SARS-CoV-2 Spike kodecytes for clinical diagnostic purposes and serosurveys in resource-poor settings
Exchange interactions and magnetic phases of transition metal oxides: benchmarking advanced ab initio methods
The magnetic properties of the transition metal monoxides MnO and NiO are
investigated at equilibrium and under pressure via several advanced
first-principles methods coupled with Heisenberg Hamiltonian MonteCarlo. The
comparative first-principles analysis involves two promising beyond-local
density functionals approaches, namely the hybrid density functional theory and
the recently developed variational pseudo-self-interaction correction method,
implemented with both plane-wave and atomic-orbital basis sets. The advanced
functionals deliver a very satisfying rendition, curing the main drawbacks of
the local functionals and improving over many other previous theoretical
predictions. Furthermore, and most importantly, they convincingly demonstrate a
degree of internal consistency, despite differences emerging due to
methodological details (e.g. plane waves vs. atomic orbitals
Sulfoglycolipids analogues as new molecules for tumor treatment
The sulfoglycolipids sulfoquinovosylacylglycerols(SQAG) are abundant sulfur-containing glycerolipids that are associated with photosynthetic organisms especially with a large number of marine algae. Their main structural feature is the anionic head group constituent sulfoquinovose, a derivative of glucose in which the 6-hydroxyl is replaced by a sulfonate group, \uf061-linked to the sn-3 position of a diacylglycerol1. Recently reported biological activities of SQAGs, including inhibitory effects on HIV-reverse transcriptase, and mammalian DNA polymerase, proliferation of some cancer cell lines, angiogenesis (especially when coupled with tumor radiotherapy), and apoptosis induction, make these compounds very attractive for their potential in cancer therapy. Also, extractive SQAG mixtures are known to inhibit in vitro TPA induced tumor promotion stage.
To obtain new active compounds for cancer therapy by structural modification of natural SQAGs, SQAG analogues have been synthesized in which the sulfoquinovose moiety is linked to the 2 position of glycerol carrying acyl chains of different length. Similar compounds in fact, with a 6\u2019-hydroxyl instead of a 6\u2019-sulfonate (namely some glycoglycerolipid analogues), are known to be active as anti-tumor-promoters in TPA promoted carcinogenesis in vitro and in vivo experiments.
A synthetic strategy has been used to selectively insert the proper chemical functionalities (i.e. sulfonate and acyl chains) at the desired positions of the previously prepared glucosylglycerol skeleton to obtain the target compounds.
Biological evaluation of anti-tumor activities will be performed including the study of their chemopreventing potential
Exceptionally strong magnetism in 4d perovskites RTcO3 (R=Ca,Sr,Ba)
The evolution of the magnetic ordering temperature of the 4d3 perovskites
RTcO3 (R=Ca,Sr,Ba) and its relation with its electronic and structural
properties has been studied by means of hybrid density functional theory and
Monte Carlo simulations. When compared to the most widely studied 3d
perovskites the large spatial extent of the 4d shells and their relatively
strong hybridization with oxygen weaken the tendency to form Jahn-Teller like
orbital ordering. This strengthens the superexchange interaction. The resulting
insulating G-type antiferromagnetic ground state is characterized by large
superexchange coupling constants (26-35 meV) and Neel temperatures (750-1200
K). These monotonically increase as a function of the R ionic radius due to the
progressive enhancement of the volume and the associated decrease of the
cooperative rotation of the TcO6 octahedra.Comment: 4 pages, 3 figure
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