126 research outputs found
NMR study of the Superconducting gap variation near the Mott transition in CsC
Former extensive studies of superconductivity in the \textit{A}C
compounds, where \textit{A} is an alkali, have led to consider that Bardeen
Cooper Schrieffer (BCS) electron-phonon pairing prevails in those compounds,
though the incidence of electronic Coulomb repulsion has been highly debated.
The discovery of two isomeric fulleride compounds CsC which
exhibit a transition with pressure from a Mott insulator (MI) to a
superconducting (SC) state clearly re-opens that question. Using pressure ()
as a single control parameter of the C balls lattice spacing, one can
now study the progressive evolution of the SC properties when the electronic
correlations are increased towards the critical pressure of the Mott
transition. We have used C and Cs NMR measurements on the cubic
phase A15-CsC just above kbar, where the SC
transition temperature displays a dome shape with decreasing cell
volume. From the dependence below of the nuclear spin lattice
relaxation rate we determine the electronic excitations in the
SC state, that is , the SC gap value. We find that
increases with decreasing towards , where decreases on the
SC dome, so that increases regularly upon approaching the
Mott transition. These results bring clear evidence that the increasing
correlations near the Mott transition are not significantly detrimental to SC.
They rather suggest that repulsive electron interactions might even reinforce
elecron-phonon SC, being then partly responsible for the large values,
as proposed by theoretical models taking the electronic correlations as a key
ingredient.Comment: 5 pages, 4 figures, Supplemental Materia
Unusual polymerization in the Li4C60 fulleride
Li4C60, one of the best representatives of lithium intercalated fullerides,
features a novel type of 2D polymerization. Extensive investigations, including
laboratory x-ray and synchrotron radiation diffraction, 13C NMR, MAS and Raman
spectroscopy, show a monoclinic I2/m structure, characterized by chains of
[2+2]-cycloaddicted fullerenes, sideways connected by single C-C bonds. This
leads to the formation of polymeric layers, whose insulating nature, deduced
from the NMR and Raman spectra, denotes the complete localization of the
electrons involved in the covalent bonds.Comment: 7 pages, 6 figures, RevTex4, submitted to Phys. Rev.
Recovering Metallicity in A4C60: The Case of Monomeric Li4C60
The restoration of metallicity in the high-temperature, cubic phase of Li4C60
represents a remarkable feature for a member of the A4C60 family (A = alkali
metal), invariably found to be insulators. Structural and resonance technique
investigations on Li4C60 at T > 600 K, show that its fcc structure is
associated with a complete (4e) charge transfer to C60 and a sparsely populated
Fermi level. These findings not only emphasize the crucial role played by
lattice symmetry in fulleride transport properties, but also re-dimension the
role of Jahn-Teller effects in band structure determination. Moreover, they
suggest the present system as a potential precursor to a new class of
superconducting fullerides.Comment: 4 pages, 3 figure
Interactive effects between carbon allotrope fillers on the mechanical reinforcement of polyisoprene based nanocomposites
Interactive effects of carbon allotropes on the mechanical reinforcement of polymer nanocomposites were investigated. Carbon nanotubes (CNT) and nano-graphite with high shape anisotropy (nanoG) were melt blended with poly(1,4- cis-isoprene), as the only fillers or in combination with carbon black (CB), measuring the shear modulus at low strain amplitudes for peroxide crosslinked composites. The nanofiller was found to increase the low amplitude storage modulus of the matrix, with or without CB, by a factor depending on nanofiller type and content. This factor, fingerprint of the nanofiller, was higher for CNT than for nanoG. The filler-polymer interfacial area was able to correlate modulus data of composites with CNT, CB and with the hybrid filler system, leading to the construction of a common master curve. Š BME-PT
Persistence of antiferromagnetic order upon La substitution in the Mott insulator CaRuO
The chemical and magnetic structures of the series of compounds
CaLaRuO [, , , ] have been
investigated using neutron diffraction and resonant elastic x-ray scattering.
Upon La doping, the low temperature S-Pbca space group of the parent compound
is retained in all insulating samples [], but with significant
changes to the atomic positions within the unit cell. These changes can be
characterised in terms of the local RuO octahedral coordination: with
increasing doping the structure, crudely speaking, evolves from an orthorhombic
unit cell with compressed octahedra to a quasi-tetragonal unit cell with
elongated ones. The magnetic structure on the other hand, is found to be
robust, with the basic , -axis antiferromagnetic order of the
parent compound preserved below the critical La doping concentration of
. The only effects of La doping on the magnetic structure are to
suppress the A-centred mode, favouring the B mode instead, and to reduce the
N\'{e}el temperature somewhat. Our results are discussed with reference to
previous experimental reports on the effects of cation substitution on the
Mott insulator CaRuO, as well as with regard to theoretical
studies on the evolution of its electronic and magnetic structure. In
particular, our results rule out the presence of a proposed ferromagnetic
phase, and suggest that the structural effects associated with La substitution
play an important role in the physics of the system.Comment: 10 pages, 9 figure
Observation of Large Topologically Trivial Fermi-Arcs in the Candidate Type-II Weyl Semimetal WTe2
We report angle-resolved photoemission experiments resolving the distinct
electronic structure of the inequivalent top and bottom (001) surfaces of WTe2.
On both surfaces, we identify a surface state that forms a large Fermi-arc
emerging out of the bulk electron pocket. Using surface electronic structure
calculations, we show that these Fermi arcs are topologically trivial and that
their existence is independent of the presence of type-II Weyl points in the
bulk band structure. This implies that the observation of surface Fermi arcs
alone does not allow the identification of WTe2 as a topological Weyl
semimetal. We further use the identification of the two different surfaces to
clarify the number of Fermi surface sheets in WTe2.Comment: Accepted in Physical Review B Rapid Communication on 16 Aug 201
Predicting needlestick and sharps injuries in nursing students: Development of the SNNIP scale
Aim: To develop an instrument to investigate knowledge and predictive factors of needlestick and sharps injuries (NSIs) in nursing students during clinical placements. Design: Instrument development and cross-sectional study for psychometric testing. Methods: A self-administered instrument including demographic data, injury epidemiology and predictive factors of NSIs was developed between October 2018âJanuary 2019. Content validity was assessed by a panel of experts. The instrument's factor structure and discriminant validity were explored using principal components analysis. The STROBE guidelines were followed. Results: Evidence of content validity was found (S-CVI 0.75; I-CVI 0.50â1.00). A three-factor structure was shown by exploratory factor analysis. Of the 238 participants, 39% had been injured at least once, of which 67.3% in the second year. Higher perceptions of âpersonal exposureâ (4.06, SD 3.78) were reported by third-year students. Higher scores for âperceived benefitsâ of preventive behaviours (13.6, SD 1.46) were reported by second-year students
A tentative application of the energy separation principle to the determination of the fracture resistance (JIc) of rubbers
In this work an innovative fracture mechanics experimental methodology for J testing of rubbers was investigated and tentatively applied. This single-specimen approach, based on the energy separation principle, would allow to identify the point of fracture initiation without any visual observation of the test, and to evaluate the material resistance to crack initiation (JIc). For comparison, fracture initiation was detected also by a methodology based on the visual observation of the crack growth by a camera. Although both methodologies evidenced that fracture initiation in rubbers is a progressive process, the energy separation based approach results to be a promising methodology
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