Investigation of the microscopic behavior of Mott insulators by means of the density functional theory and many-body methods

The objective of this work is twofold. First, we explore the performance of the density functional theory (DFT) when it is applied to solids with strong electronic correlations, such as transition metal compounds. Along this direction, particular effort is put into the refinement and development of parameterization techniques for deriving effective models on a basis of DFT calculations. Second, within the framework of the DFT, we address a number of questions related to the physics of Mott insulators, such as magnetic frustration and electron-phonon coupling (Cs2CuCl4 and Cs2CuBr4), high-temperature superconductivity (BSCCO) and doping of Mott insulators (TiOCl). In the frustrated antiferromagnets Cs2CuCl4 and Cs2CuBr4, we investigate the interplay between strong electronic correlations and magnetism on one hand and electron-lattice coupling on the other as well as the effect of this interplay on the microscopic model parameters. Another object of our investigations is the oxygen-doped cuprate superconductor BSCCO, where nano-scale electronic inhomogeneities have been observed in scanning tunneling spectroscopy experiments. By means of DFT and many-body calculations, we analyze the connection between the structural and electronic inhomogeneities and the superconducting properties of BSCCO. We use the DFT and molecular dynamic simulations to explain the microscopic origin of the persisting under doping Mott insulating state in the layered compound TiOCl

Correlation induced electron-hole asymmetry in quasi-2D iridates

We determine the motion of a charge (hole or electron) added to the Mott insulating, antiferromagnetic (AF) ground-state of quasi-2D iridates such as Ba 2 IrO 4 or Sr 2 IrO 4 . We show that correlation effects, calculated within the self-consistent Born approximation, render the hole and electron case very different. An added electron forms a spin-polaron, which closely resembles the well-known cuprates, but the situation of a removed electron is far more complex. Many-body 5d 4 configurations form which can be singlet and triplets of total angular momentum J and strongly affect the hole motion between AF sublattices. This not only has important ramifications for the interpretation of (inverse-)photoemission experiments of quasi-2D iridates but also demonstrates that the correlation physics in electron- and hole-doped iridates is fundamentally different.Comment: 11 pages main text, 11 pages supplementary, 10 figure

Distinct electridelike nature of infinite-layer nickelates and the resulting theoretical challenges to calculate their electronic structure

We demonstrate in this paper that the recently discovered infinite-layer (IL) nickelates have much in common with a class of materials known as electrides. Oxide based electrides are compounds in which topotactic removal of loosely bound oxygens leaves behind voids with a landscape of attractive potentials for electrons. We show that this is also what happens in the IL nickelates, where one of the two electrons (per formula unit) freed during the topotactic synthesis is to a large degree located in the oxygen vacancy position, occupying partially a local $s$-symmetry interstitial orbital, rather than taking part alongside the other electon in converting Ni from 3+ to a full 1+ oxidation state. We demonstrate that the interstitial orbital in question, referred to by us as the zeronium $s$ or Z $s$ orbital, forms strong covalent bonds with neighboring Ni $3d_{3z^2-r^2}$ orbitals, which in turn facilitates the one-dimensional-like dispersion of the Ni $3d_{3z^2-r^2}$ band along the $c$-axis direction, leading also to a possible large out-of-plane coupling between Ni magnetic moments. This finding, reinforced by our electron localization function analysis, points to a fundamental distinction between the nickelates and the structurally analogous cuprates, may explain the absence of superconductivity in hydrogen-poor samples, and is certainly in agreement with the observed large $z$-polarized component in the Ni $L_3$-edge x-ray absorption spectra. In addition, by using DFT+U calculations as an illustration, we show that the electride-like nature of the IL nickelates is one of the main reasons for the theoretical difficulty in determining the much debated elusive Fermi surface of these novel superconductors and aslo in exploring the possibility of them becoming excitonic insulators at low temperatures

Effect of dopant atoms on local superexchange in cuprate superconductors: a perturbative treatment

Recent scanning tunneling spectroscopy experiments have provided evidence that dopant impurities in high- Tc superconductors can strongly modify the electronic structure of the CuO2 planes nearby, and possibly influence the pairing. To investigate this connection, we calculate the local magnetic superexchange J between Cu ions in the presence of dopants within the framework of the three-band Hubbard model, up to fifth-order in perturbation theory. We demonstrate that the sign of the change in J depends on the relative dopant-induced spatial variation of the atomic levels in the CuO2 plane, contrary to results obtained within the one-band Hubbard model. We discuss some realistic cases and their relevance for theories of the pairing mechanism in the cupratesComment: 5 pages, 4 figures, revised versio