17,662 research outputs found
The role of electron-electron interactions in two-dimensional Dirac fermions
The role of electron-electron interactions on two-dimensional Dirac fermions
remains enigmatic. Using a combination of nonperturbative numerical and
analytical techniques that incorporate both the contact and long-range parts of
the Coulomb interaction, we identify the two previously discussed regimes: a
Gross-Neveu transition to a strongly correlated Mott insulator, and a
semi-metallic state with a logarithmically diverging Fermi velocity accurately
described by the random phase approximation. Most interestingly, experimental
realizations of Dirac fermions span the crossover between these two regimes
providing the physical mechanism that masks this velocity divergence. We
explain several long-standing mysteries including why the observed Fermi
velocity in graphene is consistently about 20 percent larger than the best
values calculated using ab initio and why graphene on different substrates show
different behavior.Comment: 11 pages, 4 figure
Sustainable management of miombo woodlands in the Northern part of Mozambique (Niassa National Reserve - NNR).
Poster presented at Commiting Science to Global Development. Lisbon (Portugal). 29-30 Sep 2009
Interaction driven metal-insulator transition in strained graphene
The question of whether electron-electron interactions can drive a metal to
insulator transition in graphene under realistic experimental conditions is
addressed. Using three representative methods to calculate the effective
long-range Coulomb interaction between -electrons in graphene and solving
for the ground state using quantum Monte Carlo methods, we argue that without
strain, graphene remains metallic and changing the substrate from SiO to
suspended samples hardly makes any difference. In contrast, applying a rather
large -- but experimentally realistic -- uniform and isotropic strain of about
seems to be a promising route to making graphene an antiferromagnetic
Mott insulator.Comment: Updated version: 6 pages, 3 figure
Seleção e avaliação de matrizes de guaranazeiro para produção de sementes (Paulinia cupana var. Sorbilis Mart. Duke).
bitstream/item/56904/1/PA-1553-0001.pd
Is Small Perfect? Size Limit to Defect Formation in Pyramidal Pt Nanocontacts
We report high resolution transmission electron microscopy and ab initio
calculation results for the defect formation in Pt nanocontacts (NCs). Our
results show that there is a size limit to the existence of twins (extended
structural defects). Defects are always present but blocked away from the tip
axes. The twins may act as scattering plane, influencing contact electron
transmission for Pt NC at room temperature and Ag/Au NC at low temperature.Comment: 4 pages, 3 figure
HPC Cloud for Scientific and Business Applications: Taxonomy, Vision, and Research Challenges
High Performance Computing (HPC) clouds are becoming an alternative to
on-premise clusters for executing scientific applications and business
analytics services. Most research efforts in HPC cloud aim to understand the
cost-benefit of moving resource-intensive applications from on-premise
environments to public cloud platforms. Industry trends show hybrid
environments are the natural path to get the best of the on-premise and cloud
resources---steady (and sensitive) workloads can run on on-premise resources
and peak demand can leverage remote resources in a pay-as-you-go manner.
Nevertheless, there are plenty of questions to be answered in HPC cloud, which
range from how to extract the best performance of an unknown underlying
platform to what services are essential to make its usage easier. Moreover, the
discussion on the right pricing and contractual models to fit small and large
users is relevant for the sustainability of HPC clouds. This paper brings a
survey and taxonomy of efforts in HPC cloud and a vision on what we believe is
ahead of us, including a set of research challenges that, once tackled, can
help advance businesses and scientific discoveries. This becomes particularly
relevant due to the fast increasing wave of new HPC applications coming from
big data and artificial intelligence.Comment: 29 pages, 5 figures, Published in ACM Computing Surveys (CSUR
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