Interplay between magnetism and superconductivity near the pressure induced Mott
transition in the A15-Cs3C60 compound.
H. Alloul Laboratoire de Physique des Solides, Université Paris sud Orsay(France)
In many families of compounds in which electronic correlations (EC) are of importance, the phase diagrams
exhibit magnetic phases proximate with a SC phase. It is often thought that the ECs are at the origin of thesuperconducting pairing. We have addressed the specific case of the A3C60 compounds, where A is analkali metal. Initial investigations have led one to consider that a BCS electron-phonon mechanism prevails,
with a pairing mediated by on ball optical phonon modes. However further detailed studies of A3C60
compounds gave evidences that their electronic properties cannot be explained by a simple band filling of
the C60 lowest unoccupied molecular level and it became clear that the influence of ECs generated by
Jahn-Teller Distortions (JTD) of the C60 balls should play a major role in superconductivity. The debate on thisincidence of ECs and JTD, which has been anticipated from DMFT calculations has been reopened by thediscovery of two Cs3C60 isomeric phases which present the most expanded lattice in the A3C60 family. Those
exhibit an insulator to SC transition with increasing pressure p from a Mott insulator (MI) at ambient p with adome shaped SC range.
Our NMR experiments under pressure, where p is used as a single control parameter of the C60 balls latticespacing, allowed us to probe the evolution of the SC properties when the ECs are progressively increased
toward the MI phase [1]. We have shown that the SC gap increases slightly with decreasing p towards p0,
while Tc decreases on the SC dome, so that upon approaching the Mott transition 2Δ/kBTc increases
regularly with respect to the BCS value observed in dense A3C60 compounds. A recent detailed NMR study
in the vicinity of the Mott transition [2] allowed us to evidence that, in the A15 phase (Fig.1), the insulating AF
and the SC phases coexist only in a narrow p range around p0=5.0 kbar. This establishes the first order andgenuine Mott character of the transition. We show that the step like behavior seen on the 133Cs or 13C NMR
spin-lattice relaxation rate and on the 133Cs NMR shift when decreasing pressure is the signature of arecovery of the insulating behavior and not a pseudogap like feature. Our data has led us to establish the(p,T) phase diagram presented in Fig. 2, with a critical point at pc~ 7kbar similar to that known for the liquidvapour transition.
A detailed analysis of the variation of the quadrupole split 133Cs NMR spectrum allowed us to preclude any
cell symmetry change at the 3D Mott transition. We anticipate that the slight change of the quadrupole
frequency Q detected at the transition can mostly be assigned to a variation of the cubic lattice
parameter as occurs due to a small lattice expansion when entering the insulating phase. An importantconclusion from this study is that the normal state properties do not exhibit the pseudogap phase which isquite characteristic of the cuprate phase diagram in which the insulator metal transition is controlled byhole doping.
These ensemble of results bring clear evidence that the increasing ECs near the 3D Mott transition are notsignificantly detrimental to superconductivity. They rather suggest that repulsive electron interactions mighteven reinforce electron-phonon superconductivity, being then partly responsible for the large Tc values, asproposed by theoretical models taking the ECs as a key ingredient.

Fig. 1: Unit cell structure of the A15-Cs3C60 phase. Here the body centred cell of C60 balls is completed by pairs ofCs atoms lying on the cube faces which are organized in chains in the three lattice directions.
Fig. 2: The (p,T) phase diagram of A15-Cs3C60 displays at low T a narrow first order transition at p0 between a Néel
AF and a SC state. At higher pressures the thermal expansion at fixed p induces a recovery of the insulating
paramagnetic state through a transition which broadens with increasing p up to a critical point pc ~7 kbar. (seePRL 118, 237601 (2017) for details.
(1) P. Wzietek, T. Mito, H. Alloul, D. Pontiroli, M. Aramini, and M. Ricc`o; PRL 112, 066401 (2014).
(2) H. Alloul, P. Wzietek, T. Mito, D. Pontiroli, M. Aramini, M. Ricc`o, J.P. Itie, and E. Elkaim, PRL 118, 237601 (2017).

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