5,932 research outputs found
Role of pseudospin in quasiparticle interferences in epitaxial graphene probed by high-resolution scanning tunneling microscopy
Pseudospin, an additional degree of freedom related to the honeycomb
structure of graphene, is responsible of many of the outstanding electronic
properties found in this material. This article provides a clear understanding
of how such pseudospin impacts the quasiparticle interferences of monolayer
(ML) and bilayer (BL) graphene measured by low temperature scanning tunneling
microscopy and spectroscopy. We have used this technique to map, with very high
energy and space resolution, the spatial modulations of the local density of
states of ML and BL graphene epitaxialy grown on SiC(0001), in presence of
native disorder. We perform a Fourier transform analysis of such modulations
including wavevectors up to unit-vectors of the reciprocal lattice. Our data
demonstrate that the quasiparticle interferences associated to some particular
scattering processes are suppressed in ML graphene, but not in BL graphene.
Most importantly, interferences with 2qF wavevector associated to intravalley
backscattering are not measured in ML graphene, even on the images with highest
resolution. In order to clarify the role of the pseudospin on the quasiparticle
interferences, we use a simple model which nicely captures the main features
observed on our data. The model unambiguously shows that graphene's pseudospin
is responsible for such suppression of quasiparticle interferences features in
ML graphene, in particular for those with 2qF wavevector. It also confirms
scanning tunneling microscopy as a unique technique to probe the pseudospin in
graphene samples in real space with nanometer precision. Finally, we show that
such observations are robust with energy and obtain with great accuracy the
dispersion of the \pi-bands for both ML and BL graphene in the vicinity of the
Fermi level, extracting their main tight binding parameters
The missing atom as a source of carbon magnetism
Atomic vacancies have a strong impact in the mechanical, electronic and
magnetic properties of graphene-like materials. By artificially generating
isolated vacancies on a graphite surface and measuring their local density of
states on the atomic scale, we have shown how single vacancies modify the
electronic properties of this graphene-like system. Our scanning tunneling
microscopy experiments, complemented by tight binding calculations, reveal the
presence of a sharp electronic resonance at the Fermi energy around each single
graphite vacancy, which can be associated with the formation of local magnetic
moments and implies a dramatic reduction of the charge carriers' mobility.
While vacancies in single layer graphene naturally lead to magnetic couplings
of arbitrary sign, our results show the possibility of inducing a macroscopic
ferrimagnetic state in multilayered graphene samples just by randomly removing
single C atoms.Comment: Accepted for publication in Physical Review Letter
Point defects on graphene on metals
Understanding the coupling of graphene with its local environment is critical
to be able to integrate it in tomorrow's electronic devices. Here we show how
the presence of a metallic substrate affects the properties of an atomically
tailored graphene layer. We have deliberately introduced single carbon
vacancies on a graphene monolayer grown on a Pt(111) surface and investigated
its impact in the electronic, structural and magnetic properties of the
graphene layer. Our low temperature scanning tunneling microscopy studies,
complemented by density functional theory, show the existence of a broad
electronic resonance above the Fermi energy associated with the vacancies.
Vacancy sites become reactive leading to an increase of the coupling between
the graphene layer and the metal substrate at these points; this gives rise to
a rapid decay of the localized state and the quenching of the magnetic moment
associated with carbon vacancies in free-standing graphene layers
Associations between sedentary time, physical activity and bone health among older people using compositional data analysis
Introduction : Aging is associated with a progressive decrease in bone mass (BM), and being physical active is one of the main strategies to combat this continuous loss. Nonetheless, because daily time is limited, time spent on each movement behavior is co-dependent. The aim of this study was to determine the relationship between BM and movement behaviors in elderly people using compositional data analysis.
Methods : We analyzed 871 older people [395 men (76.9 +/- 5.3y) and 476 women (76.7 +/- 4.7y)]. Time spent in sedentary behavior (SB), light physical activity (LPA) and moderate-to-vigorous physical activity (MVPA), was assessed using accelerometry. BM was determined by bone densitometry (DXA). The sample was divided according to sex and bone health indicators.
Results : The combined effect of all movement behaviors (PA and SB) was significantly associated with whole body, leg and femoral region BM in the whole sample (p<0.05), with leg and pelvic BM (p<0.05) in men and, with whole body, arm and leg BM (p<0.05) in women. In men, arm and pelvic BM were negatively associated with SB and whole body, pelvic and leg BM were positively associated with MVPA (p<0.05). In women, whole body and leg BM were positively associated with SB. Arm and whole body BM were positively associated and leg BM was negatively associated with LPA and arm BM was negatively associated with MVPA (p<0.05). Women without bone fractures spent less time in SB and more in LPA and MVPA than the subgroup with bone fractures.
Conclusion : We identified that the positive effect of MVPA relative to the other behaviors on bone mass is the strongest overall effect in men. Furthermore, women might decrease bone fracture risk through PA increase and SB reduction, despite the fact that no clear benefits of PA for bone mass were found
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