Quasiparticle bands in cuprates by quantum chemical methods: towards an ab initio description of strong electron correlations

Realistic electronic-structure calculations for correlated Mott insulators are notoriously hard. Here we present an ab initio multiconfiguration scheme that adequately describes strong correlation effects involving Cu 3d and O 2p electrons in layered cuprates. In particular, the O 2p states giving rise to the Zhang-Rice band are explicitly considered. Renormalization effects due to nonlocal spin interactions are also treated consistently. We show that the dispersion of the lowest band observed in photoemission is reproduced with quantitative accuracy. Additionally, the evolution of the Fermi surface with doping follows directly from our ab initio data. Our results thus open a new avenue for the first-principles investigation of the electronic structure of correlated Mott insulators

Near degeneracy and pseudo Jahn-Teller effects in mixed-valence ladders: the phase transition in NaV2_2O5_5

We analyze the electronic structure of a mixed-valence ladder system. We find that structural anisotropy and complex electron correlations lead to on-rung charge localization and insulating character. Charge fluctuations within the rung of the ladder interact strongly to the lattice degrees of freedom, which gives rise to large pseudo Jahn--Teller effects. The phase transition in NaV$_2$O$_5$ should be driven by this kind of mechanism.Comment: 5 pages, 2 figures, 1 table, submitted to PR

Ab initio determination of excitation energies and magnetic couplings in correlated, quasi two-dimensional iridates

To determine the strength of essential electronic and magnetic interactions in the iridates Sr$_2$IrO$_4$ and Ba$_2$IrO$_4$ - potential platforms for high-temperature superconductivity - we use many-body techniques from wavefunction-based electronic-structure theory. Multiplet physics, spin-orbit interactions, and Ir-O hybridization are all treated on equal footing, fully {\it ab initio}. Our calculations put the lowest d-d excitations of Sr$_2$IrO$_4$/Ba$_2$IrO$_4$ at 0.69/0.64 eV, substantially lower than in isostructural cuprates. Charge-transfer excitations start at 3.0/1.9 eV and the magnetic nearest-neighbor exchange coupling is 51/58 meV. Available experimental results are fully consistent with these values, which strongly constrains the parametrization of effective iridate Hamiltonians

Correlated electrons in Fe-As compounds: a quantum chemical perspective

State-of-the-art quantum chemical methods are applied to the study of the multiorbital correlated electronic structure of a Fe-As compound, the recently discovered LiFeAs. Our calculations predict a high-spin, S=2, ground-state configuration for the Fe ions, which shows that the on-site Coulomb interactions are substantial. Also, orbital degeneracy in the (xz,yz) sector and a three-quarter filling of these levels suggest the presence of strong fluctuations and are compatible with a low metallic conductivity in the normal state. The lowest electron-removal states have As 4p character, in analogy to the ligand hole states in p-type cuprate superconductors

Recording performances of perfectly aligned HSS heads

The recording characteristics of helical scan silicon (HSS) heads with perfect alignment of the magnetic poles were investigated. Their overall performances were found to be superior to those of ferrite MIG heads, especially the side writing. Erase bands of 0.25 /spl mu/m were measured using a HSS head and ME tape with 50 nm thick magnetic layer. Side writing and erasing were studied from the frequency response of microtracks, as well as from triple-track profiles

Correlation-induced corrections to the band structure of boron nitride: a wave-function-based approach

We present a systematic study of the correlation-induced corrections to the electronic band structure of zinc-blende BN. Our investigation employs an ab initio wave-function-based local Hamiltonian formalism which offers a rigorous approach to the calculation of the polarization and local charge redistribution effects around an extra electron or hole placed into the conduction or valence bands of semiconducting and insulating materials. Moreover, electron correlations beyond relaxation and polarization can be readily incorporated. The electron correlation treatment is performed on finite clusters. In conducting our study, we make use of localized Wannier functions and embedding potentials derived explicitly from prior periodic Hartree-Fock calculations. The on-site and nearest-neighbor charge relaxation bring corrections of several eV to the Hartree-Fock band gap. Additional corrections are caused by long-range polarization effects. In contrast, the dispersion of the Hartree-Fock bands is marginally affected by electron correlations. Our final result for the fundamental gap of zinc-blende BN compares well with that derived from soft x-ray experiments at the B and N K-edges.Comment: 18 pages, 8 figures; the following article has been submitted to J. Chem. Phy

Possible propagation of the Zhang-Rice singlet as a probable Cooper channel in the CuO2CuO_2 planes

The issue of how superconductivity originate in the $CuO_2$ planes believed to be crucial to understanding the high $T_c$ superconducting cuprates is still an going debate. In the wake of recent experimental observations of the the Zhang-Rice singlet (ZRS), its formation and propagation need to be revisited especially by using a simple approach almost at a phenomenological level. Within a highly simplified correlated variational approach (HSCVA) in this paper, a new formation of the ZRS as constituting the ground state of a single-band t-J model of the $CuO_2$ planes is developed. This formation is then used to demonstrate how the ZRS can be propagated as a probable Cooper channel in the $CuO_2$ planes.Comment: 10 page

Analysis of side writing asymmetry

The side writing asymmetry of a recording head was investigated using an overwrite configuration that enhances the edge effects. The track profiles of the overwrite patterns were measured for analyzing the side writing performance of the head. Magnetic force microscope (MFM) images of the overwrite patterns were studied using fast Fourier transforms (FFT), and they confirmed the profiling results. We measured weak edge effects at good pole alignment. The experiments were performed with metal evaporated (ME) and metal particle (MP) tapes having magnetic layers between 50 and 300 nm

Electron correlation effects in diamond: a wave-function quantum chemistry study of the quasiparticle band structure

The quasiparticle bands of diamond, a prototype covalent insulator, are herein studied by means of wave-function electronic-structure theory, with emphasis on the nature of the correlation hole around a bare particle. Short-range correlations are in such a system conveniently described by using a real-space representation and many-body techniques from {\it ab initio} quantum chemistry. To account for long-range polarization effects, on the other hand, we adopt the approximation of a dielectric continuum. Having as "uncorrelated" reference the Hartree-Fock band structure, the post-Hartree-Fock treatment is carried out in terms of localized Wannier functions derived from the Hartree-Fock solution. The computed correlation-induced corrections to the relevant real-space matrix elements are important and give rise to a strong reduction, in the range of $50\%$, of the initial Hartree-Fock gap. While our final results for the indirect and direct gaps, 5.4 and 6.9 eV, respectively, compare very well with the experimental data, the width of the valence band comes out by $10$ to $15\%$ too large as compared to experiment. This overestimation of the valence-band width appears to be related to size-consistency effects in the configuration-interaction correlation treatment.Comment: 16 pages, 7 figures, accepted at Phys. Rev. B (2014