Session Y23: Focus Session: Electronic Structure of Complex Oxides

Magnetic Moment Collapse-Driven Mott Transition in MnO

Metal-insulator transition in strongly correlated electron systems has been one of the central themes of condensed matter physics for a few decades. In the simplest model system, the single band Hubbard model, the transition, which is still not completely understood, is driven by the ratio of the on-site repulsion to the bare bandwidth. Real materials with multiple bands offer possibility of alternative scenarios of the metal-insulator transition. In this talk we will present a numerical study of MnO under high pressure using combination of the standard bandstructure theory and modern many-body methods (dynamical mean-field theory). Our results reveal a close relationship between the high-spin to low-spin transition and metallization, which can be interpreted as the moment collapse driving the metallization. We find an isostructural volume collapse of about 13\% accompaning the transition. While the moment collapse, which is essentially an atomic effect, is obtained by most electronic structure methods, the metal-insulator transtion can be described reliably only when dynamical correlations are taken into account. We find our results to compare very well to the available experimental data. In order to demonstrate the capability of the computational method PES and ARPES spectra obtained on the related NiO with and without hole doping will also be presented.   

Origin of magnetism in the Fe$_2$O$_3$-FeTiO$_3$ system from correlated band theory

The high remanent magnetization measured in exsolutions of the canted antiferromagnet hematite (Fe$_2$O$_3$) and room-temperature paramagnet ilmenite (FeTiO$_3$) has recently received considerable attention not only in the geoscience community [1] but also for possible spintronics applications. To resolve the microscopic origin of magnetism in this system, we have performed density functional theory calculations, varying systematically the concentration, distribution, and charge state of Ti (Fe) in a hematite (ilmenite) host. We find that including electronic correlation within the LDA+U approach is decisive to obtain the correct ground state of the end members, $\alpha$-Fe$^{3+}$$_2$O$_3$ and Fe$^{2+}$Ti$^{4+}$O$_3$. In a single Ti layer in the hematite host, Ti is not inert as commonly assumed but plays an active role in compensating the charge mismatch at the interface and the emergence of magnetism and the preferred charge state is Ti$^{3+}$, Fe$^{3+}$. As soon as a thicker ilmenite-like block forms, the most favorable compensation mechanism is through Ti$^{4+}$ and a disproportionation in the Fe contact layer in Fe$^{2+}$, Fe$^{3+}$ giving theoretical evidence for the {\sl lamellar magnetism hypothesis} [1]. The substitution of Ti (or Fe) in Fe$_2$O$_3$ (FeTiO$_3$) leads to impurity levels in the band gap and in some cases to half-metallic behavior.   

Electronic structure of Mn and Fe oxides

We present a clear, simple tight-binding representation of the electronic structure and cohesive energy (energy of atomization) of MnO, Mn$_{2}$O$_{3}$, and MnO$_{2}$, in which the formal charge states Mn$^{2+}$, Mn$^{3+}$, and Mn$^{4+}$, respectively, occur. It is based upon localized cluster orbitals for each Mn and its six oxygen neighbors. This approach is fundamentally different from local-density theory (or LDA+U), and perhaps diametrically opposite to Dynamical Mean Field Theory. Electronic states were calculated self-consistently using existing parameters [1], but it is found that the charge \textit{density} is quite insensitive to charge \textit{state}, so that the starting parameters are adequate. The cohesive energy per Mn is dominated by the transfer of two $s$ electrons to oxygen $p$ states, the same for all three compounds. The differing transfer of majority $d$ electrons to oxygen $p$ states, and the coupling between them, accounts for the observed variation in cohesion in the series. The same description applies to the perovskites, such as La$_{x}$Sr$_{1-x}$MnO$_{3}$, and can be used for FeO, Fe$_{2}$O$_{3}$ (and FeO$_{2})$, Because the formulation is local, it is equally applicable to impurities, defects and surfaces. \newline [1] Walter A. Harrison, \textit{Elementary Electronic Structure,} World Scientific (Singapore, 1999), revised edition (2004).   

Configurational Electronic Entropy and the Phase Diagram of Mixed-Valence Oxides: The Cases of Li$_x$FePO$_4$ and Fe$_3$O_4$

We demonstrate that configurational electronic entropy, previously neglected, in {\it ab initio} thermodynamics of materials can qualitatively modify the finite-temperature phase stability of mixed-valence oxides, in our case Li$_x$FePO$_4$. First-principles LDA+U calculations were performed on 245 Li$_x$FePO$_4$ structures with different lithium/vacancy and electron/hole distributions, and Monte Carlo simulations were used to determine the phase diagram based on a coupled cluster-expansion model. While transformations from low-T ordered or immiscible states are almost always driven by configurational disorder (i.e.\ random occupation of lattice sites by multiple species), in FePO$_4$--LiFePO$_4$ the formation of a solid solution is almost entirely driven by electronic, rather than ionic configurational entropy. We argue that such an electronic entropic mechanism, rather than an ionic one, may be relevant to most other mixed-valence systems. Details in Phys. Rev. Lett. {\bf 97}, 155704 (2006). Recently we have studied the Verwey transition in magnetite Fe$_3$O$_4$. The configurational entropy of the $t_{2g}$ electrons on the iron B sub-lattice is found to lead to a first-order phase transition, although the the mechanism is substantially more complicated than that Verwey originally proposed.   

Can pristine semiconducting oxides be ferromagnetic?

The recent finding of FM in HfO$_{2}$ thin films of Coey's group has urged us to re-judge the role of TM doping in introducing FM into semiconducting oxides. Our observation of FM in undoped TiO$_{2}$, HfO$_{2}$, In$_{2}$O$_{3}$, ZnO, and SnO$_{2}$ confirmed that magnetism is possible in pristine oxide thin films, and FM is likely due to oxygen vacancies. This assumption is confirmed by our XMCD measurement on TiO$_{2}$ films: The FM in TiO$_{2}$ films is indeed intrinsic, and stems from both O-2$p$ and Ti-3$d$ electrons. In semiconducting oxides, the origin of magnetism is not due to the doping, but oxygen vacancies/defects. A big issue is how to find a more appropriate model to explain better the mechanism. We propose a model based on an electronic structure calculation using the tight binding method in the confinement configuration. Vacancy sites in TiO$_{2}$, HfO$_{2}$, In$_{2}$O$_{3}$ films could create spin splitting and high spin state, so that the exchange interaction between the electrons surrounding the oxygen vacancy with the local field of symmetry could lead to a FM ground state. Calculations give the results of 3.18 $\mu _{B}$/vac for TiO$_{2}$, 3.05 $\mu _{B}$ /vac for HfO$_{2}$ and 0.16 $\mu _{B}$ /vac for In$_{2}$O$_{3}$. This model suggests that confinement effects play an important role in shaping up magnetic properties of low dimension systems.   

Charge regulation via a negative feedback: transition metal atoms in semiconductors and insulators

Transition metal (TM) atoms in semiconductors and insulators produce energy levels in the band gap, whose occupation can be altered by shifting the Fermi level e.g. via doping. Changes in level occupation correspond to changes in the formal oxidation state. Such changes are associated with inward/outward lattice relaxations recorded as ``ionic radii'', different magnetic moments, and a core shift in x-ray photoemission. We show, via density-functional calculations within the plane-wave supercell method for TM atoms including Cr, Mn, Fe, and Co in the semiconductor hosts GaAs and Cu$_2$O, as well as in the ionic insulator host MgO, that changes in gap-level occupation result in only very small changes of charge on the TM atom itself. We show that this is due to an inherent negative feedback that regulates the TM charge via a TM--ligand rehybridization. Further, the inward/outward lattice relaxations and XPS core shifts, often associated with a change of the TM charge, in fact follow from the TM-ligand rehybridization, as the TM charge is kept unchanged via the inherent negative feedback.   

Electronic Structure of Cubic Copper Monoxide

We report the calculation of the band structure and optical properties of the rocksalt form of copper monoxide. Although this particular crystal structure does not exist in bulk form for CuO, at least two groups, including ourselves, have succeeded in growing by ``forced epitaxy,'' several atomic layers of cubic CuO on rocksalt proxy substrates such as MgO and STO. For our computation, we employed the DFT/LDA+U method known to give valid results for rocksalt NiO and FeO. Our results show cubic CuO, like these two materials, to be an antiferromagnetic Mott-Hubbard insulator whose band gap is primarily determined by charge transfer between filled O 2p bands and empty Cu 4s states, with localization of the Cu 3d hole by on-site coulomb repulsion frustrating what otherwise would be metallic behavior. We compare our results with NiO, FeO and the natural form of CuO found in the monoclinic mineral tenorite.   

Instabilities of coupled Cu$_2$O$_5$ ladders

The spin-ladder compound Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$ has a complex phase diagram including charge-density-wave order as well as unconventional superconductivity under high pressure. Due to its quasi-one-dimensional nature\footnote{S. Lee, J. B. Marston, J. O. Fjaerestad, Phys. Rev. B {\bf 72}, 075126.} fundamental questions about the high-T$_c$ cuprates might be more easily addressed in this context. However, due to the spatial proximity of neighboring ladders inter-ladder Coulomb repulsion as well as hopping between ladders might still be important. Using the functional renormalization group\footnote{M. Salmhofer and C. Honerkamp, Prog. Theor. Physics {\bf{105}}, 1 (2001).} and an analysis of generalized susceptibilities \footnote{D. Zanchi and H. J. Schulz, Phys. Rev. B {\bf 61}, 13609 (2000); C. J. Halboth and W. Metzner, Phys. Rev. Lett. {\bf 85}, 5162 (2000).}, we study a model of coupled Cu$_2$O$_5$ ladders \footnote{K. Wohlfeld, A. M. Oles, and G. A. Sawatzky, Phys. Rev. B {\bf 75}, 180501(R) (2007).}. We investigate instabilities towards charge, spin, and pairing order as a function of hole doping, inter-ladder hopping, and interaction strength starting from experimentally relevant hopping parameters\footnote{T. F. A. M\"u{}ller, {\it et al.}, Phys. Rev. B {\bf 57}, R12655 (1998).}.