Electron propagation in orientationally disordered fullerides

Abstract

We study the electronic spectrum for doped electronic states in the orientationally disordered M 3 C 60 fullerides. Momentum-resolved Green's functions are calculated within a cluster-Bethe-lattice model, and compared with results from calculations on periodically repeated supercells containing quenched orientational disorder. Despite the relatively strong scattering from orientational fluctuations, the electronic states near the Fermi energy are well described by propagating states characterized by an effective Bloch wave vector, and a mean free path ℓ ≈ 20Å. The effective Fermi surface is calculated in this model. This differs from that previously calculated for the orientationally ordered crystal, but is relatively well described within a disorder-averaged virtual-crystal Hamiltonian, which we derive. Typeset using REVT E X 1

Superconductivity in fullerides

“…At low temperatures, the C 60 molecules take one of two likely orientations in an essentially random way . This orientational disorder has a substantial effect on some of the electronic properties (Gelfand and Lu, 1992ab;Mele and Erwin, 1994), but has been neglected in most calculations. A certain short-range correlation of the orientations is expected on theoretical grounds (Gunnarsson et al, 1991;Mazin et al, 1993a, Erwin and and has been observed experimentally (Teslic et al, 1995).…”

“…Since the solution, however, depends on specific features of the Fermi surface, it is not clear if it survives for the disordered system. Mele et al (1994) studied the effects of orientational disorder. They found that this introduces some gap anisotropy.…”

“…The model of Mazin et al (1993b) may therefore not be applicable to A 3 C 60 , but it is still a nice illustration of the unconventional properties that can be obtained if bosons of very different energies couple to the electrons. Mele and Erwin (1993) considered the detailed Fermi surface for a system with ordered molecules. The studied an axial, nodeless order parameter with d-symmetry.…”

A survey of the Van Hove scenario for high-tc superconductivity with special emphasis on pseudogaps and striped phases

“…Finally, Figure 17 shows a 'disorder-averaged' Fermi surface for the fullerene superconductors, M 3 C 60 (M = K, Rb, Cs) [124]. For perfectly ordered buckyballs, the Fermi surfaces depend sensitively on the relative orientations of adjacent balls.…”

“…By averaging over the disorder, a residual, but smeared Fermi surface is found, which bears a striking resemblance to that of the cuprates. In fact, Mele and Erwin [124] have introduced a disorder-averaged virtual-crystal Hamiltonian model for the bands. They find that the bands reduce to three nearly independent 2D bands, which interact by level repulsion near the points where they intersect (the three bands differ in the choice of dispersing axes, x, y → x, y, x, z, or y, z).…”