301 research outputs found
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.
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
Spin Dynamics at the Mott Transition and in the Metallic State of the Cs_{3}C_{60} Superconducting Phases
We present here ^{13}C and ^{133}Cs NMR spin lattice relaxation T_{1} data in
the A15 and fcc-Cs_{3}C_{60} phases for increasing hydrostatic pressure through
the transition at p_{c} from a Mott insulator to a superconductor. We evidence
that for p>> p_{c} the (T_{1}T)^{-1} data above T_{c} display metallic like
Korringa constant values which match quantitatively previous data taken on
other A_{3}C_{60} compounds. However below the pressure for which T_{c} goes
through a maximum, (T_{1}T)^{-1} is markedly increased with respect to the
Korringa values expected in a simple BCS scenario. This points out the
importance of electronic correlations near the Mott transition. For p > p_{c}
singular T dependences of (T_{1}T)^{-1} are detected for T >> T_{c}. It will be
shown that they can be ascribed to a large variation with temperature of the
Mott transition pressure p_{c} towards a liquid-gas like critical point, as
found at high T for usual Mott transitions.Comment: 6 pages, 6 figures, submitted to EP
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
Pressure tuning of light-induced superconductivity in K3C60
Optical excitation at terahertz frequencies has emerged as an effective means
to manipulate complex solids dynamically. In the molecular solid K3C60,
coherent excitation of intramolecular vibrations was shown to transform the
high temperature metal into a non-equilibrium state with the optical
conductivity of a superconductor. Here we tune this effect with hydrostatic
pressure, and we find it to disappear around 0.3 GPa. Reduction with pressure
underscores the similarity with the equilibrium superconducting phase of K3C60,
in which a larger electronic bandwidth is detrimental for pairing. Crucially,
our observation excludes alternative interpretations based on a high-mobility
metallic phase. The pressure dependence also suggests that transient, incipient
superconductivity occurs far above the 150 K hypothesised previously, and
rather extends all the way to room temperature.Comment: 33 pages, 17 figures, 2 table
Epidemiology of tuberculosis in a low-incidence Italian region with high immigration rates: differences between not Italy-born and Italy-born TB cases
<p>Abstract</p> <p>Background</p> <p>Emilia Romagna, a northern Italian region, has a population of 4.27 million, of which 9.7% are immigrants. The objective of this study was to investigate the epidemiology of tuberculosis (TB) during the period 1996-2006 in not Italy-born compared to Italy-born cases.</p> <p>Methods</p> <p>Data was obtained from the Regional TB surveillance system, from where personal data, clinical features and risk factors of all notified TB cases were extracted.</p> <p>Results</p> <p>5377 TB cases were reported. The proportion of immigrants with TB, over the total number of TB cases had progressively increased over the years, from 19.1% to 53.3%. In the not Italy-born population, TB incidence was higher than in Italians (in 2006: 100.7 cases per 100 000 registered not Italy-born subjects and 83.9/100 000 adding 20% of estimated irregular presences to the denominators. TB incidence among Italians was 6.5/100 000 Italians). A progressive rise in the not Italy-born incident cases was observed but associated with a decline in TB incidence. Not Italy-born cases were younger compared to the Italy-born cases, and more frequently classified as "new cases" (OR 2.0 95%CI 1.61-2.49 for age group 20-39); 60.7% had pulmonary TB, 31.6% extra pulmonary and 7.6% disseminated TB. Risk factors for TB in this population group were connected to lower income status (homeless: OR 149.9 95%CI 20.7-1083.3 for age group 40-59).</p> <p>Conclusions</p> <p>In low-incidence regions, prevention and control of TB among sub-groups at risk such as the foreign-born population is a matter of public health concern. In addition, increasing immigration rates may affect TB epidemiology. TB among immigrants is characterized by particular clinical features and risk factors, which should be analyzed in order to plan effective action.</p
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. © BME-PT
Superconducting fluctuations observed far above T<sub>c</sub> in the isotropic superconductor K<sub>3</sub>C<sub>60</sub>
Alkali-doped fullerides are strongly correlated organic superconductors that exhibit high transition temperatures, exceptionally large critical magnetic fields and a number of other unusual properties. The proximity to a Mott insulating phase is thought to be a crucial ingredient of the underlying physics, and may also affect precursors of superconductivity in the normal state above T. We report on the observation of a sizeable magneto-thermoelectric (Nernst) effect in the normal state of KC, which displays the characteristics of superconducting fluctuations. The anomalous Nernst effect emerges from an ordinary quasiparticle background below a temperature of 80K, far above T = 20K. At the lowest fields and close to T, the scaling of the effect is captured by a model based on Gaussian fluctuations. The temperature up to which we observe fluctuations is exceptionally high for a three-dimensional isotropic system, where fluctuation effects are usually suppressed
Giant resonant enhancement for photo-induced superconductivity in KC
Photo-excitation at terahertz and mid-infrared frequencies has emerged as a
new way to manipulate functionalities in quantum materials, in some cases
creating non-equilibrium phases that have no equilibrium analogue. In
KC, a metastable zero-resistance phase was documented with optical
properties and pressure dependences compatible with non-equilibrium high
temperature superconductivity. Here, we report the discovery of a dominant
energy scale for this phenomenon, along with the demonstration of a giant
increase in photo-susceptibility near 10 THz excitation frequency. At these
drive frequencies a metastable superconducting-like phase is observed up to
room temperature for fluences as low as ~400 . These findings shed
light on the microscopic mechanism underlying photo-induced superconductivity.
They also trace a path towards steady state operation, currently limited by the
availability of a suitable high-repetition rate optical source at these
frequencies.Comment: 35 pages, 13 figures, including supplementar
Engineered yeast for enhanced CO2 mineralization
In this work, a biologically catalysed CO2 mineralization process for the capture of CO2 from point sources was designed, constructed at a laboratory scale, and, using standard chemical process scale-up protocols, was modelled and evaluated at an industrial scale. A yeast display system in Saccharomyces cerevisae was used to screen several carbonic anhydrase isoforms and mineralization peptides for their impact on CO2 hydration, CaCO3 mineralization, and particle settling rate. Enhanced rates for each of these steps in the CaCO3 mineralization process were confirmed using quantitative techniques in lab-scale measurements. The effect of these enhanced rates on the CO2 capture cost in an industrial scale CO2 mineralization process using coal fly ash as the CaO source was evaluated. The model predicts a process using bCA2-yeast and fly ash is [similar]10% more cost effective per tonne of CO2 captured than a process with no biological molecules, a savings not realized by wild-type yeast and high-temperature stable recombinant CA2 alone or in combination. The levelized cost of electricity for a power plant using this process was calculated and scenarios in which this process compares favourably to CO2 capture by MEA absorption process are presented.MIT Energy InitiativeEni S.p.A. (Firm)National Institutes of Health (U.S.) (NIH Biotechnology Training Program)Thomas and Stacey Siebel Foundatio
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