We study a class of variational wave functions for strongly interacting one-dimensional lattice fermions in which correlations among the particles are specified by a single variational parameter. We find that the wave functions describe the ground-state properties of the one-dimensional t-J model remarkably well over the entire phase diagram in which interaction strength and density are varied. Specifically the wave function describes a Tomonaga-Luttinger liquid at low Jit, a phase-separated state at large Jit, and a stable state with infinite compressibility between these phases at low densities.

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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...

Atomic displacement-displacement correlation functions for the surface atoms of the Si(111)2X 1 and Si(001) 2X 1 surfaces are calculated using phonon energies and eigenvectors obtained from a tight-binding theory of the lattice dynamics of Si. The temperature dependence and the possible anisotropies of the amplitudes of vibration of the surface atoms are investigated. For the m-bonded chain model of Si(111) 2X1 it is found that the surface atoms have larger vibrational amplitudes perpendicular to the surface than parallel to it, and this anisotropy (1.8 in the mean-squared amplitudes at 270 K) increases rapidly with increasing temperature. This is attributed to the existence of a small-wavelength acoustic phonon with an unusually low energy. For the surface atoms on Si(001) 2X 1 the anisotropy between perpendicular and parallel vibrations is not as marked. The interatomic vibrational correlations between surface atoms reveal the strength of the new bonds that are formed in the reconstruction of these surfaces.

Rapid Communications are intended for the accelerated publication of important new results and are therefore given priority treatment both in the editorial office and in production A. Rapid Communication in Physical Review 8 should be no longer than 4 printed pages and must be accompanied by an abstract Pag.e proofs are sent to authorsWe study a class of variational wave functions for strongly correlated systems by expanding the electron operators as composites of spin-2 Fermi fields and spinless Fermi fields. The composite particles automatically satisfy the local constraint of no double occupancy and include correlations between opposite-spin particles in a very physical way. We calculate the energy and correlation functions for the one-dimensional