Structural and electronic response of overexpanded superconducting fullerides close to the Mott insulator boundary

Abstract

The ternary fulleride, Rb[Formula: see text]Cs[Formula: see text]C[Formula: see text], is the most expanded member of the family of face-centered cubic (fcc) structured superconducting fullerides ever accessed with superconductivity surviving at ambient pressure closest to the Mott insulator boundary. Here, we study the evolution of its structural and electronic properties with temperature. At ambient temperature, Rb[Formula: see text]Cs[Formula: see text]C[Formula: see text] lies in the Mott–Jahn–Teller (MJT) insulating part of the phase diagram. High-resolution synchrotron X-ray diffraction shows that its structure remains strictly cubic at all temperatures, but the transition to the metallic state at [Formula: see text] 50 K — evident in the evolution of the magnetic susceptibility with temperature — is accompanied by a lattice collapse, [Formula: see text]V/V0 of [Formula: see text]. Bulk superconductivity then emerges on further cooling with a T[Formula: see text] of 25.9 K. The results permit the extension of the electronic phase diagram of A3C[Formula: see text] fullerides as close as possible to the metal–insulator (M–I) crossover.

Synthesis and structural analysis of nonstoichiometric ternary fulleride K1.5Ba0.25CsC60

“…Tc of 35 K at ~7 kbar [8] and fcc RbxCs3−xC60 (0.35 ≤ x ≤ 2) with Tc varies between 25.9 K and 32.9 K at ambient pressure [9]). Since then extensive research has been carried out on fcc A3C60 and AxCs3−xC60 fullerides to discover superconductors with higher Tc and understand their molecular electronic structure [8][9][10][11][12]. Current understanding proves that the A3C60 superconducting fullerides belong to the family of unconventional superconductivity where electron correlations play an important role for the pairing mechanism [8][9][10][11][13][14][15][16][17][18].…”

“…Since then extensive research has been carried out on fcc A3C60 and AxCs3−xC60 fullerides to discover superconductors with higher Tc and understand their molecular electronic structure [8][9][10][11][12]. Current understanding proves that the A3C60 superconducting fullerides belong to the family of unconventional superconductivity where electron correlations play an important role for the pairing mechanism [8][9][10][11][13][14][15][16][17][18]. Besides that, molecular electronic structure [9,19,20] and cation specific effects [12] are crucially important in producing the unconventional superconductivity in the A3C60 family.…”

Caged-Electron States in Endohedral Li Fullerenes

“…The already wide range of applicability of fullerenes has dramatically further grown since the discovery of endohedral fullerenes. − Owing to their broader range of electronic properties, they may even play a more important role than their parent fullerenes and exhibit electronic phenomena that the parent fullerenes cannot have, such as, for instance, interatomic Coulombic decay . Among the many applications of fullerenes and their derivatives, we just briefly mention organic solar cells, − supercapacitors, − catalyzers, − and superconductive agents. − …”