Size and symmetry of the superconducting gap in the f.c.c. Cs3C60 polymorph close to the metal-Mott insulator boundary

作者：Potocnik Anton; Krajnc Andraz; Jeglic Peter; Takabayashi Yasuhiro; Ganin Alexey Y; Prassides Kosmas; Rosseinsky Matthew J; Arcon Denis^*

来源：Scientific Reports, 2014, 4(1): 4265.

DOI：10.1038/srep04265

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, Delta, 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.) Cs3C60 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) Delta with a large gap-to-superconducting transition temperature ratio, 2 Delta(0)/k(B)T(c) = 5.3(2) [Delta(0) = Delta(0 K)]. 2 Delta(0)/k(B)T(c) decreases continuously upon pressurization until it approaches a value of similar to 3.5, characteristic of weak-coupling BCS theory of superconductivity despite the dome-shaped dependence of T-c on interfullerene separation. The results indicate the importance of the electronic correlations for the pairing interaction as the metal/superconductor-insulator boundary is approached.

• 出版日期2014-3-3