Doping-Induced Spectral Shifts in Two Dimensional Metal Oxides

Doping of strongly layered ionic oxides is an established paradigm for creating novel electronic behavior. This is nowhere more apparent than in superconductivity, where doping gives rise to high temperature superconductivity in cuprates (hole-doped) and to surprisingly high Tc in HfNCl (Tc=25.5K, electron-doped). First principles calculations of hole-doping of the layered delafossite CuAlO2 reveal unexpectedly large doping-induced shifts in spectral density, strongly in opposition to the rigid band picture that is widely used as an accepted guideline. These spectral shifts, of similar origin as the charge transfer used to produce negative electron affinity surfaces and adjust Schottky barrier heights, drastically alter the character of the Fermi level carriers, leading in this material to an O-Cu-O molecule-based carrier (or polaron, at low doping) rather than a nearly pure-Cu hole as in a rigid band picture. First principles linear response electron-phonon coupling (EPC) calculations reveal, as a consequence, net weak EPC and no superconductivity rather than the high Tc obtained previously using rigid band expectations. These specifically two-dimensional dipole-layer driven spectral shifts provides new insights into materials design in layered materials foe functionalities besides superconductivity.Comment: 6 pages, 8 figures,1 tabl

Evaluation of Compensated Magnetism in La2_2VCuO6_6: Exploration of Charge States

We present an ab initio study of double perovskite La$_2$VCuO$_6$, which was one of the earliest compounds suggested as a potential compensated half-metal. Two charge and spin configurations close in energy have been identified. (i) The originally envisioned spin-compensated V$^{4+}$:d$^1$/Cu$^{2+}$:d$^1$ configuration is comprised of antialigned S=1/2 cations. This state is a spin-compensated half-metal for moderate values of U (the on-site Coulomb repulsion strength on the metal ions) and is insulating for larger values of U. (ii) An unanticipated non-magnetic solution V$^{5+}$:d$^0$/Cu$^{+}$:d$^{10}$ consists of an empty- and a full-shell ion, both spherically symmetric, that leads to a band insulator. This ionic band insulator is calculated to be the ground state at small and moderate values of U. The different distortions of the perovskite structure that occur for each state are central in determining the energy differences. Treating the Cu$^{2+}$ Jahn-Teller distortion self-consistently is particularly important for the magnetic solution.Comment: 5 pages, 2 figures, 1 tabl

Quantum Confinement Induced Molecular Mott Insulating State in La4_4Ni3_3O8_8

The recently synthesized layered nickelate La$_4$Ni$_3$O$_8$, with its cuprate-like NiO$_2$ layers, seemingly requires a Ni1 ($d^8$)+2Ni2 ($d^9$) charge order, together with strong correlation effects, to account for its insulating behavior. Using density functional methods including strong intra-atomic repulsion (Hubbard U), we obtain an insulating state via a new mechanism: {\it without charge order}, Mott insulating behavior arises based on quantum coupled, spin-aligned Ni2-Ni1-Ni2 $d_{z^2}$ states across the trilayer (rather than based on atomic states), with antiferromagnetic ordering within layers. The weak and frustrated magnetic coupling between cells may account for the small spin entropy that is removed at the N\'eel transition at 105 K and the lack of any diffraction peak at the N\'eel point.Comment: 4 pages, 3 figure