Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session G39: Invited Session: Advanced First-Principles Methods for Complex Oxides |
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Sponsoring Units: DCMP DCOMP Chair: James Rondinelli, Drexel University Room: Mile High Ballroom 2A-3A |
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G39.00001: Formal Valence, $3d$ Occupation, and Charge Ordering Transitions Invited Speaker: Warren Pickett The metal-insulator transition (MIT), discovered by Verwey in the late 1930s, has been thought to be one of the best understood of MITs, the other ones being named after Wigner, Peierls, Mott, and Anderson. Continuing work on these transitions finds in some cases less and less charge to order, raising the fundamental question of just where the entropy is coming from, and just what is ordering. To provide insight into the mechanism of charge-ordering transitions, which conventionally are pictured as a disproportionation, I will (1) review and reconsider the charge state (or oxidation number) picture itself, (2) introduce new theoretical results for the rare earth nickelates (viz. YNiO$_3$), the putative charge ordering compound AgNiO$_2$, and the dual charge state insulator AgO, and (3) analyze cationic occupations of actual (not formal) charge, and work to reconcile the conundrums that arise. Several of the clearest cases of charge ordering transitions involve no disproportion; moreover, the experimental data used to support charge ordering can be accounted for within density functional based calculations that contain no charge transfer The challenge of modeling charge ordering transitions with model Hamiltonians will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:27PM |
G39.00002: Controlling structural complexity as a path towards new multifunctional correlated materials Invited Speaker: Craig Fennie Perovskite ABO$_3$ oxides display an amazing variety of phenomena that can be altered by subtle changes in the chemistry and internal structure. Most undergo non-polar structural distortions associated with a rotation of the BO$_6$ octahedra about one or more of the crystal axes. These distortions are well known to control the charge/orbital, magnetic and electronic degrees of freedom. This strong coupling represents an opportunity to understand and create new functional materials that respond to an external perturbation in a useful way. For example, if octahedral rotations can be designed to induce ferroelectricity, an applied electric field would be able to directly couple to the BO$_6$ octahedra, thereby controlling emergent phenomena such as magnetism, and possibly controlling metal/insulator transition. In this talk I will discuss our recent work in this area, highlighting the opportunities and the challenges to realizing such materials. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 1:03PM |
G39.00003: Controlling the two-dimensional electron gas at complex oxide interfaces Invited Speaker: Anderson Janotti Heterostructures of complex oxides have attracted great interest since the demonstration of a high-density two-dimensional electron gas (2DEG) at the SrTiO$_3$/LaAlO$_3$ (STO/LAO) interface. Still, the density of the 2DEG is only one tenth of what was expected from simple electron counting, i.e., 1/2 electron per unit-cell area. Since then, the origin and amount of the charge, the electrical properties of the 2DEG, the role of native defects, and the abrupt variation of the electron density with the thickness of the LAO top layer have been the subject of numerous theoretical and experimental studies. More recently, a 2DEG with the full density of 1/2 electron per unit cell area has been observed at the interface between the band insulator STO and the Mott insulator GdTiO$_3$ (GTO) [1], shedding additional light on the origin of the 2DEG, and raising important questions on the differences between the STO/LAO and STO/GTO heterostructures. Here we will discuss the similarities of the 2DEG at the STO/LAO and STO/GTO heterostructures from the perspective of first-principles simulations. We will address the differences in band alignments in the STO/LAO and STO/GTO heterostructures, and how the 2DEG is affected by the surface of the LAO top layer in the STO/LAO, but apparently not in the STO/GTO case [2]. Finally, we will also discuss how heterostructures can be used to drastically alter the electronic structure of STO, transforming it from a band insulator into a Mott insulator. \\[4pt] [1] P. Moetakef, T. A. Cain, D. G. Ouellette, J. Y. Zhang, D. O. Klenov, A. Janotti, C. G. Van de Walle, S. Rajan, S. J. Allen, and S. Stemmer, App. Phys. Lett. {\bf 99}, 232116 (2011). \newline [2] A. Janotti, L. Bjaalie, L. Gordon, and C. G. Van de Walle, Phys. Rev. B {\bf 86}, 241108(R) (2012). [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:39PM |
G39.00004: One-Dimensional Electron Gas at the Steps of a LaAlO3-SrTiO3 Interface Invited Speaker: Emilio Artacho Thin films of LaAlO$_3$ (LAO) epitaxially grown on SrTiO$_3$ (STO) substrates give rise to a two-dimensional electron gas that has spurred lots of interest and activity, given its application possibilities (if, for instance, coupled to any of the many interesting effects displayed by perovskite materials as LAO and STO), and the fundamental questions it poses. The gas originates as a consequence of the polarisation discontinuity at the interface: Although both materials are centrosymmetric, one (STO) belongs to the category of such insulators with zero bulk dielectric polarisation, while LAO is a member of the family that displays half a polarisation quantum along its $\langle 001 \rangle$ direction. The lattice of polarisation values associated to each material allows the prediction of the different polarisation discontinuities for different interface directions. This includes vicinal interfaces, which would be expected to display a structure of terraces of lower index interfaces separated by steps. This enables predicting the effect of steps in the electrostatics across these films, and the possibility of charge carriers being attracted to them. Large-scale first-principles calculations based on density functional theory were performed to substantiate such predictions, allowing to expect the formation of one-dimensional electron gases associated to steps at well-chosen interfaces of the two materials. Such gases should prove to be of fundamental interest since they are expected to show highly correlated electron carriers. The polarisation analysis and the results of the calculations will be presented along related and supporting results. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 2:15PM |
G39.00005: DFT+DMFT calculations in oxide superlattices Invited Speaker: Chris Marianetti While density functional theory (DFT) is a useful tool for understanding transition metal oxides and their heterostructures, it can break down qualitatively for certain properties or in specific transition metal oxides. We present a dual variable theory, density functional theory plus dynamical mean-field theory (DFT+DMFT), which remedies many of the shortcomings of DFT. Our full implementation of this method uses a plane wave basis and maximally localized Wannier functions to create the correlated subspace. We will demonstrate the ability to compute not only spectra and low energy properties, but also fully charge self-consistent total energies. We will explain the role of the double-counting correction, and introduce a more optimal approach. The method will be applied to various nickelate superlattices, and we will identify a new class of polar Mott insulators. Additionally, we will address the ``thinness'' driven metal-insulator transition which is observed in nickelates with very few layers. [Preview Abstract] |
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