Showing papers on "Charge transfer insulators published in 2004"

Orbital ordering in charge transfer insulators

Abstract: We discuss a new mechanism of orbital ordering, which in charge transfer insulators is more important than the usual exchange interactions and which can make the very type of the ground state of a charge transfer insulator, i.e., its orbital and magnetic ordering, different from that of a Mott-Hubbard insulator. This purely electronic mechanism allows us to explain why orbitals in Jahn-Teller materials typically order at higher temperatures than spins, and to understand the type of orbital ordering in a number of materials, e.g., K2CuF4, without invoking the electron-lattice interaction.

Electronic structure of half-metallic ferromagnets and spinel ferromagnetic insulators

Abstract: We discuss an application of the self-interaction-corrected local spin density (SIC-LSD) approximation to study electronic structure of some half-metallic ferromagnets and ferromagnetic insulators of current interest in spintronics. Both d- and f-electron materials are considered, and we concentrate on the nominal valence and ground state properties of these systems.

Quantum phases and phase transitions of Mott insulators

Abstract: This article contains a theoretical overview of the physical properties of antiferromagnetic Mott insulators in spatial dimensions greater than one. Many such materials have been experimentally studied in the past decade and a half, and we make contact with these studies. Mott insulators in the simplest class have an even number of S=1/2 spins per unit cell, and these can be described with quantitative accuracy by the bond operator method: we discuss their spin gap and magnetically ordered states, and the transitions between them driven by pressure or an applied magnetic field. The case of an odd number of S=1/2 spins per unit cell is more subtle: here the spin gap state can spontaneously develop bond order (so the ground state again has an even number of S=1/2 spins per unit cell), and/or acquire topological order and fractionalized excitations. We describe the conditions under which such spin gap states can form, and survey recent theories of the quantum phase transitions among these states and magnetically ordered states. We describe the breakdown of the Landau-Ginzburg-Wilson paradigm at these quantum critical points, accompanied by the appearance of emergent gauge excitations.

Charge transport of electron-doped Mott insulators on a triangular lattice

Abstract: The charge transport of electron doped Mott insulators on a triangular lattice is investigated within the t-J model based on the partial charge-spin separation fermion-spin theory. The conductivity spectrum shows a low-energy peak and the unusual midinfrared band, while the resistivity is characterized by a crossover from the high temperature metallic-like to the low temperature insulating-like behavior, in qualitative agreement with experiments. Our results also show that the mechanism that cause this unusual charge transport is closely related with a competition between the kinetic energy and magnetic energy in the system.

Absence of asymptotic freedom in doped mott insulators: breakdown of strong coupling expansions.

Abstract: We show that doped Mott insulators such as the copper-oxide superconductors are asymptotically slaved in that the quasiparticle weight $Z$ near half-filling depends critically on the existence of the high-energy scale set by the upper Hubbard band. In particular, near half-filling, the following dichotomy arises: $Z\ensuremath{ e}0$ when the high-energy scale is integrated out but $Z=0$ in the thermodynamic limit when it is retained. Slavery to the high-energy scale arises from quantum interference between electronic excitations across the Mott gap. Broad spectral features seen in photoemission in the normal state of the cuprates are argued to arise from high-energy slavery.

Charge-density distributions in doped antiferromagnetic insulators

Abstract: We consider the form of the charge-density nanoscale configurations in underdoped states of planar antiferromagnetic insulators in the framework of a soft variant of Faddeev-Niemi model. It is shown that there is such a level of doping and a temperature range, where charge-density distributions in the form of rings appear to be more preferable than open distributions in the form of stripes.

Quantum effects on charge order

Abstract: Charge ordering in strongly correlated 1/4-filled systems, such as charge transfer organic conductors and transition metal oxides, is discussed from a theoretical point of view. Strong electron-electron Coulomb interaction especially that between neighboring molecules induces Wigner crystal-type charge order, which competes with another insulating state due to strong correlation in these systems, i.e., the dimer Mott insulating state. Based on such idea, new aspects of the charge ordering phenomenon are pointed out, where full quantum mechanical treatments are required. One case is the interplay between electron-phonon interaction leading to a novel co-existence of charge order and spin-Peierls lattice distortion, and the other is the effect of geometrical frustration resulting in a quantum melting of the charge order.

Orbital ordering in charge transfer insulators

Abstract: We discuss a new mechanism of orbital ordering, which in charge transfer insulators is more important than the usual exchange interactions and which can make the very type of the ground state of a charge transfer insulator, i.e., its orbital and magnetic ordering, different from that of a Mott-Hubbard insulator. This purely electronic mechanism allows us to explain why orbitals in Jahn-Teller materials typically order at higher temperatures than spins, and to understand the type of orbital ordering in a number of materials, e.g., K2CuF4, without invoking the electron-lattice interaction.