title: Size and symmetry of the superconducting gap in the f.c.c. Cs3C60 polymorph close to the metal-Mott insulator boundary.
abstract: The alkali fullerides, A(3)C(60) (A = alkali metal) are molecular superconductors that undergo a transition to a magnetic Mott-insulating state at large lattice parameters. However, although the size and the symmetry of the superconducting gap, Δ, are both crucial for the understanding of the pairing mechanism, they are currently unknown for superconducting fullerides close to the correlation-driven magnetic insulator. Here we report a comprehensive nuclear magnetic resonance (NMR) study of face-centred-cubic (f.c.c.) Cs(3)C(60) polymorph, which can be tuned continuously through the bandwidth-controlled Mott insulator-metal/superconductor transition by pressure. When superconductivity emerges from the insulating state at large interfullerene separations upon compression, we observe an isotropic (s-wave) Δ with a large gap-to-superconducting transition temperature ratio, 2Δ0/k(B)T(c) = 5.3(2) [Δ0 = Δ(0 K)]. 2Δ0/k(B)T(c) decreases continuously upon pressurization until it approaches a value of ~3.5, characteristic of weak-coupling BCS theory of superconductivity despite the dome-shaped dependence of Tc on interfullerene separation. The results indicate the importance of the electronic correlations for the pairing interaction as the metal/superconductor-insulator boundary is approached.
authors.affiliation: Jožef Stefan Institute, Jamova c. 39, SI-1000 Ljubljana, Slovenia.
authors.forename: Anton
authors.initials: A
authors.lastname: Potočnik
authors.affiliation: Jožef Stefan Institute, Jamova c. 39, SI-1000 Ljubljana, Slovenia.
authors.forename: Andraž
authors.initials: A
authors.lastname: Krajnc
authors.affiliation: 1] Jožef Stefan Institute, Jamova c. 39, SI-1000 Ljubljana, Slovenia [2] EN-FIST Centre of Excellence, Dunajska c. 156, SI-1000 Ljubljana, Slovenia.
authors.forename: Peter
authors.initials: P
authors.lastname: Jeglič
authors.affiliation: Department of Chemistry, Durham University, Durham DH1 3LE, UK.
authors.forename: Yasuhiro
authors.initials: Y
authors.lastname: Takabayashi
authors.affiliation: Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, UK.
authors.forename: Alexey Y
authors.initials: AY
authors.lastname: Ganin
authors.affiliation: 1] Department of Chemistry, Durham University, Durham DH1 3LE, UK [2] WPI Research Center, Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan.
authors.forename: Kosmas
authors.initials: K
authors.lastname: Prassides
authors.affiliation: Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, UK.
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Rosseinsky
authors.affiliation: 1] Jožef Stefan Institute, Jamova c. 39, SI-1000 Ljubljana, Slovenia [2] Faculty of mathematics and physics, University of Ljubljana, Jadranska c. 19, SI-1000 Ljubljana, Slovenia.
authors.forename: Denis
authors.initials: D
authors.lastname: Arčon