740 research outputs found
Negative-U properties for substitutional Au in Si
The isolated substitutional gold impurity in bulk silicon is studied in
detail using electronic structure calculations based on density-functional
theory. The defect system is found to be a non-spin-polarized negative-U
centre, thus providing a simple solution to the long-standing debate over the
electron paramagnetic resonance signal for gold in silicon. There is an
excellent agreement (within 0.03 eV) between the well-established experimental
donor and acceptor levels and the predicted stable charge state transition
levels, allowing for the unambiguous assignment of the two experimental levels
to the (1+/1-) and (1-/3-) transitions, respectively, in contrast to previously
held assumptions about the system.Comment: 6 pages, 5 figure
Improving the conductance of carbon nanotube networks through resonant momentum exchange
We present a mechanism to improve the conductivity of carbon nanotube (CNT)
networks by improving the conductance between CNTs of different chirality. We
argue generally that a weak perturbation can greatly improve the inter-tube
conductance by allowing momentum-conserving tunnelling. The mechanism is
verified with a tight-binding model, allowing an investigation of its impact
for a network containing a range of chiralities. We discuss practical
implementations, and conclude that it may be effected by weak physical
interactions, and therefore does not require chemical bonding to the CNTs.Comment: 6 pages, 4 figure
Multiscale molecular simulations of the formation and structure of polyamide membranes created by interfacial polymerization
Large scale molecular simu lations to model the formation of polyamide membranes have been carried out using a procedure that mimics experimental interfacial polymerization of trimesoyl chloride (TMC) and metaphenylene diamine (MPD) monomers. A coarse - grained representation of the m onomers has been developed to facilitate these simulations, which captures essential features of the stereochemistry of the monomers and of amide bonding between them. Atomic models of the membranes are recreated from the final coarse - grained representatio ns. Consistent with earlier treatments, membranes are formed through the growth and aggregation of oligomer clusters. The membranes are inhomogeneous, displaying opposing gradients of trapped carboxyl and amine side groups, local density variations, and r egions where the density of amide bonding is reduced as a result of the aggregation process. We observe the interfacial polymerization reaction is self - limiting and the simulated membranes display a thickness of 5 – 10 nm. They also display a surface roughn ess of 1 – 4 nm. Comparisons are made with recently published experimental results on the structure and chemistry of these membranes and some interesting similarities and differences are found
Twist-angle dependence of electron correlations in moir\'e graphene bilayers
Motivated by the recent observation of correlated insulator states and
unconventional superconductivity in twisted bilayer graphene, we study the
dependence of electron correlations on the twist angle and reveal the existence
of strong correlations over a narrow range of twist-angles near the magic
angle. Specifically, we determine the on-site and extended Hubbard parameters
of the low-energy Wannier states using an atomistic quantum-mechanical
approach. The ratio of the on-site Hubbard parameter and the width of the flat
bands, which is an indicator of the strength of electron correlations, depends
sensitively on the screening by the semiconducting substrate and the metallic
gates. Including the effect of long-ranged Coulomb interactions significantly
reduces electron correlations and explains the experimentally observed
sensitivity of strong correlation phenomena on twist angle.Comment: 17 pages, 6 figure
Van der Waals interactions in DFT made easy by Wannier functions
Ubiquitous Van der Waals interactions between atoms and molecules are
important for many molecular and solid structures. These systems are often
studied from first principles using the Density Functional Theory (DFT).
However, the commonly used DFT functionals fail to capture the essence of Van
der Waals effects. Many attempts to correct for this problem have been
proposed, which are not completely satisfactory because they are either very
complex and computationally expensive or have a basic semiempirical character.
We here describe a novel approach, based on the use of the Maximally-Localized
Wannier functions, that appears to be promising, being simple, efficient,
accurate, and transferable (charge polarization effects are naturally
included). The results of test applications are presented.Comment: submitted to Phys. Rev. Let
Generalized Wannier functions: a comparison of molecular electric dipole polarizabilities
Localized Wannier functions provide an efficient and intuitive means by which
to compute dielectric properties from first principles. They are most commonly
constructed in a post-processing step, following total-energy minimization.
Nonorthogonal generalized Wannier functions (NGWFs) [Skylaris et al., Phys.
Rev. B 66, 035119 11 (2002); Skylaris et al., J. Chem. Phys. 122, 084119
(2005)] may also be optimized in situ, in the process of solving for the
ground-state density. We explore the relationship between NGWFs and
orthonormal, maximally localized Wannier functions (MLWFs) [Marzari and
Vanderbilt, Phys. Rev. B 56, 12847 (1997); Souza, Marzari, and Vanderbilt,
ibid. 65, 035109 (2001)], demonstrating that NGWFs may be used to compute
electric dipole polarizabilities efficiently, with no necessity for
post-processing optimization, and with an accuracy comparable to MLWFs.Comment: 5 pages, 1 figure. This version matches that accepted for Physical
Review B on 4th May 201
Subspace representations in ab initio methods for strongly correlated systems
We present a generalized definition of subspace occupancy matrices in ab
initio methods for strongly correlated materials, such as DFT+U and DFT+DMFT,
which is appropriate to the case of nonorthogonal projector functions. By
enforcing the tensorial consistency of all matrix operations, we are led to a
subspace projection operator for which the occupancy matrix is tensorial and
accumulates only contributions which are local to the correlated subspace at
hand. For DFT+U in particular, the resulting contributions to the potential and
ionic forces are automatically Hermitian, without resort to symmetrization, and
localized to their corresponding correlated subspace. The tensorial invariance
of the occupancies, energies and ionic forces is preserved. We illustrate the
effect of this formalism in a DFT+U study using self-consistently determined
projectors.Comment: 15 pages, 8 figures. This version (v2) matches that accepted for
Physical Review B on 15th April 201
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Multiple cerebral cavernous malformations in association with a Dubowitz-like syndrome.
Cerebral cavernous malformations (CCMs) are proliferative sinusoidal vascular lesions and are the most common vascular malformations of the brain. They can occur sporadically or secondary to an underlying genetic predisposition where multiple lesions are commonly seen. Dubowitz syndrome is a clinically-diagnosed rare genetic disorder with an unknown molecular basis. An association between these conditions has not been reported previously. A 30-year-old woman with a Dubowitz-like syndrome presented with acute left leg weakness, gait ataxia and transient loss of consciousness. Imaging revealed five CCMs with recent hemorrhage in relation to one lesion in the left middle cerebellar peduncle. A recurrent hemorrhage from the same lesion occurred ten weeks later and she underwent microsurgical excision of this malformation. Genetic analysis revealed an unbalanced chromosomal rearrangement involving partial deletion of chromosome 7q21, the locus of the CCM1/KRIT1 gene known to be associated with familial CCMs. This is the first description of CCMs in association with the Dubowitz phenotype. The genetic basis of Dubowitz syndrome may be heterogeneous but, for the first time, overlap is demonstrated between this condition and multiple CCMs, with a possible common genetic etiology. Knowledge of this association may be of help in the management of acute neurological presentations in Dubowitz-like syndromes. Keywords: Hemangioma, Cavernous, Central nervous system, Dubowitz syndrome, Genetics
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