David Logan

Abstract:

A theory of the dipolar Frenkel excitonic insulator (EI) phase developed in an earlier paper is extended to spatially disordered systems. Using the Hartree approximation studied previously, the authors derive, for a given atomic centre-of-mass configuration, a self-consistency equation for the atomic dipole moments, a non-zero solution to which indicates an EI phase. They obtain as a special case the microscopic Yvon-Kirkwood equations of classical dielectric theory. For the experimentally relevant case of an impurity at infinite dilution in a solvent or disordered matrix, they derive an explicit expression for the impurity dipole moment. To take into account the ensemble of atomic configurations a mean field approximation is developed, numerical results for which, within the class of linear approximations of classical liquid state theory, will be given in a subsequent paper. The authors also examine the dynamic response of the impurity system to an oscillating electric field. They locate the lowest excited state of the system in both the normal insulating and dipolar EI phases, and show that it is degenerate with the ground state at the EI transition, thus making contact with exciton theories of the EI phase.

Abstract:

In a previous paper the authors have developed a mean field theory for the dipolar Frenkel excitonic insulator (EI) phase of an impurity at infinite dilution in a liquid solvent or disordered matrix, a situation of experimental relevance. Based on this, they here present numerical results for the location of the EI transition, and the impurity dipole moment in the dipolar EI phase, using linear classical liquid state theories. For a non-polar polarizable solvent, the authors consider impurity and solvent atoms of (i) identical hard-sphere diameter, and (ii) differing hard-sphere diameter. They also generalize the previously studied model to allow for dipolar polarizable solvent molecules, and present example results. The authors consider in particular the case of alkali metal atoms dissolved in methylamine, and conclude that a dipolar EI phase is possible for Li and Cs.

Abstract:

To obtain a broad understanding of the combined effects of disorder and electron interactions, the phase diagram of a half-filled Gaussian site-disordered Anderson-Hubbard model is assessed numerically on a simple cubic lattice, at the unrestricted Hartree-Fock mean field level. Metallic/insulating and magnetic phases are considered. Paramagnetic, disordered antiferromagnetic and spin glass magnetic phases are found. Particular attention is given to obtaining a microscopic picture of the interplay between disorder and interaction-induced local moment formation, which underlies the metal-(gapless) insulator transition. © 1993.

Abstract:

The pseudorotating clusters, Li5 and K7, have been identified in an adamantane matrix by EPR spectroscopy. These new clusters were synthesised by depositing the parent alkali metal atoms at 77 K in the hydrocarbon matrix followed by careful annealing to temperatures above 200 K. © 1993.

Abstract:

We consider a numerical mean-field study, at the unrestricted Hartree-Fock level, of a Gaussian site-disordered Anderson-Hubbard model on a simple cubic lattice. The phase diagram at half filling is obtained, including magnetic and metallic-insulating phases, and all relevant phase boundaries are found to occur in a relatively weak-coupling regime. Variation with filling fraction y is also considered, with particular reference to the y-differential disorder-induced enhancement of electron interactions that lead to site-differential local-moment formation. The inhomogeneous distribution of local charges and magnetic moments over the sites is found to influence strongly the distribution and character of pseudoparticle states. This enables a simple microscopic rationalization of the metal-insulator transition and its phase boundaries in terms of an interplay between disorder and electron interactions, giving in addition a natural explanation for the two-fluid-like coexistence of delocalized charge-carrying states and strong-local-moment sites. © 1993 The American Physical Society.