NMR study of the Superconducting gap variation near the Mott transition in Cs3_{3}C60_{60}

Former extensive studies of superconductivity in the \textit{A}$_{3}$C$_{60}$ 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 Cs$_{3}$C$_{60}$ which exhibit a transition with pressure from a Mott insulator (MI) to a superconducting (SC) state clearly re-opens that question. Using pressure ($p$) as a single control parameter of the C$_{60}$ balls lattice spacing, one can now study the progressive evolution of the SC properties when the electronic correlations are increased towards the critical pressure $p_{c}$ of the Mott transition. We have used $^{13}$C and $^{133}$Cs NMR measurements on the cubic phase A15-Cs$_{3}$C$_{60}$ just above $p_{c}=5.0(3)$ kbar, where the SC transition temperature $T_{c}$ displays a dome shape with decreasing cell volume. From the $T$ dependence below $T_{c}$ of the nuclear spin lattice relaxation rate $(T_{1})^{-1}$ we determine the electronic excitations in the SC state, that is $2\Delta$, the SC gap value. We find that $2\Delta$ increases with decreasing $p$ towards $p_{c}$, where $T_{c}$ decreases on the SC dome, so that $2\Delta /k_{B}T_{c}$ 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 $T_{c}$ 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

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

Mott Transition in the A15 Phase of Cs3C60 : Absence of a Pseudogap and Charge Order

We present a detailed NMR study of the insulator-to-metal transition induced by an applied pressure p in the A15 phase of Cs3C60. We evidence that the insulating antiferromagnetic (AFM) and superconducting (SC) phases coexist only in a narrow p range. At fixed p, in the metallic state above the SC transition T-c, the Cs-133 and C-13 NMR spin-lattice relaxation data are seemingly governed by a pseudogaplike feature. We prove that this feature, also seen in the (CsNMR)-Cs-133 shift data, is rather a signature of the Mott transition which broadens and smears out progressively for increasing (p,T). The analysis of the variation of the quadrupole splitting nu(Q) of the Cs-133 NMR spectrum precludes any cell symmetry change at the Mott transition and only monitors a weak variation of the lattice parameter. These results open an opportunity to consider theoretically the Mott transition in a multiorbital three-dimensional system well beyond its critical point.Peer reviewe

Superconducting fluctuations observed far above T_c in the isotropic superconductor K₃C₆₀

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$_\text{c}$. We report on the observation of a sizeable magneto-thermoelectric (Nernst) effect in the normal state of K$_3$C$_{60}$, 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$_\text{c}$ = 20K. At the lowest fields and close to T$_\text{c}$, 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