8,640 research outputs found
On the properties of surface reconstructed silicon nanowires
We study by means of density-functional calculations the role of lateral
surface reconstructions in determining the electrical properties of
silicon nanowires. The different lateral reconstructions are explored by
relaxing all the nanowires with crystalline bulk silicon structure and all
possible ideal facets that correspond to an average diameter of 1.5 nm. We show
that the reconstruction induces the formation of ubiquitous surface states that
turn the wires into semi-metallic or metallic
Determination of the magnetic anisotropy axes of single-molecule magnets
Simple methods are presented allowing the determination of the magnetic
anisotropy axes of a crystal of a single-molecule magnet (SMM). These methods
are used to determine an upper bound of the easy axis tilts in a standard
Mn12-Ac crystal. The values obtained in the present study are significately
smaller than those reported in recent high frequency electron paramagnetic
resonance (HF-EPR) studies which suggest distributions of hard-axes tilts.Comment: 10 pages, 6 figure
Density, structure and dynamics of water: the effect of Van der Waals interactions
It is known that ab initio molecular dynamics (AIMD) simulations of liquid
water, based on the generalized gradient approximation (GGA) to density
functional theory (DFT), yield structural and diffusive properties in
reasonable agreement with experiment only if artificially high temperatures are
used in the simulations. The equilibrium density, at normal conditions, of DFT
water has been recently shown by Schmidt et al. [J. Phys. chem. B, 113, 11959
(2009)] to be underestimated by different GGA functionals for exchange and
correlation, and corrected by the addition of interatomic pair potentials to
describe van derWaals (vdW) interactions. In this contribution we present a
DFTAIMD study of liquid water using several GGA functionals as well as the van
der Waals density functional (vdW-DF) of Dion et al. [Phys. Rev. Lett. 92,
246401(2004)]. As expected, we find that the density of water is grossly
underestimated by GGA functionals. When a vdW-DF is used, the density improves
drastically and the experimental diffusivity is reproduced without the need of
thermal corrections. We analyze the origin of the density differences between
all the functionals. We show that the vdW-DF increases the population of
non-H-bonded interstitial sites, at distances between the first and second
coordination shells. However, it excessively weakens the H-bond network,
collapsing the second coordination shell. This structural problem is partially
associated to the choice of GGA exchange in the vdW-DF. We show that a
different choice for the exchange functional is enough to achieve an overall
improvement both in structure and diffusivity.Comment: 11 pages, 9 figures, submitted. Revised versio
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Fostering an EU strategy for security sector reform in the Mediterranean: learning from Turkish and Palestinian police reform experiences
In the absence of an overarching strategy, Turkey and the Palestinian territories depict how the EU has adopted different approaches to security sector reform (SSR) which have not facilitated the consolidation of a common EU foreign policy, though the situation might soon change given respective SSR-related documents from the Council and Commission. This report contributes to the security sector debate by stressing that in conflict and post-conflict scenarios endurable SSR requires fomenting synergies between the police and judicial sectors and the inclusion of DDR, in tandem with the institutional implementation of transparent, accountable and democratic oversight mechanisms. There is an adamant need for constructive consistency when applying this central facet of EU foreign policy in the Mediterranean basin and beyond.
SSR is an emerging phenomenon in conflict, post-conflict and development scenarios that has acquired a prominent role within the policy agendas of key international actors. As a prelude to the two case studies, and in order to better understand the EUâs end goal, a brief analysis of the two pivotal European SSR documents is provided with particular emphasis on their contribution to develop a more coherent and effective EU presence in this field. More specific consideration is then given to the role played by SSR-related matters within the framework of two EU foreign policy mechanisms towards the Mediterranean: the Euro-Mediterranean Partnership (EMP), also known as the Barcelona Process, and the European Neighbourhood Policy (ENP).
The two case studies - Turkey and the Palestinian territories - are exceptional due to the nature of their geopolitical position in regional and international politics. Both demonstrate different levels of state development and different degrees of monopoly over the legitimate use of force, which have contributed to the development of different EU approaches to SSR, tailored to the specific needs of the local context in which the Union is operating. Both cases also demonstrate how the EU emphasizes democracy or security depending on the respective circumstantial differences. Turkey exemplifies by and large the EU accession process of fulfilling democratic reform in return for EU membership. The Palestinian case illustrates how security demands precede democratic apertures. Together, both highlight the lack of a consistent and comprehensive EU SSR strategy, the lack of which continues to impede the emergence of a common EU foreign policy.
This study makes a set of recommendations for the two case studies, and concludes with more general ones applicable to the broader Euro-Mediterranean area. These are mainly addressed to the European Commission, Council of the EU (and thus, Member States) and European Parliament. Moreover, the conclusions and recommendations included in this report could inspire the work of various advocacy groups in the fields of SSR, conflict resolution and democracy and human rights promotion given that, based on the âhuman securityâ logic, this report departs from the assumption that police reform encapsulates both modernising and democratising processes
An Imprint of Molecular Cloud Magnetization in the Morphology of the Dust Polarized Emission
We describe a morphological imprint of magnetization found when considering
the relative orientation of the magnetic field direction with respect to the
density structures in simulated turbulent molecular clouds. This imprint was
found using the Histogram of Relative Orientations (HRO): a new technique that
utilizes the gradient to characterize the directionality of density and column
density structures on multiple scales. We present results of the HRO analysis
in three models of molecular clouds in which the initial magnetic field
strength is varied, but an identical initial turbulent velocity field is
introduced, which subsequently decays. The HRO analysis was applied to the
simulated data cubes and mock-observations of the simulations produced by
integrating the data cube along particular lines of sight. In the 3D analysis
we describe the relative orientation of the magnetic field with
respect to the density structures, showing that: 1.The magnetic field shows a
preferential orientation parallel to most of the density structures in the
three simulated cubes. 2.The relative orientation changes from parallel to
perpendicular in regions with density over a critical density in the
highest magnetization case. 3.The change of relative orientation is largest for
the highest magnetization and decreases in lower magnetization cases. This
change in the relative orientation is also present in the projected maps. In
conjunction with simulations HROs can be used to establish a link between the
observed morphology in polarization maps and the physics included in
simulations of molecular clouds.Comment: (16 pages, 11 figures, submitted to ApJ 05MAR2013, accepted
07JUL2013
Structural relaxations in electronically excited poly(para-phenylene)
Structural relaxations in electronically excited poly(para-phenylene) are
studied using many-body perturbation theory and density-functional-theory
methods. A sophisticated description of the electron-hole interaction is
required to describe the energies of the excitonic states, but we show that the
structural relaxations associated with exciton formation can be obtained quite
accurately within a constrained density-functional-theory approach. We find
that the structural relaxations in the low-energy excitonic states extend over
about 8 monomers, leading to an energy reduction of 0.22 eV and a Stokes shift
of 0.40 eV.Comment: 4 pages, 3 figure
Electronic structure interpolation via atomic orbitals
We present an efficient scheme for accurate electronic structure
interpolations based on the systematically improvable optimized atomic
orbitals. The atomic orbitals are generated by minimizing the spillage value
between the atomic basis calculations and the converged plane wave basis
calculations on some coarse -point grid. They are then used to calculate the
band structure of the full Brillouin zone using the linear combination of
atomic orbitals (LCAO) algorithms. We find that usually 16 -- 25 orbitals per
atom can give an accuracy of about 10 meV compared to the full {\it ab initio}
calculations. The current scheme has several advantages over the existing
interpolation schemes. The scheme is easy to implement and robust which works
equally well for metallic systems and systems with complex band structures.
Furthermore, the atomic orbitals have much better transferability than the
Shirley's basis and Wannier functions, which is very useful for the
perturbation calculations
Quantum phase interference (Berry phase) in single-molecule magnets of Mn12
Magnetization measurements of a molecular clusters Mn12 with a spin ground
state of S = 10 show resonance tunneling at avoided energy level crossings. The
observed oscillations of the tunnel probability as a function of the magnetic
field applied along the hard anisotropy axis are due to topological quantum
phase interference of two tunnel paths of opposite windings. Mn12 is therefore
the second molecular clusters presenting quantum phase interference.Comment: 3 pages, 4 figures, MMM'01 conference (12-16 Nov.
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