2,617 research outputs found
van der Waals density functionals built upon the electron-gas tradition: Facing the challenge of competing interactions
The theoretical description of sparse matter attracts much interest, in
particular for those ground-state properties that can be described by density
functional theory (DFT). One proposed approach, the van der Waals density
functional (vdW-DF) method, rests on strong physical foundations and offers
simple yet accurate and robust functionals. A very recent functional within
this method called vdW-DF-cx [K. Berland and P. Hyldgaard, Phys. Rev. B 89,
035412] stands out in its attempt to use an exchange energy derived from the
same plasmon-based theory from which the nonlocal correlation energy was
derived. Encouraged by its good performance for solids, layered materials, and
aromatic molecules, we apply it to several systems that are characterized by
competing interactions. These include the ferroelectric response in PbTiO,
the adsorption of small molecules within metal-organic frameworks (MOFs), the
graphite/diamond phase transition, and the adsorption of an aromatic-molecule
on the Ag(111) surface. Our results indicate that vdW-DF-cx is overall well
suited to tackle these challenging systems. In addition to being a competitive
density functional for sparse matter, the vdW-DF-cx construction presents a
more robust general purpose functional that could be applied to a range of
materials problems with a variety of competing interactions
Effect of uniaxial strain on plasmon excitations in graphene
Uniaxial strain is known to modify significantly the electronic properties of
graphene, a carbon single layer of atomic width. Here, we study the effect of
applied strain on the composite excitations arising from the coupling of charge
carriers and plasmons in graphene, i.e. the plasmarons. Specifically, we
predict that the plasmaron energy dispersion, which has been recently observed
experimentally in unstrained graphene, is shifted and broadened by applied
uniaxial strain. Thus, strain constitutes an additional parameter which may be
useful to tune graphene properties in plasmaronic devices.Comment: Invited oral lecture at the 23rd AIRAPT International Conference on
"High Pressure Science and Technology", Mumbai (India), September 25-30,
2011. To be published in J. Phys.: Conf. Series (2012
High Resolution Spectroscopy of Balmer-Dominated Shocks in the RCW 86, Kepler and SN 1006 Supernova Remnants
We report results from high resolution optical spectroscopy of three
non-radiative galactic supernova remnants, RCW 86, Kepler's supernova remnant
and SN 1006. We have measured the narrow component H-alpha line widths in
Balmer-dominated filaments in RCW 86 and SN 1006, as well as the narrow
component width in a Balmer-dominated knot in Kepler's SNR. The narrow
component line widths measured in RCW 86 and Kepler's SNR show FWHM of 30-40
km/s, similar to what has been seen in other Balmer-dominated remnants. Of the
remnants in our sample, SN 1006 is the fastest shock (~3000 km/s). The narrow
component H-alpha and H-beta lines in this remnant have a FWHM of merely 21
km/s. Comparing the narrow component widths measured in our sample with those
measured in other remnants shows that the width of the narrow component does
not correlate in a simple way with the shock velocity. The implications for the
pre-heating mechanism responsible for the observed line widths are discussed.Comment: Accepted by A&
Influence of van der Waals forces on the adsorption structure of benzene on silicon studied using density functional theory
Two different adsorption configurations of benzene on the Si(001)-(2 x 1) surface, the tight-bridge and butterfly structures, were studied using density functional theory. Several exchange and correlation functionals were used, including the recently developed van der Waals density functional (vdW-DF), which accounts for the effect of van der Waals forces. In contrast to the Perdew-Burke-Ernzerhof (PBE), revPBE, and other generalized-gradient approximation functionals, the vdW-DF finds that, for most coverages, the adsorption energy of the butterfly structure is greater than that of the tight-bridge structure
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Massive Open Online Courses and economic sustainability
Millions of users around the world have registered on Massive Open Online Courses (MOOCs) offered by hundreds of universities (and other organizations) worldwide. Creating and offering these courses costs thousands of pounds. However, at present, revenue generated by MOOCs is not sufficient to offset these costs. The sustainability of MOOCs is a pressing concern as they incur not only upfront creation costs but also maintenance costs to keep content relevant, as well as on-going facilitation support costs while a course is running and re-running. At present, charging a fee for certification seems to be a popular business model adopted by leading platform providers.
In this position paper, the authors explore possible business models for courses, along with their advantages and disadvantages, by conducting a literature study and applying personal insights gained from attending various MOOC discussion fora. Some business models discussed here are: the Freemium model, sponsorships, initiatives and grants, donations, merchandise, the sale of supplementary material, selective advertising, data-sharing, follow-on events, and revenue from referrals. This paper looks at the sustainability of MOOCS as opposed to the sustainability of MOOC platforms, while observing the tight link between them
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Who are with us: MOOC learners on a FutureLearn course
Massive Open Online Courses (MOOCs) attract learners with a variety of backgrounds. Engaging them using game development was trialled in a beginner’s programming course, “Begin programming: build your first mobile game”, on FutureLearn platform. The course has completed two iterations: first in autumn 2013 and second in spring 2014 with thousands of participants. This paper explores the characteristics of learner groups attracted by these two consecutive runs of the course and their perceptions of the course using pre- and post-course surveys. Recommendations for practitioners are offered, including when the audience is different to the one expected. A MOOC is unlikely to please everyone, especially with such large cohorts. Nevertheless, this course, using game development as a vehicle to teach programming, seems to have offered a balanced learning experience to a diverse group of learners. However, MOOC creators and facilitators should accept that a course cannot be made to please everyone and try to communicate clearly who the intended audience for the course are
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