340 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
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
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.
Structure and dynamics of the fullerene polymer Li4 C60 studied with neutron scattering
The two-dimensional polymer structure and lattice dynamics of the superionic
conductor Li4 C60 are investigated by neutron diffraction and spectroscopy. The
peculiar bonding architecture of this compound is definitely confirmed through
the precise localisation of the carbon atoms involved in the intermolecular
bonds. The spectral features of this phase are revealed through ab-initio
lattice dynamics calculations and inelastic neutron scattering experiments. The
neutron observables are found to be in very good agreement with the simulations
which predict a partial charge transfer from the Li atoms to the C60 cage. The
absence of a well defined band associated to one category of the Li atoms in
the experimental spectrum suggests that this species is not ordered even at the
lowest temperatures. The calculations predict an unstable Li sublattice at a
temperature of 200 K, that we relate to the large ionic diffusivity of this
system. This specificity is discussed in terms of coupling between the low
frequency optic modes of the Li ions to the soft structure of the polymer.Comment: 29 pages, 13 Figure
Flexural and fracture behaviour of a cement-based material reinforced with GO nanoplates
In the present research work, the mechanical properties of a cement-based material reinforced with Graphene Oxide (GO) nanoplates are experimentally investigated. In particular, a detail experimental campaign, consisting of three-point bending tests on both unnotched and edge-notched specimens, is performed in order to determine flexural strength and fracture toughness. More precisely, the flexural strength is computed as a function of the experimental values of the peak load according to UNI EN Recommendation, whereas the fracture toughness is analytically determined according to the Modified Two-Parameter Mode
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
Zn-doped titania nanoparticles as building blocks for solid foam filters of water and air via photocatalytic oxidation
Photocatalytic oxidation (PCO) could provide energy-efficient purification of water and air. Its efficacy is constrained mainly by limited photocatalytic activity and active surface. To address both, solid foams with hierarchic porous structures spanning multiple length-scales, stabilized by photocatalytic Zn-doped titania nanoparticles (NP) were synthesized and tested. The NP were characterized by SEM, EDS, DLS, XRD, Raman and UV–Vis spectroscopies. Solid foams were stabilized by NP complexes with cationic surfactants. The foam morphology was characterized and photocatalytic activity was demonstrated in water. The present work paves the way for the development of efficient systems for air and water purification in demanding technological sectors, such as aerospace
Pathogen quantitation in complex matrices: a multi-operator comparison of DNA extraction methods with a novel assessment of PCR inhibition
This is the final version of the article. Available from the publisher via the DOI in this record.BACKGROUND: Mycobacterium bovis is the aetiological agent of bovine tuberculosis (bTB), an important recrudescent zoonosis, significantly increasing in British herds in recent years. Wildlife reservoirs have been identified for this disease but the mode of transmission to cattle remains unclear. There is evidence that viable M. bovis cells can survive in soil and faeces for over a year. METHODOLOGY/PRINCIPAL FINDINGS: We report a multi-operator blinded trial for a rigorous comparison of five DNA extraction methods from a variety of soil and faecal samples to assess recovery of M. bovis via real-time PCR detection. The methods included four commercial kits: the QIAamp Stool Mini kit with a pre-treatment step, the FastDNA® Spin kit, the UltraClean™ and PowerSoil™ soil kits and a published manual method based on phenol:chloroform purification, termed Griffiths. M. bovis BCG Pasteur spiked samples were extracted by four operators and evaluated using a specific real-time PCR assay. A novel inhibition control assay was used alongside spectrophotometric ratios to monitor the level of inhibitory compounds affecting PCR, DNA yield, and purity. There were statistically significant differences in M. bovis detection between methods of extraction and types of environmental samples; no significant differences were observed between operators. Processing times and costs were also evaluated. To improve M. bovis detection further, the two best performing methods, FastDNA® Spin kit and Griffiths, were optimised and the ABI TaqMan environmental PCR Master mix was adopted, leading to improved sensitivities. CONCLUSIONS: M. bovis was successfully detected in all environmental samples; DNA extraction using FastDNA® Spin kit was the most sensitive method with highest recoveries from all soil types tested. For troublesome faecal samples, we have used and recommend an improved assay based on a reduced volume, resulting in detection limits of 4.25×10(5) cells g(-1) using Griffiths and 4.25×10(6) cells g(-1) using FastDNA® Spin kit.This work was supported by Biotechnology and Biological Sciences Research Council BBSRCBB/E020925/1 grant to OC and EMHW, and Department for
Environment, Food and Rural Affairs, DEFRA SE3231 grant to EMHW and OC. The funders had no role in study design, data collection and analysis, decision to
publish, or preparation of the manuscript
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
An optically stimulated superconducting-like phase in K3C60 far above equilibrium Tc
The control of non-equilibrium phenomena in complex solids is an important
research frontier, encompassing new effects like light induced
superconductivity. Here, we show that coherent optical excitation of molecular
vibrations in the organic conductor K3C60 can induce a non-equilibrium state
with the optical properties of a superconductor. A transient gap in the real
part of the optical conductivity and a low-frequency divergence of the
imaginary part are measured for base temperatures far above equilibrium Tc=20
K. These findings underscore the role of coherent light fields in inducing
emergent order.Comment: 40 pages, 23 figure
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