Showing papers on "Charge transfer insulators published in 2006"

Charge localization or itineracy at LaAlO3/SrTiO3 interfaces: Hole polarons, oxygen vacancies, and mobile electrons

Abstract: While correlated electron behavior is to be expected at oxide interfaces (IFs) involving Mott insulators, we show how strong correlations in the oxygen 2p states may be necessary to account for observed insulating behavior at charged (001)-IFs between the band insulators LaAl O3 and SrTi O3. Using correlated band theory applied to the O 2p states, an insulating p -type IF is obtained only when a disproportionated, charge-, orbital-, and spin-ordered O Pπ magnetic hole is formed, centered between Al3+ ions in the Al O2 layer at the IF. As an alternative explanation, charge compensation by oxygen vacancies that accommodate the holes as charge-conjugate F centers is modeled. For the n -type IF, a charge disproportionated Ti4+ + Ti3+ layer is obtained with ferromagnetic alignment of the spins resulting from occupied dxy orbitals at checkerboard arranged Ti3+ sites. Electron hopping on a 50\% occupied Ti sublattice (a quarter-filled band) and/or lattice relaxations are discussed as origin of the measured conductivity.

Localized orbital description of electronic structures of extended periodic metals, insulators, and confined systems: Density functional theory calculations

Abstract: We present a simple and general method for construction of localized orbitals to describe an 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, ${\mathrm{PbTiO}}_{3}$, and molecules, such as ethane and diborane.

Light absorption spectrum of two-dimensional Mott insulators

Abstract: We theoretically investigate the light absorption spectrum of two-dimensional (2D) Mott insulators. We employ the numerical diagonalization method to calculate the light absorption spectrum with using the effective Hamiltonian for the extended Hubbard model, which is valid when the on-site Coulomb interaction energy $U$ is much larger than the nearest-neighbor transfer integral $t$ and the nearest-neighbor Coulomb interaction energy $V$. For $V=0$, the absorption spectrum consists of a broad band with a width of about $16t$ and a sharp central peak, and the energy eigenstates contributing to the absorption spectrum do not have the antiferromagnetic (AF) spin order, when $U$ is sufficiently larger than $t$. These features result from the spin-charge interplay inherent in the very strong correlation region where charge transfer term is dominant. With decreasing $U∕t$, the peak structure in absorption spectrum becomes unclear and some low energy eigenstates have the AF spin order, as a result of the spin-spin interaction term. For large $V∕t$, the dominant peak arises in the lower energy region of the spectrum. A large number of holon-doublon bound states, which have nearly the same charge but different spin structures, are responsible for this peak, in contrast to the conventional exciton state. This is also in contrast to the charge bound states in one-dimensional (1D) Mott insulators, where a single energy eigenstate dominates the optical transition moment as a result of the spin-charge separation. The essentially different absorption spectra between the 1D and 2D Mott insulators originate from the difference in the coupling between spin and charge degrees of freedom.

Online detecting composite insulators by two dimensions electric field distribution

Abstract: Electric field method is one of the most efficient methods to find conductive defects on composite insulators. This method has already been used in many places including China. If there is a defect on an insulator, the distribution of the electric field along the insulator contrasts with that of a good insulator. The electric fields covered in this paper include that along the axis direction of the insulators, that on the radial direction of the insulators and the synthetic electric field. The characteristics and sensitivity of each kind of electric field distribution along the insulators were studied by experiments in the laboratory to detect defects at the high voltage part and the middle part of insulators. It was found that the electric field along the axis direction is more sensitive to defects at the high voltage part of the insulator and can find 5 cm defects when the insulator does not have a corona ring. The electric field in the radial direction can find 10 cm defects at the middle part and 5 cm defects at the high voltage part when the insulator has a corona ring.

Symmetry of photoexcited states and large-shift raman scattering in two-dimensional mott insulators

Abstract: Symmetry of photoexcited states with two photoinduced carriers in two-dimensional Mott insulators is examined by applying the numerically exact diagonalization method to finite-size clusters of a half-filled Hubbard model in the strong-coupling limit. The symmetry of minimum-energy bound state is found to be s -wave, which is different from a d x 2 - y 2 wave of a two-hole pair in doped Mott insulators. We demonstrate that the difference is originated from an exchange of fermions due to the motion of a doubly occupied site. Correspondingly large-shift Raman scattering across the Mott gap exhibits a minimum-energy excitation in the A 1 ( s -wave) channel. We discuss implications of the results for the Raman scattering and other optical experiments.

Scaling Relation of Two-Photon Absorption in Antiferromagnetic Insulators

Abstract: We discuss some properties of the nonlinear absorption spectrum of insulators with a gap originating from the electron correlation We formulate a method of evaluating the nonlinear susceptibility and apply this to the antiferromagnetic insulator A scaling relation for two-photon absorption is derived and applied to a quantitative estimation, which gives values comparable to those of experiments on Mott insulators The direct transition term is found to be dominant in many-body systems It is also shown that our formulation naturally includes the final-states interaction (the charge fluctuation and the excitonic effect), which gives a sufficient contribution to the two-photon absorption spectrum

High-Gradient Insulators

Abstract: High voltage systems operated in vacuum require insulating materials to maintain spacing between conductors held at different potentials. Traditional vacuum insulators consist of a single material, often machined with a 45deg angle to suppress surface flashover. However, insulating structures composed of alternating layers of dielectric and metal can also be built, and have been experimentally shown to have higher breakdown voltages than conventional insulators. These "high-gradient insulators" allow closer spacing of components in vacuum, and therefore; have application to a wide range of high-voltage vacuum systems where compact size is important. This paper describes ongoing simulations and experimental work on these structures, as well as the theoretical understanding driving this research

Introduction of Conductivity in Molecular Assemblies

Abstract: Diversity of electronic properties of molecular solids has been briefly summarized with special emphasis on the electrical conduction in the A2B type charge transfer salts. It has been demonstrated that electrons in these systems are generally strongly correlated and that the characteristic features are systematically understood based on the extended Hubbard model on the basis of molecular orbitals.