Showing papers on "Charge transfer insulators published in 2005"

Hybrid LDA and generalized tight-binding method for electronic structure calculations of strongly correlated electron systems

Abstract: A novel hybrid scheme is proposed. The {\it ab initio} LDA calculation is used to construct the Wannier functions and obtain single electron and Coulomb parameters of the multiband Hubbard-type model. In strong correlation regime the electronic structure within multiband Hubbard model is calculated by the Generalized Tight-Binding (GTB) method, that combines the exact diagonalization of the model Hamiltonian for a small cluster (unit cell) with perturbation treatment of the intercluster hopping and interactions. For undoped La$_2$CuO$_4$ and Nd$_2$CuO$_4$ this scheme results in charge transfer insulators with correct values of gaps and dispersions of bands in agreement to the ARPES data.

Direct observation of excitons and a continuum of one-dimensional Mott insulators: a reflection-type third-harmonic-generation study of Ni-halogen chain compounds.

Abstract: The third-harmonic-generation (THG) spectrum was measured for a NiBr-chain compound, which is a one-dimensional Mott insulator, in a reflection configuration. A sharp peak and a shoulder structure in the THG spectrum are attributed to three-photon resonance to an exciton and a continuum, respectively. The band-edge energy, the exciton binding energy, and the spectral weights for the exciton and the continuum were determined from comparative studies of linear absorption, THG, and electroreflectance spectra. The excitonic effect is more pronounced in the NiCl chain than in the NiBr chain.

Single-particle spectra of charge-transfer insulators by cluster perturbation theory: The correlated band structure of NiO

Abstract: We propose a many-body method for band-structure calculations in strongly correlated electron systems and apply it to NiO. The method may be viewed as a translationally invariant version of the cluster method of Fujimori and Minami. Thereby the Coulomb interaction within the $d$-shells is treated by exact diagonalization and the $d$-shells then are coupled to a solid by an extension of the cluster perturbation theory due to Senechal et al. The method is computationally no more demanding than a conventional band structure calculation and for NiO we find good agreement between the calculated single particle spectral function and the experimentally measured band structure.

Mechanism of Ambipolar Field-Effect Carrier Injections in One-Dimensional Mott Insulators

Abstract: To clarify the mechanism of recently reported, ambipolar carrier injections into quasi-one-dimensional Mott insulators on which field-effect transistors are fabricated, we employ the one-dimensiona...

Suppression of Mott-Hubbard states and metal-insulator transitions in the two-band Hubbard model

Abstract: I investigate band and Mott insulating states in a two-band Hubbard model, with the aim of understanding the differences between the idealized one-orbital model and the more realistic multi-band case. Using a projection ansatz I show that additional orbitals suppress the metal–insulator transition, leading to a critical coupling of approximately eight times the bare bandwidth. I also demonstrate the effects of orbital ordering, which hinder Mott–Hubbard states and open a bandgap. Since multi-band correlations are common in real materials, this work suggests that very strongly correlated band insulators may be more common than Mott–Hubbard insulators.

Charge transfer by contact between metallic samples and insulating coatings

Abstract: The paper deals with experiments carried out in ambient air to study charge transfer in a quasi equilibrium state from metallic samples into insulating layers covering their surface. Experimental results concern the surface potential variations undergone when the thickness of these layers is increased. They show that below a critical thickness xcrit depending on the layers nature, the electrical behaviour of the layers is governed by a surface charge, and that beyond this thickness value, it becomes space charge dominated. A model fitting with these results is proposed.

Photoinduced dynamics and nonequilibrium characteristics in quasi-one-dimensional effectron systems: Mott insulators vs band insulators

Abstract: Effectron-effectron interactions play an important role in nonequilibrium properties of molecular materials. First, we show differences between photoinduced ionic-to-neutral and neutral-to-ionic transitions in quasi-one-dimensional extended Peierls Hubbard models with alternating potentials. Cooperative dynamics lead to nonlinear ionicity in the former, while uncooperative dynamics lead to quite linear ionicity in the latter, as a function of the energy supplied from the oscillating effectric field. Interchain effectron-effectron interactions bring about initial competition among metastable and stable domains in neighboring chains, slowing down the phase transition. Interchain elastic couplings are necessary to form a ferroeffectric long-range order. Second, we show differences between field-effect characteristics of Mott insulators and those of band insulators in one-dimensional Hubbard models, to which tight-binding models are attached for metallic effectrodes and scalar potentials are added for interfacial barriers. Ambipolar characteristics are found in the former, while unipolar characteristics generally appear in the latter. In the former, charge transport is cooperative so that the drain current is insensitive to the difference between the work function of the channel and that of the effectrodes, and thus insensitive to the polarity of the gate bias.

Localized Orbital Description of Electronic Structure

Abstract: We present a simple and general method for construction of localized orbitals to describe electronic structure of extended periodic metals and insulators as well as confined systems. Spatial decay of these orbitals is found to exhibit exponential behavior for insulators and power law for metals. While these orbitals provide a clear description of bonding, they can be also used to determine polarization of insulators. Within density functional theory, we illustrate applications of this method to crystalline Aluminium, Copper, Silicon, PbTiO$_3$ and molecules such as ethane and diborane.

Single-particle spectra of charge transfer insulators

Abstract: We propose a many-body method for band-structure calculations in strongly correlated electron systems and apply it to NiO. The method may be viewed as a translationally invariant version of the cluster method of Fujimori and Minami, wherein the Coulomb interaction within the d-shells is treated by exact diagonalization and the d-shells then are coupled to a solid by an extension of the cluster perturbation theory (CPT). The method is computationally no more demanding than a conventional band structure calculation and for NiO, we find good agreement between the calculated single-particle spectral function and the experimentally measured band structure.

Defect charge states for classical modeling of diffusion processes in insulators

Abstract: Some point defect properties can be accurately modeled by describing the ions classically, in terms of the shell model. This is particularly the case for point-defect diffusion in strongly ionic crystals. First, we discuss the issue of what ionic charge should be attributed to shell-model ions, including the possibility of partly covalent materials. We then discuss the issue of what defect charge states are likely to be of experimental interest and at the same time amenable to classical modeling. Finally, we discuss the rather common case where the defect charge is not well localized at a single ionic site, and where the electronic charge distribution of the defect and its neighbors will not remain fixed throughout a diffusion step.