An effective medium theory for studying the electronic structure of the orientationally disordered M3C60 fullerides is developed and applied to study various normal state properties. The theory is based on a cluster-Bethe-lattice method in which the disordered medium is modeled by a threeband Bethe lattice, into which we embed a molecular cluster whose scattering properties are treated exactly. Various single-particle properties and the frequency-dependent conductivity are calculated in this model, and comparison is made with numerical calculations for disordered lattices, and with experiment.PACS numbers: 71.25. Pi, 71.25.Tn, 74.70.Wz In the metallic fullerides M3C60 doped with the large alkali metals, the fullerene molecules are quenched into an orientationally disordered state [1]. In this state, each of the Ceo molecules is centered on the sites of an fee Bravais lattice and adopts a high symmetry setting, with the twofold symmetry axes aligned along [001] crystal directions. There are two inequivalent ways of achieving this orientation on each lattice site, and the intensities of x-ray reflections measured for these solids are well described by a merohedrally disordered structural model in which the choice of setting varies randomly from site to site in the solid [1]. It is now widely accepted that merohedral disorder is either a dominant or a contributing factor to a number of the observed electronic properties of these doped systems [2]. In this paper we apply an effective medium theory to study the conduction-electron states in these disordered solids.The amplitude for electronic hopping across bond r between neighboring fullerene sites in these systems can be represented by a real 3x3 matrix, TJ^r), in which each of the matrix elements corresponds to a possible choice of orbital polarization JJL(V) = x,y,z in the t\ u manifold on the initial and final sites [3][4][5][6]. Orientational disorder on the fullerene sites leads to a model with offdiagonal disorder in a Hamiltonian for an electron which carries with it an internal "orbital" degree of freedom, represented by a three component field. Gelfand and Lu [3] made the interesting observation that the sizes and signs of the various matrix elements in T can be changed by changing the relative molecular orientations on the terminal sites. This is a representation-dependent statement, since the signs and sizes of the various elements in T are also changed by any local rotation of the internal ti u orbital bases on either of the terminal sites. Of course, physically measurable quantities in this problem can always be expressed in a representation-independent (gauge invariant) manner that does not refer to a specific convention for defining the orbital polarizations on the various molecular sites. For example, the total density of states is obtained from a trace over the single-particle Green's function, N(E) --(l/7r)ImtrG+(i£), where the trace requires a sum over both sites and over the orbital degrees of freedom on a single site. If one considers a momen...

We calculate electrostatic multipole moments of C 60 up to l=18 using the quantum-mechanical charge distribution with icosahedral symmetry obtained from ab initio calculations. It is found that the second nonzero moment (l=10) is comparable to the first nonzero moment (l=6). The values of several low-order multipole moments are almost 10 times smaller than those found from the charge distribution of recently proposed potential models and thus the actual Coulomb interaction between C 60 molecules is much smaller than previously predicted. Much better agreement with calculated multipoles is obtained from a model which introduces point charges at the center of hexagonal and pentagonal plaquettes, following the physical arguments of David et al. [Nature 353, 147 (1991)]. We show that a multipole expansion including only l=6 and 10 moments can predict the potential due to a C 60 molecule at distances R≥2R 0 within an error of about 5%, where R 0 is the radius of the C 60 molecule. At distances less than R<3/2R 0 the multipole expansion is qualitatively incorrect even if one includes the terms up to l=18, indicating the importance of short-range quantum effects at these distances. The Coulomb interaction we obtain predicts two nearly degenerate, locally stable configurations for solid C 60 : (1) a metastable structure with Pa3 symmetry and setting angle φ=23.3°, close to experimentally observed value, and (2) a global minimum with the Pa3 structure but a setting angle φ=93.6°. We give physical arguments for expecting two such configurations and give a qualitative explanation for their near degeneracy. We conclude that a satisfactory intermolecular potential requires a first-principles calculation of the quantum-mechanical short-range repulsive interactions. Disciplines Physics | Quantum Physics

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

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