2,188 research outputs found
Absence of stable atomic structure in fluorinated graphene
Based on the results of first-principles calculations we demonstrate that
significant distortion of graphene sheets caused by adsorption of fluorine
atoms leads to the formation of metastable patterns for which the next step of
fluorination is considerably less energetically favorable. Existence of these
stable patterns oriented along the armchair direction makes possible the
synthesis of various CFx structures. The combination of strong distortion of
the nonfluorinated graphene sheet with the doping caused by the polar nature of
C-F bonds reduces the energy cost of migration and the energy of migration
barriers, making possible the migration of fluorine atoms on the graphene
surface as well as transformation of the shapes of fluorinated areas. The
decreasing energy cost of migration with increasing fluorine content also leads
to increasing numbers of single fluorine adatoms, which could be the source of
magnetic moments.Comment: 16 pages, 6 figures (one figure added), accepted in PCC
Atomic and electronic structure of nitrogen- and boron-doped phosphorene
First principle modeling of nitrogen- and boron-doped phosphorene
demonstrates the tendency toward formation of highly ordered structures.
Nitrogen doping leads to the formation of -N-P-P-P-N- lines. Further
transformation to -P-N-P-N- lines across the chains of phosphorene occurs with
increasing band gap and increasing nitrogen concentration, which coincides with
the decreasing chemical activity of N-doped phosphorene. In contrast to the
case of nitrogen, boron atoms prefer to form -B-B- pairs with the further
formation of -P-P-B-B-P-P- patterns along the phosphorene chains. The low
concentration of boron dopants converts the phosphorene from a semiconductor
into a semimetal with the simultaneous enhancement of its chemical activity.
Co-doping of phosphorene by both boron and nitrogen starts from the formation
of -B-N- pairs, which provide flat bands and the further transformation of
these pairs to hexagonal BN lines and ribbons across the phosphorene chains.Comment: 21 pages, 8 figures, 2 tables, to appear at PCC
A narrow-band unfitted finite element method for elliptic PDEs posed on surfaces
The paper studies a method for solving elliptic partial differential
equations posed on hypersurfaces in , . The method allows
a surface to be given implicitly as a zero level of a level set function. A
surface equation is extended to a narrow-band neighborhood of the surface. The
resulting extended equation is a non-degenerate PDE and it is solved on a bulk
mesh that is unaligned to the surface. An unfitted finite element method is
used to discretize extended equations. Error estimates are proved for finite
element solutions in the bulk domain and restricted to the surface. The
analysis admits finite elements of a higher order and gives sufficient
conditions for archiving the optimal convergence order in the energy norm.
Several numerical examples illustrate the properties of the method.Comment: arXiv admin note: text overlap with arXiv:1301.470
Structured Sparse Modelling with Hierarchical GP
In this paper a new Bayesian model for sparse linear regression with a
spatio-temporal structure is proposed. It incorporates the structural
assumptions based on a hierarchical Gaussian process prior for spike and slab
coefficients. We design an inference algorithm based on Expectation Propagation
and evaluate the model over the real data.Comment: SPARS 201
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