191 research outputs found
Pressure screening in the interior of primary shells in double-wall carbon nanotubes
The pressure response of double-wall carbon nanotubes has been investigated
by means of Raman spectroscopy up to 10 GPa. The intensity of the radial
breathing modes of the outer tubes decreases rapidly but remain observable up
to 9 GPa, exhibiting a behavior similar (but less pronounced) to that of
single-wall carbon nanotubes, which undergo a shape distortion at higher
pressures. In addition, the tangential band of the external tubes broadens and
decreases in amplitude. The corresponding Raman features of the internal tubes
appear to be considerably less sensitive to pressure. All findings lead to the
conclusion that the outer tubes act as a protection shield for the inner tubes
whereas the latter increase the structural stability of the outer tubes upon
pressure application.Comment: PDF with 15 pages, 3 figures, 1 table; submitted to Physical Review
Genetic vs community diversity patterns of macrobenthic species: preliminary results from the lagoonal ecosystem
1 - The use of molecular data derived from multispecies assemblages in order to test ecological theory has only recently been introduced in the scientific literature.2 - As a first step, we compared patterns of abiotic environment, polychaeta distribution and their genetic diversity in five lagoon ecosystems in Greece. Our results confirm the hypothesis that higher genetic diversity is expected in the populations of the species occurring in the transitional waters rather than of those occurring in the marine environment.3 - Patterns derived from the polychaete community level and from the mitochondrial DNA (16S rRNA) obtained from Nephtys hombergii and Hediste diversicolor showed convergence, indicating the potential use of molecular matrices as surrogates in community analysis.4 - Finally, the high correlation between the genetic diversity pattern of H. diversicolor and the phosphorus concentration in the sediments may imply the broadening of the hierarchic-response-tostress hypothesis towards lower than species level
Raman spectra of MgB2 at high pressure and topological electronic transition
Raman spectra of the MgB2 ceramic samples were measured as a function of
pressure up to 32 GPa at room temperature. The spectrum at normal conditions
contains a very broad peak at ~590 cm-1 related to the E2g phonon mode. The
frequency of this mode exhibits a strong linear dependence in the pressure
region from 5 to 18 GPa, whereas beyond this region the slope of the
pressure-induced frequency shift is reduced by about a factor of two. The
pressure dependence of the phonon mode up to ~ 5GPa exhibits a change in the
slope as well as a "hysteresis" effect in the frequency vs. pressure behavior.
These singularities in the E2g mode behavior under pressure support the
suggestion that MgB2 may undergo a pressure-induced topological electronic
transition.Comment: 2 figure
O papel do “Mineral Resources Expert Group” dos EuroGeoSurveys no contexto mineral Europeu
ABSTRACT: Europe shows an inevitably growing and accelerating consumption
of mineral commodities and the high import dependence of strategic and
critical raw materials has a serious impact on the sustainability of the EU
manufacturing industry. The European Commission has long recognized
the challenge of sustainable supply of mineral raw materials and steps are
implemented to try to deal with this. In this context, EuroGeoSurveys and
its Mineral Resources Expert Group (MREG) serve as one of the contact
points for the Commission. The MREG is a group of earth scientists, experts
in Economic Geology that act under the umbrella body of EuroGeoSurveys.
The group acts upon requests and queries received by member states and
the European Commission and provides the best available mineral expertise
and information based on the knowledge of member Geological Surveys,
for policy, communication, public awareness and education purposes
at European level.RESUMO: A Europa mostra um inevitável e acelerado aumento de consumo
de recursos minerais. A alta dependência das importações de matérias-primas
estratégicas e críticas tem um forte impacto na sustentabilidade da indústria
transformadora da UE. A produção de muitas matérias primas depende apenas
de alguns países. O desafio do abastecimento sustentável tem sido reconhecido pela Comissão Europeia e foram implementadas etapas no sentido de
tal ser assegurado. Neste contexto, o EuroGeoSurveys e o seu Grupo de
Peritos em Recursos Minerais (MREG) servem como um dos pontos de
contacto para a Comissão. O MREG dos EuroGeoSurveys é um grupo de
Geocientistas, especialistas em Geologia Económica que trabalham sobre
a tutela dos EuroGeoSurveys. O grupo atua com base em solicitações e
consultas recebidas pelos Estados membros e pela Comissão Europeia e
fornece os melhores conhecimentos e informações disponíveis com base
no conhecimento geológico dos recursos minerais, nas esferas da política,
e de comunicação, conscientização pública e educação a nível europeu.info:eu-repo/semantics/publishedVersio
muSR study of carbon-doped MgB2 superconductors
The evolution of the superconducting properties of the carbon-doped MgB2
superconductors, MgB(2-x)Cx (x= 0.02, 0.04, 0.06) have been investigated by the
transverse-field muon spin rotation (TF-muSR) technique. The low-temperature
depolarisation rate, sigma(0) at 0.6 T which is proportional to the second
moment of the field distribution of the vortex lattice decreases monotonically
with increasing electron doping and decreasing Tc. In addition, the temperature
dependence of sigma(T) has been analysed in terms of a two-gap model. The size
of the two superconducting gaps decreases linearly as the carbon content
increases, while the doping effect is more pronounced for the smaller gap
related to the 3D pi-sheets of the Fermi surface.Comment: 7 pages, 2 Figures, 1 Table, Europhys. Lett. in pres
Superconductivity: Exotic Commonalities in Phase and Mode
Recent muon and neutron experiments on the new FeAs-based superconductors
revealed phase diagrams characterized by first-order evolution from
antiferromagnetic to superconducting states, and an inelastic magnetic
resonance mode whose energy scales as . These features
exhibit striking commonalities with cuprate, backyball, organic, and
heavy-fermion superconductors as well as superfluid He.Comment: 4 pages, 2 figures, original version of News and Views for Nature
Materials submitted on Feb. 8, 2009; revised version will appear as Nature
Materials 8 (April, 2009) 253-25
B NMR study of pure and lightly carbon doped MgB superconductors
We report a B NMR line shape and spin-lattice relaxation rate
() study of pure and lightly carbon doped MgBC for
, 0.02, and 0.04, in the vortex state and in magnetic field of 23.5 kOe.
We show that while pure MgB exhibits the magnetic field distribution from
superposition of the normal and the Abrikosov state, slight replacement of
boron with carbon unveils the magnetic field distribution of the pure Abrikosov
state. This indicates a considerable increase of with carbon doping
with respect to pure MgB. The spin-lattice relaxation rate
demonstrates clearly the presence of a coherence peak right below in pure
MgB, followed by a typical BCS decrease on cooling. However, at
temperatures lower than K strong deviation from the BCS behavior is
observed, probably from residual contribution of the vortex dynamics. In the
carbon doped systems both the coherence peak and the BCS temperature dependence
of weaken, an effect attributed to the gradual shrinking of the
hole cylinders of the Fermi surface with electron doping.Comment: 8 pages, 6 figures, submitted to Phys. Rev.
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