Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session V40: Magnetic Oxides: TheoryFocus

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Sponsoring Units: GMAG DMP DCOMP Chair: David Mandrus, Oak Ridge National Lab Room: BCEC 208 
Thursday, March 7, 2019 2:30PM  3:06PM 
V40.00001: Intertwined orbital and magnetic order in 5d^{1} and 5d^{2} Double Perovskite Mott Insulators Invited Speaker: Mohit Randeria In this talk I summarize our work on understanding the unusual properties of Mottinsulating double perovskites A_{2}BB'O_{6} where the B' sites are 5d magnetic ions with either 1 or 2 electrons in t_{2g} orbitals and the B sites are nonmagnetic. Our theory is motivated by several experimental puzzles, including: (i) Why do almost all cubic 5d^{1} materials exhibit ferromagnetic (FM) order, rare in Mott insulators, while all the 5d^{2} materials have antiferromagnetic (AFM) ground states? (ii) Why is only partial (log 2) entropy recovered above the magnetic transition, rather than the expected log 4 for j = 3/2, in cubic 5d^{1} materials? (iii) Why do the cubic 5d^{1} materials exhibit a high temperature magnetic susceptibility that deviates from a CurieWeiss form? We derive and analyze lowenergy effective Hamiltonians for these systems that include spinorbit coupling, superexchange, intersite Coulomb interactions and Hund’s coupling. We predict that novel orbital order sets in at a high temperature T_{o} and strongly constrains the noncollinear magnetic order that appears at a much lower T_{c}. Our results allow us to understand all the puzzles noted above and make predictions for new experiments. This research was done in collaboration with W. Zhang, C. Svoboda, P. M. Woodward and N. Trivedi. 
Thursday, March 7, 2019 3:06PM  3:18PM 
V40.00002: Entangled spin orbital order in 5d^{1} and 5d^{2} Double Pervoskite Mott Insulators Nandini Trivedi, WENJUAN ZHANG, Christopher Svoboda, Mohit Randeria We theoretically investigate the unusual properties of Mottinsulating double perovskites A2BB'O6 where the magnetic B' ions have 1 or 2 electrons in the 5d shell and the B sites are nonmagnetic. We derive a lowenergy effective Hamiltonian that includes spinorbit coupling, superexchange, intersite Coulomb interactions and Hund’s coupling, and analyze it within mean field theory. We show that orbital order sets in at a high temperature To and strongly constrains the noncollinear magnetic order that appears at a much lower Tc.Our results give insight into several experimental puzzles. The prediction of orbital ordering well above Tc explains the puzzle of the missing entropy above the magnetic transition. Orbital order is also responsible for the deviations of the high temperature magnetic susceptibility from a CurieWeiss form. Finally, we show why cubic 5d1 materials most often exhibit canted ferromagnetism, which is rare in Mott insulators, while the 5d2 and distorted 5d1 materials are all antiferromagnetically ordered. 
Thursday, March 7, 2019 3:18PM  3:30PM 
V40.00003: Deterioration of SpinOrbit Transitions in Mott Insulating CoO Paul Sarte, Roger A Cowley, Ka Hou Hong, Manila Songvilay, Russell A Ewings, Dharmalingam Prabhakaran, ZahraSadat Yamani, William J L Buyers, John Paul Attfield, Chris Stock Despite a myriad of measurements spanning several decades, the low energy magnetic excitations of the classical Mott insulator CoO in the Néel regime are still poorly understood. As a result of the strong molecularfield induced entanglement of various j_{eff} manifolds, the establishment of a clear model for the low energy magnetic excitation spectrum of this deceptively simple monoxide has proven intractable using conventional spin wave approaches so far. Having extracted estimates for the exchange constants via the dilute monoxide Co_{0.03}Mg_{0.97}O (Sarte et al. PRB 98, 024415 (2018)), we have employed a random phasetype approximation in the method of Green's functions to model the rich low energy magnetic excitation spectrum of antiferromagnetically ordered CoO. The multilevel spin wave model successfully accounts for the temporally sharp spinorbit transitions consistent with orbital ordering observed near the magnetic zone center. However, the model fails to account for higher energy transfers, where welldefined spin waves are replaced by energy and momentum broadened excitations, characterized by steeply dispersive columns of scattering. The failure of the model and breakdown of spinorbit excitations are discussed in terms of coupling to a higher energy process. 
Thursday, March 7, 2019 3:30PM  3:42PM 
V40.00004: Hund nodal line semimetals: The case of twisted magnetic phase in the doubleexchange model Richard Geilhufe, Francisco Guinea, Vladimir Juricic In this talk, we discuss a class of topological metals, which we dub Hund nodal line semimetals, arising from the strong Coulomb interaction encoded in the Hund's coupling between itinerant electrons and localized spins [1]. We consider a particular twisted spin configuration, which is realized in the double exchange model describing the manganite oxides. As we show, the resulting effective tetragonal lattice of electrons with hoppings tied to the local spin features an antiunitary nonsymmorphic symmetry that in turn, together with another nonsymmorphic but unitary glide mirror symmetry protects crossings of a double pair of bands along a highsymmetry line on the Brillouin zone boundary. We also discuss the stability of Hund nodal line semimetal with respect to symmetry breaking from various perturbations of the twisted phase. Our results motivate further studies of other realizations of this state of matter, for instance in different spin backgrounds and properties of its drumhead surface states. 
Thursday, March 7, 2019 3:42PM  3:54PM 
V40.00005: Structural and electronic properties of doped NiO from density functional theory and quantum Monte Carlo simulations Olle Heinonen, Hyeondeok Shin, Jaron Krogel, Panchapakesan Ganesh, Friederike Wrobel, Anand Bhattacharya, Paul Kent NiO is a prototypical strongly correlated oxide. According to band filling, it should be a metal, but correlations drive the ground state to an antiferromagnetic insulator. The general question of how doping affects the electronic – and chemical – structure of correlated oxides is of great fundamental interest, but also important for the realization of electronics,”Mottronics”, based on correlated materials. We are studying hole and electrondoped NiO using density functional theory (DFT) methods and much more accurate quantum Monte Carlo simulations and compare our results directly with experimental results on highquality thin films grown by molecular beam epitaxy. One surprising result is that DFT in all flavors we have used fails to properly account for the KO bond distance, and underestimates it by over 0.3 A compared to analysis based on extended Xray absorption fine structure. Preliminary results using QMC show much better agreement with experiments, indicating that correlation effects beyond DFT have a dramatic effect on the energy landscape around the dopant. 
Thursday, March 7, 2019 3:54PM  4:06PM 
V40.00006: First Principles Electronic Structure Study of Ca_{2}CuO_{2}Cl_{2} Matthew Matzelle, Cheng Hu, Christopher Lane, Robert Markiewicz, Jianwei Sun, Xingjiang Zhou, Arun Bansil We discuss Density Functional Theory (DFT) based results on the oxychloride cuprate Ca_{2}CuO_{2}Cl_{2} (CCOC), which are obtained by using the recently constructed StronglyConstrainedandAppropriatelyNormed (SCAN) functional. Theoretical results are compared and contrasted with the corresponding angleresolved photoemission (ARPES) measurements. Previous firstprinciples DFT studies have found the ground state of the halffilled CCOC to be metallic in sharp disagreement with the experimentally observed insulating state. Although the insulating behavior can be captured by introducing an empirical Hubbard U parameter in firstprinciples computations, that reduces the predictive power of the theory. In sharp contrast, the SCAN functional yields the antiferromagnetic insulator phase with a gap in good agreement with optical conductivity studies without the need to invoke the Hubbard U. We also discuss how the electronic structure of CCOC evolves with hole doping. 
Thursday, March 7, 2019 4:06PM  4:18PM 
V40.00007: Strange metallicity in the doped Hubbard model Edwin Huang, Ryan Sheppard, Brian Moritz, Thomas Devereaux Strange or bad metallic transport, defined by its incompatibility with conventional quasiparticle pictures, is a theme common to strongly correlated materials and ubiquitous in many high temperature superconductors. The Hubbard model represents a minimal starting point for modeling strongly correlated systems. Here we demonstrate strange metallic transport in the doped twodimensional Hubbard model using determinantal quantum Monte Carlo calculations. Over a wide range of doping, we observe resistivities exceeding the MottIoffeRegel limit with linear temperature dependence. The temperatures of our calculations extend to as low as 1/40 the noninteracting bandwidth, placing our findings in the degenerate regime relevant to experimental observations of strange metallicity. Our results provide a foundation for connecting theories of strange metals to models of strongly correlated materials. 
Thursday, March 7, 2019 4:18PM  4:30PM 
V40.00008: A twopronged approach to the cuprate pseudogap: A comparison of modecoupling and firstprinciples (SCAN) results Robert Markiewicz, Christopher Lane, Yubo Zhang, James Furness, Bernardo Barbiellini, Arun Bansil, Jianwei Sun Density functional theory using the new SCAN exchangecorrelation functional has successfully described the antiferromagnetic ground states of undoped cuprate superconductors. In YBa_{2}Cu_{3}O_{7} (YBCO_{7}), the energy landscape involves a competition between many different nearly degenerate states – a mixture of antiferromagnetic and stripe states[1]. The organizing principle of these states remains Mottlike: the lowerenergy phases tend to have a larger planar copper magnetic moment. In particular, all these phases have substantially lower energy than the nonmgnetic Fermi liquid phase found in most previous DFT calculations, showing that it can play no role in the lowenergy properties of YBCO_{7}. 
Thursday, March 7, 2019 4:30PM  4:42PM 
V40.00009: Tunable band gaps from anion ordering and octahedral tilting in oxyfluorides Richard J Saballos, James M Rondinelli Heteroanionic materials are currently receiving increased interest because they may support superior functionality than singleanion materials [1]. In perovskite structures, anion ordering combined with octahedral tilting provides new degreesoffreedom (DOFs) from which to manipulate the chemical, physical and electronic properties. This control has been shown to tune the band gap in oxynitrides, but oxyfluorides remain underexplored [2,3]. Here we discuss the results of firstprinciples calculations on the experimentally known wideband gap semiconductor, KNaNbOF5 [4], and show that octahedral tilting and anion ordering can be used to tune the band gap. We show how the combination of these DOFs make it possible to decrease the band gap so it resides in the visible range. Finally, these results will be used to propose a design strategy for creating novel photovoltaic materials. 
Thursday, March 7, 2019 4:42PM  4:54PM 
V40.00010: DFT+U+J electronic structure calculations of correlated Bi_{2}CrAl_{3}O_{9} Jaylyn Umana, Alicia Baccarella, Lucia Steinke, Meigan Aronson, Jack Simonson Firstprinciples calculations allow for the prediction and interpretation of the intrinsic properties of a system. Density functional theory calculations of the electronic structure of the multiband magnetic insulator Bi_{2}CrAl_{3}O_{9} fail to corroborate experimental observations that suggest a magnetic ground state. Spinpolarized DFT calculations find basic agreement with antiferromagnetic order, which has been putatively observed at temperatures below T=79 ± 3K. We report here realignment of our results with experiment by inclusion of a Hubbard parameter U and Hund's exchange J found via linear response methods — suggesting that the properties of Bi_{2}CrAl_{3}O_{9} are the product of interatomic and intraatomic electronelectron correlations. 
Thursday, March 7, 2019 4:54PM  5:06PM 
V40.00011: Fourspin ring interaction as a source of unconventional magnetic orders in orthorhombic perovskite manganites Natalya Fedorova, Amadé Bortis, Christoph Findler, Nicola Spaldin We use ab initio electronic structure calculations and Monte Carlo simulations to investigate the magnetic and ferroelectric properties of bulk orthorhombic HoMnO_{3} and ErMnO_{3}. Our goals are to explain the inconsistencies in the measured magnetic properties of the orthorhombic perovskite manganites (oRMnO_{3}) with small rareearth (R) cations or Y, as well as the contradictions between the directions and amplitudes of the electric polarizations reported by different experimental groups. We find that several unconventional magnetic orders (socalled wspiral, HAFM and IAFM) can be stabilized in these materials due to strong fourspin ring exchange interactions. We show that the presence of these orders resolves the contradictions in the measured magnetic and ferroelectric properties of oRMnO_{3}. 
Thursday, March 7, 2019 5:06PM  5:18PM 
V40.00012: Spinorbitalentangled J_{eff}=1/2 state in 3d transition metal oxide CuAl_{2}O_{4} Choong Hyun Kim, Hwanbeom Cho, Santu Baidya, Vladimir Gapontsev, Sergey Streltsov, Daniel Khomskii, JeGuen Park, Ara Go, Hosub Jin We show that CuAl_{2}O_{4} spinel can host a spinorbitalentangled J_{eff}=1/2 state. During the long history of the 3d transition metal study, spinorbit coupling has never been a dominating energy scale. Here we propose on the CuAl_{2}O_{4} as the first example of a J_{eff}=1/2 Mott insulator in 3d transition metal compounds. From single crystal Xray diffraction data, our CuAl_{2}O_{4} is confirmed to have cubic structure without JahnTeller distortion and it is consistent with firstprinciples total energy calculations. Density functional theory combined with dynamical mean field theory calculations reveal that the J_{eff}=1/2 state survives the competition with the orbitalmomentumquenched S=1/2 state with the help of strong electron correlation. 
Thursday, March 7, 2019 5:18PM  5:30PM 
V40.00013: Multiloop contributions in the pseudofermion functional renormalization group for quantum spin systems: implementation and consequences Tobias Müller, Yasir Iqbal, Johannes Reuther, Ronny Thomale We extend the pseudofermion functional renormalization group (PFFRG) treatment of quantum spin systems by including diagrammatic higher loop contributions into the renormalization group flow. This allows us to consistently account for all contributions of parquettype diagrams in the twoparticle vertex and selfenergy derivatives within the twoparticle truncated PFFRG flow. We will discuss the impact of these corrections in different quantum spin models within PFFRG, especially in the light of the MerminWagner theorem. 
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