Unconventional high-Tc superconductivity in fullerides

Abstract

A3C60 molecular superconductors share a common electronic phase diagram with unconventional high-temperature superconductors such as the cuprates: superconductivity emerges from an antiferromagnetic strongly correlated Mott-insulating state upon tuning a parameter such as pressure (bandwidth control) accompanied by a dome-shaped dependence of the critical temperature, Tc However, unlike atom-based superconductors, the parent state from which superconductivity emerges solely by changing an electronic parameter-the overlap between the outer wave functions of the constituent molecules-is controlled by the C60 (3-) molecular electronic structure via the on-molecule Jahn-Teller effect influence of molecular geometry and spin state. Destruction of the parent Mott-Jahn-Teller state through chemical or physical pressurization yields an unconventional Jahn-Teller metal, where quasi-localized and itinerant electron behaviours coexist. Localized features gradually disappear with lattice contraction and conventional Fermi liquid behaviour is recovered. The nature of the underlying (correlated versus weak-coupling Bardeen-Cooper-Schrieffer theory) s-wave superconducting states mirrors the unconventional/conventional metal dichotomy: the highest superconducting critical temperature occurs at the crossover between Jahn-Teller and Fermi liquid metal when the Jahn-Teller distortion melts.This article is part of the themed issue 'Fullerenes: past, present and future, celebrating the 30th anniversary of Buckminster Fullerene'.

Keywords: Jahn–Teller effect; Mott insulator; antiferromagnetism; electron correlation; fullerenes; superconductivity.

Figure 1.

Crystalstructures of superconducting A₃C₆₀ (A = alkali metal) compositions. (a) Face-centred cubic (fcc) unit cell (space group ). Two orientations related by 44°23′ rotation about the [111] crystal axis exist in a disordered manner—only one of these is shown at each site for clarity. The A⁺ ions are depicted as red and green spheres to signify symmetry-inequivalent positions in the unit cell—octahedral and tetrahedral sites, respectively. (b) Primitive cubic unit cell (space group ). The C₆₀³⁻ anions are orientationally ordered and their orientations are optimized for an anticlockwise rotation about [111] of approximately 98°. Two crystallographically distinct A⁺ sites are present as in the fcc analogues. (c) A15 unit cell (space group ) based on body-centred cubic (bcc) anion packing. One unique orientation of the C₆₀³⁻ anions and a single crystallographically distinct A⁺ (tetrahedral) site are present.

Figure 2.

(a) Molecular orbital energy scheme for the C₆₀³⁻ anion and the corresponding valence and conduction bands for solid A₃C₆₀. (b) Variation of the superconducting transition temperature T_c with cubic lattice parameter a₀ for various compositions of fcc A₃C₆₀ and primitive cubic Na₂A′C₆₀ (A′=Rb, Cs). The dotted line is the T_c−a₀ relationship expected from BCS theory using N(ε_F) values obtained by LDA calculations, while the straight line is a guide to the eye. (Online version in colour.)

Figure 3.

(a) K⁺–NH₃ and K⁺–NH₂–CH₃ units occupying the octahedral sites of expanded orthorhombic fullerides with stoichiometries (NH₃)A₃C₆₀ and (CH₃NH₂)A₃C₆₀ (A = alkali metal), respectively. (b) Electronic phase diagram of fullerides including the superconducting T_c values for fcc-structured A₃C₆₀ superconductors (open circles) and the Néel T_N values for the fco-structured antiferromagnets, (NH₃)A₃C₆₀ (open/solid squares) and (CH₃NH₂)K₃C₆₀ (triangle). The LUMO schemes are illustrated for the cubic metallic/superconducting (retention of orbital degeneracy) and the orthorhombic AFM insulating (degeneracy-lifting, low-spin magnetic state) compositions. The shaded area denotes the metal–antiferromagnetic insulator boundary. (Online version in colour.)

Figure 4.

Superconducting transition temperature, T_c, as a function of pressure for (a) fcc- and (b) A15-structured Cs₃C₆₀ [40,42]. (c) T_c as a function of volume occupied per fulleride anion, V , at low temperature for selected fcc-structured Rb_xCs_3−xC₆₀ (0≤x≤2) compositions [43].

Figure 5.

Electronic phase diagram of A15 Cs₃C₆₀ showing the evolution of the Néel temperature T_N (squares) and T_c (circles) as a function of volume occupied per fulleride anion, V , at low temperature [42]. AFI, antiferromagnetic insulating state; SC, superconducting state.

Figure 6.

Molecular orbitals of C₆₀³⁻ for (a) undistorted I_h symmetry with an unsplit triply degenerate t_1u LUMO and (b) Jahn–Teller-distorted D_2h symmetry with threefold splitting (b_1u, b_2u and b_3u) of the LUMO [51]. (Online version in colour.)

Figure 7.

Globalelectronic phase diagram of fcc-structured Rb_xCs_3−xC₆₀ showing the evolution of T_c (ambient P, solid triangles; high P, unfilled triangles) and the MJT insulator-to-JT metal crossover temperature, T′ (synchrotron X-ray diffraction, squares; χ(T), stars; ¹³C, ⁸⁷Rb and ¹³³Cs NMR spectroscopy, hexagons with white, colour and black edges, respectively; IR spectroscopy, diamonds), as a function of V per C₆₀ [43]. Within the metallic (superconducting) regime, gradient shading from orange to green (purple) schematically illustrates the JT metal to conventional metal (unconventional to weak-coupling BCS conventional SC) crossover.

Figure 8.

Contrasting dependence of the normalized superconducting gap, 2Δ₀/k_BT_c, and of the superconducting transition temperature, T_c, on fulleride packing density, V [38,43,53]. (Online version in colour.)