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 5d1 and 5d2 Double Perovskite Mott Insulators Invited Speaker: Mohit Randeria In this talk I summarize our work on understanding the unusual properties of Mott-insulating double perovskites A2BB'O6 where the B' sites are 5d magnetic ions with either 1 or 2 electrons in t2g orbitals and the B sites are non-magnetic. Our theory is motivated by several experimental puzzles, including: (i) Why do almost all cubic 5d1 materials exhibit ferromagnetic (FM) order, rare in Mott insulators, while all the 5d2 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 5d1 materials? (iii) Why do the cubic 5d1 materials exhibit a high temperature magnetic susceptibility that deviates from a Curie-Weiss form? We derive and analyze low-energy effective Hamiltonians for these systems that include spin-orbit coupling, superexchange, inter-site Coulomb interactions and Hund’s coupling. We predict that novel orbital order sets in at a high temperature To and strongly constrains the non-collinear magnetic order that appears at a much lower Tc. 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 5d1 and 5d2 Double Pervoskite Mott Insulators Nandini Trivedi, WENJUAN ZHANG, Christopher Svoboda, Mohit Randeria We theoretically investigate the unusual properties of Mott-insulating double perovskites A2BB'O6 where the magnetic B' ions have 1 or 2 electrons in the 5d shell and the B sites are non-magnetic. We derive a low-energy effective Hamiltonian that includes spin-orbit coupling, super-exchange, inter-site 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 non-collinear 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 Curie-Weiss 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 Spin-Orbit Transitions in Mott Insulating CoO Paul Sarte, Roger A Cowley, Ka Hou Hong, Manila Songvilay, Russell A Ewings, Dharmalingam Prabhakaran, Zahra-Sadat 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 molecular-field induced entanglement of various jeff 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 Co0.03Mg0.97O (Sarte et al. PRB 98, 024415 (2018)), we have employed a random phase-type approximation in the method of Green's functions to model the rich low energy magnetic excitation spectrum of antiferromagnetically ordered CoO. The multi-level spin wave model successfully accounts for the temporally sharp spin-orbit transitions consistent with orbital ordering observed near the magnetic zone center. However, the model fails to account for higher energy transfers, where well-defined 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 spin-orbit 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 double-exchange 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 non-symmorphic symmetry that in turn, together with another non-symmorphic but unitary glide mirror symmetry protects crossings of a double pair of bands along a high-symmetry 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 electron-doped NiO using density functional theory (DFT) methods and much more accurate quantum Monte Carlo simulations and compare our results directly with experimental results on high-quality 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 K-O bond distance, and underestimates it by over 0.3 A compared to analysis based on extended X-ray 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 Ca2CuO2Cl2 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 Ca2CuO2Cl2 (CCOC), which are obtained by using the recently constructed Strongly-Constrained-and-Appropriately-Normed (SCAN) functional. Theoretical results are compared and contrasted with the corresponding angle-resolved photoemission (ARPES) measurements. Previous first-principles DFT studies have found the ground state of the half-filled 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 first-principles 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 two-dimensional Hubbard model using determinantal quantum Monte Carlo calculations. Over a wide range of doping, we observe resistivities exceeding the Mott-Ioffe-Regel limit with linear temperature dependence. The temperatures of our calculations extend to as low as 1/40 the non-interacting 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 two-pronged approach to the cuprate pseudogap: A comparison of mode-coupling and first-principles (SCAN) results Robert Markiewicz, Christopher Lane, Yubo Zhang, James Furness, Bernardo Barbiellini, Arun Bansil, Jianwei Sun Density functional theory using the new SCAN exchange-correlation functional has successfully described the antiferromagnetic ground states of undoped cuprate superconductors. In YBa2Cu3O7 (YBCO7), 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 Mott-like: the lower-energy 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 low-energy properties of YBCO7. |
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 single-anion materials [1]. In perovskite structures, anion ordering combined with octahedral tilting provides new degrees-of-freedom (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 first-principles calculations on the experimentally known wide-band 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 Bi2CrAl3O9 Jaylyn Umana, Alicia Baccarella, Lucia Steinke, Meigan Aronson, Jack Simonson First-principles calculations allow for the prediction and interpretation of the intrinsic properties of a system. Density functional theory calculations of the electronic structure of the multi-band magnetic insulator Bi2CrAl3O9 fail to corroborate experimental observations that suggest a magnetic ground state. Spin-polarized 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 Bi2CrAl3O9 are the product of inter-atomic and intra-atomic electron-electron correlations. |
Thursday, March 7, 2019 4:54PM - 5:06PM |
V40.00011: Four-spin 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 HoMnO3 and ErMnO3. Our goals are to explain the inconsistencies in the measured magnetic properties of the orthorhombic perovskite manganites (o-RMnO3) with small rare-earth (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 (so-called w-spiral, H-AFM and I-AFM) can be stabilized in these materials due to strong four-spin ring exchange interactions. We show that the presence of these orders resolves the contradictions in the measured magnetic and ferroelectric properties of o-RMnO3. |
Thursday, March 7, 2019 5:06PM - 5:18PM |
V40.00012: Spin-orbital-entangled Jeff=1/2 state in 3d transition metal oxide CuAl2O4 Choong Hyun Kim, Hwanbeom Cho, Santu Baidya, Vladimir Gapontsev, Sergey Streltsov, Daniel Khomskii, Je-Guen Park, Ara Go, Hosub Jin We show that CuAl2O4 spinel can host a spin-orbital-entangled Jeff=1/2 state. During the long history of the 3d transition metal study, spin-orbit coupling has never been a dominating energy scale. Here we propose on the CuAl2O4 as the first example of a Jeff=1/2 Mott insulator in 3d transition metal compounds. From single crystal X-ray diffraction data, our CuAl2O4 is confirmed to have cubic structure without Jahn-Teller distortion and it is consistent with first-principles total energy calculations. Density functional theory combined with dynamical mean field theory calculations reveal that the Jeff=1/2 state survives the competition with the orbital-momentum-quenched S=1/2 state with the help of strong electron correlation. |
Thursday, March 7, 2019 5:18PM - 5:30PM |
V40.00013: Multi-loop contributions in the pseudo-fermion functional renormalization group for quantum spin systems: implementation and consequences Tobias Müller, Yasir Iqbal, Johannes Reuther, Ronny Thomale We extend the pseudo-fermion 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 parquet-type diagrams in the two-particle vertex and self-energy derivatives within the two-particle truncated PFFRG flow. We will discuss the impact of these corrections in different quantum spin models within PFFRG, especially in the light of the Mermin-Wagner theorem. |
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