Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session F21: First Principles Design of Magnetic OxidesFocus
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Sponsoring Units: GMAG DMP DCOMP Chair: Turan Birol, Univ of Minnesota - Twin Cities Room: LACC 309 |
Tuesday, March 6, 2018 11:15AM - 11:51AM |
F21.00001: Towards a comprehensive DFT theory of the anti-ferromagnetic and paramagnetic phases of the classic Mott insulators MnO, FeO, CoO, and NiO Invited Speaker: Giancarlo Trimarchi The insulating character of the antiferromagnetic (AFM) and paramagnetic (PM) phases of the classic NaCl-structure Mott insulators MnO, FeO, CoO, and NiO is traditionally explained as a manifestation of strong correlation. Within this view, the gap originates from electrons moving in the lattice and forming states with doubly occupied d orbitals on certain metal sites and empty d orbitals on others, even without spatial symmetry breaking. The need for a correlated picture is usually justified, at least in part, by the failure of band theory to predict a gap for the PM phases when modeled by a non-magnetic NaCl structure, where all transition metal (TM) sites are symmetry equivalent (a monomorphous description). Here, we seek to understand the minimum theoretical description needed to capture the gapping and moment formation in these classic Mott systems. As noted by previous authors, band theory predicts a gap for the spin-ordered AFM phases. For the spin-disordered PM phases, we use large NaCl-type supercells where each TM site can have different spin directions and local bonding environments with zero total magnetization. Such a polymorphous description accommodates symmetry-breaking lattice distortions and in open-shell systems allows for full occupation of just some of the components of a degenerate level rather than partial occupation of all of them. In such supercells the degeneracy of the d orbitals can be lifted, thus allowing for on-site magnetic moments to develop. For this polymorphous description we use special quasi-random structures (SQSs), supercells defined so as to provide the best approximations of average properties of random configurations (not individual snapshots). Single determinant band theory (based on DFT+U) applied to the SQS models predicts significant (1-3 eV) band gaps and large local moments in the PM phases of MnO, FeO, CoO, and NiO, in agreement with experiment. |
Tuesday, March 6, 2018 11:51AM - 12:03PM |
F21.00002: Orbital re-polarization transition in multiferroic Ca3Ru2O7 Zheting Jin, Wei Ku
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Tuesday, March 6, 2018 12:03PM - 12:15PM |
F21.00003: Interplay between pressure, anion order, and octahedral tilts in perovskite oxyfluorides Richard Saballos, James Rondinelli Mechanisms governing octahedral rotations in homoanionic perovskites(ABO3,ABF3)is well understood;cation size mismatch drive tilts of BO6 and BF6 octahedra with angles that linearly depend on the electron population in the π-bonding t2gorbital of the transition metal[1].However, the applicability of this mechanism to octahedral tilting in heteroanionic materials is less established.Here we use a combination of symmetry analysis and density functional theory calculations to investigate how anion order affects octahedral tilting under hydrostatic pressure in the anion ordered double perovskite KNaNbOF5.Consistent with earlier reports, we find that the multiple connected NaOF5 and NbOF5 anionic groups exhibiting tilt distortions expands the phase space from 12 to 14[2].We also find that KNaNbOF5 undergoes multiple pressure-driven tilt transitions, which we explain based on the aforementioned mechanism for octahedral tilting.This would show a path to create a pressure-stabilized phase that may exhibit interesting properties. |
Tuesday, March 6, 2018 12:15PM - 12:27PM |
F21.00004: Accurate first-principles description of the antiferromagnetic state of La2CuO4 Christopher Lane, James Furness, Ioana Buda, Yubo Zhang, Robert Markiewicz, Bernardo Barbiellini, Jianwei Sun, Arun Bansil We show how an accurate first-principles treatment of the antiferromagnetic (AFM) state of La2CuO4 can be obtained without invoking any free parameters such as the Hubbard U. Both the magnitude and alignment of our theoretically predicted magnetic moment of 0.495 µB on Cu-sites along the (100) direction are in excellent accord with experimental results. The computed values of the AFM band gap (1 eV) and the exchange-coupling (-138 meV) match the corresponding experimental values. The magnetic form factor obtained from neutron scattering experiments is also well described by our calculations. Our study opens a new pathway for first-principles investigations of electronic structures and phase diagrams of cuprates and other complex materials. |
Tuesday, March 6, 2018 12:27PM - 12:39PM |
F21.00005: Magnetically-driven orbital-selective insulator-to-metal transition in double perovskite oxides Hanghui Chen Interaction-driven metal-insulator transitions or Mott transitions are widely observed in condensed-matter systems. In multi-orbital systems, many-body physics is richer in which an orbital-selective metal-insulator transition is an intriguing and unique phenomenon. Here we use first-principles calculations to show that a magnetic transition (from paramagnetic to long-range magnetically ordered) can simultaneously induce an orbital-selective insulator-to-metal transition in rock-salt ordered double perovskite oxides A2BB′O6 where B is a nonmagnetic ion and B′ a magnetic ion with a d3 electronic configuration (Tc4+, Ru5+, Os5+ etc.). The orbital selectivity originates from geometry frustration of a face-centered-cubic lattice on which the magnetic ions B′ reside. Including realistic structural distortions and spin-orbit interaction do not affect the transition. Our work shows that by exploiting geometry frustration on non-bipartite lattices, novel electronic/magnetic/orbital-coupled phase transitions can occur in correlated materials that are in the vicinity of metal-insulator phase boundary. |
Tuesday, March 6, 2018 12:39PM - 12:51PM |
F21.00006: Effects of "Stuffing'' on the Atomic and Electronic Structure of the Pyrochlore Yb2Ti2O7 Soham Ghosh, Efstratios Manousakis The ground state of the magnetic rare earth pyrochlore Yb2Ti2O7 is on the boundary between competing states. We have carried out ab initio density functional calculations of pure and "stuffed" systems (with an Yb replacement on a Ti site) to determine the most stable chemical formula and structure as a function of the oxygen chemical potential. In the "stuffed" crystal, we studied the effect of oxygen vacancies on the electronic properties of the system. In addition, with the inclusion of the contribution of spin-orbit-coupling (SOC) on top of the GGA+U approach, we investigated the electronic structure and the magnetic moments of the most stable "stuffed" system. Our first-principle findings should form a foundation for effective models describing the low-temperature properties of this material whose true ground state remains controversial. |
Tuesday, March 6, 2018 12:51PM - 1:03PM |
F21.00007: Ferroelectric Behavior of PZT-BCO-BFO Solid Solutions David Parker, Simuck Yuk, Valentino Cooper The ferroelectric behavior of 75% PbZr0.5Ti0.5O3 - 25% BiMO3 (where M=Fe,Co) (PZT-BMO) was studied using first principles density functional theory calculations. Despite the large rhombohedral and tetragonal polar distortions expected for the BiFeO3 (BFO) and |
Tuesday, March 6, 2018 1:03PM - 1:15PM |
F21.00008: Nonlinear Optical Susceptibility and Ferroic Domains in Polar Metals Haricharan Padmanabhan, Shiming Lei, Zhiqiang Mao, Jak Chakhalian, Venkatraman Gopalan Polar metals are materials that are simultaneously metallic and exhibit ferroelectric-like polar order, despite the presence of free charges that typically screen long-range electrostatic forces. The existence of a ferroelectric-like polar state in a conductor leads to the question – how do properties typical of insulating ferroelectrics manifest themselves in a metal? In this work, we investigate two such properties – nonlinear optical susceptibility, and domain formation. |
Tuesday, March 6, 2018 1:15PM - 1:27PM |
F21.00009: Density-Functional Description of Bond Disproportionation in ABO3 Compounds Into Different Local Environments for the Same B Chemical Element Gustavo Dalpian, Qihang Liu, Alex Zunger Some ABO3 oxides can contain an identical B element observed to exist in two different local environments (LE). Traditionally, the two B atoms were designated by different formal oxidation states (FOS) (sometimes called Charge Ordering) such as Bi3+ and Bi5+ in the case of Ba2Bi2O6. Close examination shows that the physical charge around such atoms, calculated by DFT, is almost identical, as the ligands conspire to change their hybridization to maintain nearly constant charge —the self-regulating response (Raebiger, Lany and Zunger Nature 453, 763, 2008). Such bond disproportionation—identical elements appearing in the solid with different LE at constant charge, while opening a band gap (metal-nonmetal transition) and concomitantly lowering the total energy—is described via DFT as energy-lowering broken symmetry. We demonstrate this via DFT calculations in CsTlF3, SmNiO3, CaFeO3, PbCoO3 and Ba2Nb5O15, comparing total energies, geometries and total charge distributions. Although FOS are commonly used to understand and explain these compounds, we will show that the actual physical property that changes is not the atomic charges, but the local structural environment. |
Tuesday, March 6, 2018 1:27PM - 1:39PM |
F21.00010: Upper Limits of Magnetization, Curie Temperature, and Magnetic Anisotropy of Permanent Magnet Materials Hisazumi Akai Calculation based on LDA/GGA is performed for the purpose of estimating possible upper limits of saturation magnetic polarization JS, Curie temperature TC, and magnetocrystaline anisotropy constant K1 of permanent magnet materials. It was concluded that the upper limits of JS, TC, and K1 could be ~2.7 T, ~2000 K, and ~1000 MJm-3, respectively. K1 obtained assuming that the 4f orbitals are bound to the lattice seems too large: one order of magnitude smaller than that might be plausible. |
Tuesday, March 6, 2018 1:39PM - 1:51PM |
F21.00011: Theoretical approach to control tetragonal distortion in Heuslers for large magnetocrystalline anisotropy Yu-ichiro Matsushita, Galia Madjarova, John Dewhurst, Sangeeta Sharma, Eberhard K Gross Recently, Heusler materials have got much attention due to the intriguing magnetic properties. However, most of them prefer cubic structure and consequently large magnetocrystalline anisotropy is not expected. To realize the large magnetocrystalline anisotropy in Heuslers, controlling tetragonal distortion plays an important role. In this study, we have successfully clarified the microscopic mechanisms of tetragonal distortion in Heusler materials by the electronic structure calculations on the basis of the density-functional theory. Subsequently, we have suggested a way to control the distortion [1]. |
Tuesday, March 6, 2018 1:51PM - 2:03PM |
F21.00012: Temperature dependent Ab initio study of magnetic materials Matthew Heine, Olle Hellman, David Broido Magnetic materials are important to a wide array of applications from steel to computer memory technology. Understanding the thermodynamics of these materials in such applications is often crucial. In this study, we apply the Temperature Dependent Effective Potential (TDEP) method to perform temperature-dependent ab initio studies of magnetic materials. Noncolinear constrained magnetic moment calculations within the framework of Density Function Theory (DFT) are used. The effects of the thermal environment on material properties are demonstrated. |
Tuesday, March 6, 2018 2:03PM - 2:15PM |
F21.00013: First-principles studies of the electronic and structural properties of vanadium dioxide and characterization of VO2 (011) surfaces Jakub Planer, Josef Redinger, Florian Mittendorfer Vanadium dioxide is used in various applications such as field-effect transistors or ultra-fast optoelectronic switches. Standard GGA functionals (PBE, PBESol) fail to describe the band gap of 0.6 - 0.7 eV and predict the monoclinic phase to be metallic. Another contradiction to experiments is that the rutile phase is calculated to be 65 - 95 meV / f.u. more stable than the monoclinic (M1) phase. Using the meta-GGA+U (SCAN) functional with the treatment of van der Waals interactions (rVV 10) or hybrid functional (PBE + 7% HF exchange) calculations, we obtained the band gap of 0.65 eV (hybrid) and 0.73 eV (meta-GGA) which is in a good agreement with optical photoemission experiments. Furthermore, hybrid functional calculations improve the description of the band width in the rutile phase. However, both functionals still predict the rutile phase to be 406 meV / f.u. (meta-GGA) and 65 meV / f.u. (hybrid) more stable than monoclinic (M1) phase. |
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