Pressure tuning of light-induced superconductivity in K3C60

Optical excitation at terahertz frequencies has emerged as an effective means to dynamically manipulate complex materials. In the molecular solid K₃C₆₀, short mid-infrared pulses transform the high-temperature metal into a non-equilibrium state with the optical properties of a superconductor. Here we tune this effect with hydrostatic pressure and find that the superconducting-like features gradually disappear at around 0.3 GPa. Reduction with pressure underscores the similarity with the equilibrium superconducting phase of K₃C₆₀, in which a larger electronic bandwidth induced by pressure is also 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.

NMR study of the superconducting gap variation near the Mott transition in Cs₃C₆₀

Former extensive studies of superconductivity in the A3C60 compounds, where A is an alkali metal, have led one to consider that Bardeen-Cooper-Schrieffer 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 Cs3C60 which exhibit a transition with pressure from a Mott insulator (MI) to a superconducting (SC) state clearly reopens that question. Using pressure (p) as a single control parameter of the C60 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 13C and 133Cs NMR measurements on the cubic phase A15-Cs3C60 just above p(c)=5.0(3) kbar, where the SC transition temperature Tc displays a dome shape with decreasing cell volume. From the T dependence below T(c) of the nuclear spin lattice relaxation rate (T1)(-1) we determine the electronic excitations in the SC state, that is 2Δ, the gap value. The latter is found to be largely enhanced with respect to the Bardeen-Cooper-Schrieffer value established in the case of dense A3C60 compounds. It even increases slightly with decreasing p towards p(c), where T(c) decreases on the SC dome, so that 2Δ/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 superconductivity. They rather suggest that repulsive electron interactions might even reinforce elecron-phonon superconductivity, being then partly responsible for the large T(c) values, as proposed by theoretical models taking the electronic correlations as a key ingredient.

NMR study of the Mott transitions to superconductivity in the two Cs3C60 phases

We report a NMR and magnetometry study on the expanded intercalated fulleride Cs3C60 in both its A15 and face centered cubic structures. NMR allowed us to evidence that both exhibit a first-order Mott transition to a superconducting state, occurring at distinct critical pressures p{c} and temperatures T{c}. Though the ground state magnetism of the Mott phases differs, their high T paramagnetic and superconducting properties are found similar, and the phase diagrams versus unit volume per C60 are superimposed. Thus, as expected for a strongly correlated system, the interball distance is the relevant parameter driving the electronic behavior and quantum transitions of these systems.

Superionic conductivity in the Li4C60 fulleride polymer

We report on the extraordinary superionic conductivity in the fulleride polymer Li4C60, a crystalline material with no disorder. 7Li, NMR, and dc frequency dependent conductivity show uncorrelated ionic hopping across small energy barriers (DeltaE_{a} approximately 200 meV) and an ionic conductivity of 10;{-2} S/cm at room temperature, higher than in "standard" ionic conductors. Ab initio calculations of the molecular structure find intrinsic unoccupied interstitial sites that can be filled by Li+ cations in stoichiometric Li4C60 even at low temperatures. The low energy required for the occupation of these sites allows a sizable Li+ diffusion above 130 K. The results suggest novel application of lithium intercalated fullerides as electrodes in Li ions batteries.