Title: Interfaces in coexisting metals and Mott insulators

The NiS{sub 2{minus}x}Se{sub x} system represents one of the best examples of a Mott-Hubbard system, i.e., a system in which, under appropriate conditions of concentration, temperature, or pressure, a metal-insulator transition driven by electron-electron interaction takes place. Here, the metallic phase is either antiferromagnetic (for 0.44 {le} x {le} 1) or paramagnetic (for x {ge} 1), whereas the insulating phase is as a rule antiferromagnetic (including the spin-canted phases). In this paper the authors review both the physical properties and outline the basic features of the theoretical approach to those correlated electron systems. Emphasis is placed on a qualitative understandingmore » of the observed transformation of the system from semiconductor (or magnetic insulator) to metal.« less

Mott insulators are identified here with ordinary magnetic insulators. The insulating gap, local moment, and effective spin hamiltonian aspects are qualitatively explained by means of a novel set of solutions of the Hartree-Fock equations. The apparent conflict between Bloch's theorem and localized-electron phenomenology is thereby resolved in an elementary manner. This Hartree-Fock approach also sheds considerable light on the physical mechanisms responsible for the associated metal-insulator (Mott) and other related phase transitions, as observed in V/sub 2/O/sub 3/ and several other materials. With some generalizations and refinements, this theoretical picture is shown to also account semiquantitatively for a number ofmore » detailed properties of NiO and CoO, two of the most extensively studied Mott insulator materials. A wide variety of experimental data for NiO is surveyed in order to determine reasonable values for its effective Hubbard hamiltonian parameters, suitably generalized for the 3d electrons. The problems of formally deriving effective spin hamiltonians for macroscopic magnetic insulator systems are also carefully examined. The old nonorthogonality catastrophe is fully resolved by means of a degenerate (open-shell) analogue of the linked cluster perturbation expansion of Brueckner and Goldstone. Although many quantitative issues remain, these results indicate that there is now a reasonably adequate conceptual understanding of the Mott insulating state.« less

One of the unresolved issues in the understanding of the Cuprate Superconductors is the theory of their unusual normal state properties. One of the more promising developments in their understanding is the gauge model of doped Mott Insulators where the charge degrees of freedom are coupled to the chiral fluctuations of the spins. Here the authors study the consequences of this coupling via numerical tests to determine whether it can account for anomalous properties such as the diamagnetic response and the temperature dependence of the Hall coefficient.