3,513 research outputs found
Adhesion and electronic structure of graphene on hexagonal boron nitride substrates
We investigate the adsorption of graphene sheets on h-BN substrates by means
of first-principles calculations in the framework of adiabatic connection
fluctuation-dissipation theory in the random phase approximation. We obtain
adhesion energies for different crystallographic stacking configurations and
show that the interlayer bonding is due to long-range van der Waals forces. The
interplay of elastic and adhesion energies is shown to lead to stacking
disorder and moir\'e structures. Band structure calculations reveal substrate
induced mass terms in graphene which change their sign with the stacking
configuration. The dispersion, absolute band gaps and the real space shape of
the low energy electronic states in the moir\'e structures are discussed. We
find that the absolute band gaps in the moir\'e structures are at least an
order of magnitude smaller than the maximum local values of the mass term. Our
results are in agreement with recent STM experiments.Comment: 8 pages, 8 figures, revised and extended version, to appear in Phys.
Rev.
Sustaining entrepreneurial business: a complexity perspective on processes that produce emergent practice
This article examines the management practices in an entrepreneurial small firm which sustain the business. Using a longitudinal qualitative case study, four general processes are identified (experimentation, reflexivity, organising and sensing), that together provide a mechanism to sustain the enterprise. The analysis draws on concepts from entrepreneurship and complexity science. We suggest that an entrepreneur’s awareness of the role of these parallel processes will facilitate their approaches to sustaining and developing enterprises. We also suggest that these processes operate in parallel at multiple levels, including the self, the business and inter-firm networks. This finding contributes to a general theory of entrepreneurship. A number of areas for further research are discussed arising from this result
Orbital moment of a single Co atom on a Pt(111) surface - a view from correlated band theory
The orbital magnetic moment of a Co adatom on a Pt(111) surface is calculated
in good agreement with experimental data making use of the LSDA+U method. It is
shown that both electron correlation induced orbital polarization and
structural relaxation play essential roles in orbital moment formation. The
microscopic origins of the orbital moment enhancement are discussed
Enhanced Screening in Chemically Functionalized Graphene
Resonant scatterers such as hydrogen adatoms can strongly enhance the low
energy density of states in graphene. Here, we study the impact of these
impurities on the electronic screening. We find a two-faced behavior: Kubo
formula calculations reveal an increased dielectric function upon
creation of midgap states but no metallic divergence of the static
at small momentum transfer . This bad metal behavior
manifests also in the dynamic polarization function and can be directly
measured by means of electron energy loss spectroscopy. A new length scale
beyond which screening is suppressed emerges, which we identify with the
Anderson localization length.Comment: 5 pages, 4 figure
Unified character of correlation effects in unconventional Pu-based superconductors and \delta-Pu
Electronic structure calculations combining the local-density approximation
with an exact diagonalization of the Anderson impurity model show an
intermediate 5f^5-5f^6-valence ground state and delocalization of the 5f^5
multiplet of the Pu atom 5f-shell in PuCoIn_5, PuCoGa_5, and \delta-Pu. The
5f-local magnetic moment is compensated by a moment formed in the surrounding
cloud of conduction electrons. For PuCoGa_5 and \delta-Pu the compensation is
complete and the Anderson impurity ground state is a singlet. For PuCoIn_5 the
compensation is partial and the Pu ground state is magnetic. We suggest that
the unconventional d-wave superconductivity is likely mediated by the 5f-states
antiferromagnetic fluctuations in PuCoIn_5, and by valence fluctuations in
PuCoGa_5.Comment: 5 pages, 3 figure
Effect of ligand substitution on the exchange interactions in {Mn12}-type single-molecule magnets
We investigate how the ligand substitution affects the intra-molecular spin
exchange interactions, studying a prototypal family of single-molecule magnets
comprising dodecanuclear cluster molecules [Mn12O12(COOR)16]. We identify a
simple scheme based on accumulated Pauling electronegativity numbers (a.e.n.)
of the carboxylate ligand groups (R). The redistribution of the electron
density, controlled by a.e.n. of a ligand, changes the degree of hybridization
between 3d electrons of manganese and 2p electrons of oxygen atoms, thus
changing the exchange interactions. This scheme, despite its conceptual
simplicity, provides a strong correlation with the exchange energies associated
with carboxylate bridges, and is confirmed by the electronic structure
calculations taking into account the Coulomb correlations in magnetic
molecules.Comment: 18 pages, 1 table, 4 figures. Accepted to "Inorganic Chemistry
Plane-wave based electronic structure calculations for correlated materials using dynamical mean-field theory and projected local orbitals
The description of realistic strongly correlated systems has recently
advanced through the combination of density functional theory in the local
density approximation (LDA) and dynamical mean field theory (DMFT). This
LDA+DMFT method is able to treat both strongly correlated insulators and
metals. Several interfaces between LDA and DMFT have been used, such as (N-th
order) Linear Muffin Tin Orbitals or Maximally localized Wannier Functions.
Such schemes are however either complex in use or additional simplifications
are often performed (i.e., the atomic sphere approximation). We present an
alternative implementation of LDA+DMFT, which keeps the precision of the
Wannier implementation, but which is lighter. It relies on the projection of
localized orbitals onto a restricted set of Kohn-Sham states to define the
correlated subspace. The method is implemented within the Projector Augmented
Wave (PAW) and within the Mixed Basis Pseudopotential (MBPP) frameworks. This
opens the way to electronic structure calculations within LDA+DMFT for more
complex structures with the precision of an all-electron method. We present an
application to two correlated systems, namely SrVO3 and beta-NiS (a
charge-transfer material), including ligand states in the basis-set. The
results are compared to calculations done with Maximally Localized Wannier
functions, and the physical features appearing in the orbitally resolved
spectral functions are discussed.Comment: 15 pages, 17 figure
In-plane magnetic anisotropy of Fe atoms on BiSe(111)
The robustness of the gapless topological surface state hosted by a 3D
topological insulator against perturbations of magnetic origin has been the
focus of recent investigations. We present a comprehensive study of the
magnetic properties of Fe impurities on a prototypical 3D topological insulator
BiSe using local low temperature scanning tunneling microscopy and
integral x-ray magnetic circular dichroism techniques. Single Fe adatoms on the
BiSe surface, in the coverage range are heavily relaxed
into the surface and exhibit a magnetic easy axis within the surface-plane,
contrary to what was assumed in recent investigations on the opening of a gap.
Using \textit{ab initio} approaches, we demonstrate that an in-plane easy axis
arises from the combination of the crystal field and dynamic hybridization
effects.Comment: 5 pages, 3 figures, typos correcte
On Embeddability of Buses in Point Sets
Set membership of points in the plane can be visualized by connecting
corresponding points via graphical features, like paths, trees, polygons,
ellipses. In this paper we study the \emph{bus embeddability problem} (BEP):
given a set of colored points we ask whether there exists a planar realization
with one horizontal straight-line segment per color, called bus, such that all
points with the same color are connected with vertical line segments to their
bus. We present an ILP and an FPT algorithm for the general problem. For
restricted versions of this problem, such as when the relative order of buses
is predefined, or when a bus must be placed above all its points, we provide
efficient algorithms. We show that another restricted version of the problem
can be solved using 2-stack pushall sorting. On the negative side we prove the
NP-completeness of a special case of BEP.Comment: 19 pages, 9 figures, conference version at GD 201
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