Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session U47: Multipolar order and strongly spin-orbit coupled oxidesFocus
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Sponsoring Units: GMAG DMP DCOMP Chair: Rebecca Dally, National Institute of Standards and Technology Room: 710/712 |
Thursday, March 5, 2020 2:30PM - 3:06PM |
U47.00001: The Case for Octupolar Order in Cubic 5d2 Double Perovskites Invited Speaker: Bruce D. Gaulin We have carried out neutron spectroscopic and diffraction studies on powder samples of the cubic 5d2 family of double perovskites, Ba2ZnOsO6, Ba2MgOsO6, and Ba2CaOsO6. These materials all display thermodynamic phase transitions at T*=30 K, 49 K, and 50 K, respectively, where ~ R ln(2) in entropy is released on passing through T*. All three display Curie-Weiss susceptibilities with antiferromagnetic Curie-Weiss constants of ~ -150 K, and show oscillations in low temperature zero longitudinal μSR, indicating time reversal symmetry breaking. Our inelastic neutron scattering shows the development of a gapped spin excitation spectrum, where a gap of ~ 15 meV develops below T*. This spectral weight is centred at wavevectors typical of type I AF order, however no magnetic Bragg diffraction is observed at low temperatures. Our neutron powder diffraction measurements place an upper limit on any possible ordered dipole moment of ~ 0.1 μB. To understand these results, we introduce a model for orbital repulsion between the two 5d electrons in Os6+ in these materials. This results in a non-Kramer's doublet and excited triplet of states for the ion, and ferro-octupolar order below T*, consistent with the full set of experimental observations. We believe this likely constitutes the first compelling case for octupolar order in a d-orbital Mott insulators. |
Thursday, March 5, 2020 3:06PM - 3:18PM |
U47.00002: Octupolar order in d-orbital Mott insulators. Arun Paramekanti, Dalini D Maharaj, Bruce D. Gaulin We discuss the quantum magnetism of $J=2$ ions which can be realized in spin-orbit coupled oxides with $5d^2$ transition metal ions. We discuss a microscopic mechanism where the combination of orbital repulsion and antiferromagnetic spin interactions leads to ferro-octupolar coupling, potentially stabilizing ferro-octupolar order. We discuss experimental signatures of octupolar order. We uncover a a gapped and dispersive magnetic exciton in the ferro-octupolar state using a slave-boson approach, which can be observed in neutron scattering. For sufficiently strong magnetic exchange, this dispersive exciton can condense, leading to conventional type-I antiferromagnetic order which can coexist with or even preempt octupolar order. Our proposal for ferrooctupolar order provides a comprehensive understanding of thermodynamics, $\mu$SR, X-ray diffraction, and inelastic neutron scattering measurements on a range of cubic $5d^2$ double perovskite materials including Ba$_2$ZnOsO$_6$, Ba$_2$CaOsO$_6$, and Ba$_2$MgOsO$_6$. Our proposal for exciton condensation leading to type-I magnetic ordering is potentially relevant to materials such as Sr$_2$MgOsO$_6$. |
Thursday, March 5, 2020 3:18PM - 3:30PM |
U47.00003: Neutron scattering study of magnetic excitation in spinel structure spin-orbital coupling compound CuGa2O4 Zhijun Xu, Guangyong Xu, Aashish Sapkota, yimin qiu, Genda Gu, John Tranquada We report neutron scattering measurements on a single crystal CuGa2O4 sample. The spinel structure of CuGa2O4 can cause strong spin-orbit coupling in this compound. From DC susceptibility and elastic neutron measurements, we found no sign of long- or short-range magnetic order down to 0.1K. Measurements on low energy magnetic excitations show ring shape intensity distribution in momentum space from this sample at low temperature. With increasing temperature, the magnetic excitation intensity gradually goes away. |
Thursday, March 5, 2020 3:30PM - 3:42PM |
U47.00004: d-Orbital Occupancy Systematics of 4d/5d Row Transition Metal Oxides by X-Ray Absorption Near Edge Spectra (XANES) Eric Kurywczak, Mehmet Alper Sahiner, Mark Clyde Croft XANES L2 and L3 edge X-Ray Absorption Near Edge Spectra (XANES) for 4d and 5d row transition metals (TM) oxides are assumed to be directly reflecting unoccupied d-orbitals influenced by the local symmetry of the metal ion1. XANES L2 and L3 edge data analysis through non-linear curve fitting allows for a unique, efficient look at the structural eccentricities of transition metal oxides. In this way it is possible to determine the oxidation state of a material as well as its site symmetry. We have used non-linear least-squares fitting across the near-edge region of the various 4d row and 5d row TM oxides in order to investigate the systematics of the d-orbital occupancy and the XANES white line features for t2g and eg symmetry orbitals. The statistics have already shown promising trends to be expected in 4d and 5d row level transition metal oxides. The systematics obtained by these results provide crucial information for the synthesis of the new materials with specific electronic structures and crystal symmetries. |
Thursday, March 5, 2020 3:42PM - 4:18PM |
U47.00005: From multipolar to spin-lattice disordered states in Mott insulators Invited Speaker: George Jackeli We theoretically explore how in the Mott insulating compounds the spin-orbit coupling could give rise to unusual disordered (glass or liquid) states of spin-orbital or spin-lattice degrees of freedom depending on the local d-electron counting. We discuss d1 and d2 transition metal compounds, such as molybdenum oxides with fcc and pyrochlore lattices, as candidate materials and consider experimental rezults from this perspective. |
Thursday, March 5, 2020 4:18PM - 4:30PM |
U47.00006: Observation of Spin Glassy Behavior in Sr3(Ru0.64Mn0.36)2O7 Roshan Nepal, Lingyi Xing, Rongying Jin One of the most accessible knobs to tune the ground state properties of a material system is chemical doping. Double-layered Ruddlesden-Popper ruthenate Sr3Ru2O7 is known to respond dramatically to the substitution of Ru by Mn leading to a rich phase diagram. We report the magnetic properties of 36% Mn doped single-crystalline Sr3(Ru1-xMnx)2O7 (x = 0.36). The temperature dependence of the zero-field cooled magnetic susceptibility shows a significant downturn akin to the collective freezing of the spins below a freezing temperature Tf, which is suppressed by the applied magnetic field. The onset of the spin freezing is accompanied by the development of large magnetic memory, magnetic relaxation, and exchange bias effects, which suggests Sr3(Ru0.64Mn0.36)2O7 has the spin glass (SG) ground state. Further analysis reveals the presence of de Almeida-Thouless phase line separating the high temperature paramagnetic (PM) and low temperature SG phases with TSG ~ 32 K. The nature of the SG state and its implication will be discussed. |
Thursday, March 5, 2020 4:30PM - 4:42PM |
U47.00007: A muSR study of current-induced destruction of order in Ca2RuO4 Matthew Nugent, Graeme Luke, James W Beare, Markus Braden, Kevin Jenni, Sudarshan Sharma, Megan Rutherford, Qi Sheng Past work has shown that Ca2RuO4 is a Mott insulator, but can be switched to a semimetallic state with the application of a reasonable amount of current. The material is an antiferromagnet below 113K in the insulating state, but achieving the semimetallic state causes a destruction of order, instead giving a diamagnetic response [1]. For this muSR study on the material, we used a 1% Ti doped Ca2RuO4 sample, which provided improved structural stability without significantly impacting magnetic ordering temperature [1]. We measured the sample under 0mA, 30mA, and 100mA of current, demonstrating both the presence of an antiferromagnetic ordering and its destruction in the semimetallic state when a current is applied down to 15K. |
Thursday, March 5, 2020 4:42PM - 4:54PM |
U47.00008: Electronic and magnetic properties in barium iridium oxides Xiang Chen, Dongsheng Yuan, Edith Bourret-Courchesne, Robert J Birgeneau The strongly spin orbit coupled Jeff = 1/2 spin residing on various lattice motifs are promising playgrounds for realizing assorted unusual ground states, such as Kitaev quantum spin liquid in frustrated Kagome lattice, proximity to unconventional superconducting state in layered square lattice. The recently proposed exotic quantum liquid state in facing sharing spin chain Ba4Ir3O10 might offer another platform for novel frustration. Here we introduce detailed experimental studies on those spin chain barium iridium oxides, which might be another intriguing system for engineering unusual electronic and magnetic ground states. |
Thursday, March 5, 2020 4:54PM - 5:06PM |
U47.00009: First-Principles Calculations of the atomic and magnetic structure of perovskite oxides SrMoO3, PbMoO3 and LaMoO3 Jeremy Lee-Hand, Cyrus Dreyer Perovskite oxides have a characteristic ABO3 structure that is able to accommodate a large number of different cations in the A and B locations. In particular, perovskites with Mo on the B site, including SrMoO3 and PbMoO3 have recently been fabricated. SrMoO3 has been shown to have high electron mobility, and PbMoO3 exhibits metallic behavior to low temperatures with a resistivity displaying a sublinear dependence on temperature. We use first-principles calculations based on density-functional theory (DFT) plus Hubbard U to investigate the family of perovskite molybdenates: SrMoO3, PbMoO3, and LaMoO3 in order to determine their ground-state atomic and magnetic structures. We determine the dependence with the choice of U, exchange-correlation functional, and implementation with respect to DFT code. Finally, we interpret the ground-state structures in terms of unstable phonons of higher symmetry structures including cubic Pm-3m. |
Thursday, March 5, 2020 5:06PM - 5:18PM |
U47.00010: Random singlet state in the spin liquid candidate Ba5CuIr3O12 Denis Gorbunov, Pavel Volkov, Choong-Jae Won, Jaewook Kim, Mai Ye, Heung-Sik Kim, Jed Pixley, Sang-Wook Cheong, Girsh Blumberg Understanding the role of disorder is crucial for the realization of quantum spin liquid (QSL) states in frustrated magnets, as it can lead to states mimicking QSL, but devoid of long-range entanglement. We study the thermodynamic and high magnetic field properties of the magnetic insulator Ba5CuIr3O12, a QSL candidate showing no magnetic order down to 2 K. The temperature dependencies of the magnetic susceptibility and the specific heat suggest weak antiferromagnetic correlations, in stark contrast to the magnetization that does not saturate up to a field of 59 Tesla. We show that these results can be reconciled only within the framework of a disorder-dominated random singlet state. The obtained exchange coupling distribution P(J) is found to be consistent with the power-law form P(J)∼J-α with α≈0.6. Our work highlights the use of high magnetic field measurements for distinguishing QSL candidates from disorder-dominated states and characterizing the latter. |
Thursday, March 5, 2020 5:18PM - 5:30PM |
U47.00011: Valence bond solids and hidden order phases in a pyrochlore U(1) quantum spin liquid Hyeok-Jun Yang, SungBin Lee Antiferromagnetic spin-1/2 system on a pyrochlore lattice has been extensively studied as a host of the exotic phase of matter, U(1) quantum spin liquid (QSL). In a low-energy sector, the spin degrees of freedom are mapped to the emergent U(1) gauge fields and fractionalized excitations. As the kinematics of the fractionalized excitations dominates the system, the emergent gauge structure is broken and the QSL encounters the magnetic instability. Beyond the U(1) QSL and the conventional orderings, the realization of unconventional phases such as the spin nematic, multiple order etc., in frustrated magnet opens an interesting question. Here, we study the pyrochlore XXZ model with farther and competing exchange interactions. Analyzing the effective spinon actions, we schematically depict the richer phase diagram exhibiting the valence bond solids, spin nematic, and inversion breaking hidden orders. |
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