Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session P23: Magnetic Phenomena in Bulk Oxides: TheoryFocus

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Sponsoring Units: GMAG DMP DCOMP Chair: Hyowon Park, University of Illinois at Urbana–Champaign Room: LACC 402B 
Wednesday, March 7, 2018 2:30PM  2:42PM 
P23.00001: Percolation of frustrated polarons in doped cobaltite La_{1x}Sr_{x}CoO_{3δ} (LSCO) Peter Orth, Daniel Phelan, Chris Leighton, Rafael Fernandes Due to fascinating phenomena such as magnetoelectronic phase separation and Co ion spinstate transitions, the archetypal cobaltite La_{1x}Sr_{x}CoO_{3δ} (LSCO) remains of high interest. Replacing La by Sr in the diamagnetic and insulating parent compound LaCoO_{3δ} introduces holes and magnetic moments. As Co undergoes spinstate transitions, 7site spin polarons form at small x with a spinperSr of S=12.5. Further doping leads to a glassy magnetic state and finally to a ferromagnetic (FM) metal at x=0.18. As simple statistical considerations predict a percolation of polarons already at x=0.05, the question arises what suppresses the formation of a macroscopic FM cluster. 
Wednesday, March 7, 2018 2:42PM  2:54PM 
P23.00002: Why rareearth ferromagnets are so rare: insights from the pwave Kondo model Onur Erten, Shadab Ahamed, Roderich Moessner Magnetic exchange in Kondo lattice systems is of the RudermanKittelKasuyaYosida type, whose sign depends on the Fermi wave vector, k_{F}. In the simplest setting, for small k_{F} , the interaction is predominately ferromagnetic, whereas it turns more antiferromagnetic with growing k_{F}. It is remarkable that even though k_{F} varies vastly among the rareearth systems, an overwhelming majority of lanthanide magnets are in fact antiferromagnets. To address this puzzle, we investigate the effects of a pwave form factor for the Kondo coupling pertinent to nearly all rareearth intermetallics. We show that this leads to interference effects which for small k_{F} are destructive, greatly reducing the size of the RKKY interaction in the cases where ferromagnetism would otherwise be strongest. By contrast, for large k_{F}, constructive interference can enhance antiferromagnetic exchange. Based on this, we propose a new route for designing ferromagnetic rareearth magnets. 
Wednesday, March 7, 2018 2:54PM  3:06PM 
P23.00003: Landau Theory of Multipolar Order in Pr(TM)2X20 Kondo materials SungBin Lee, Frederic Freyer, Jan Attig, Arun Paramekanti, Simon Trebst, YongBaek Kim A series of Pr(TM)2X20 (with TM=Ti,V,Rh,Ir and X=Al,Zn) Kondo materials exhibits exotic behavior such as quadrupolar order, superconductivity and nonFermi liquid behavior. In particular, nonKramers Pr3+ 4f2 moments show interesting magnetic field dependence with multipolar orderings. 
Wednesday, March 7, 2018 3:06PM  3:18PM 
P23.00004: Ferromagnetism of local uranium moments in UBeGe Andreas LeitheJasper, Roman Gumeniuk, Walter Schnelle, Michael Nicklas, Alexander Yaresko, Kristina Kvashnina, Christoph Henning, Yuri Grin The new intermetallic uranium beryllium germanide UBeGe and its thorium reference compound ThBeGe crystallize with the hexagonal ZrBeSi type of structure. Studies of magnetic, thermal and transport properties reveal UBeGe to be a strongly uniaxial ferromagnet with two magnetic transitions at T_{C}(1) =160 K and T_{C}(2) =150 K. ThBeGe is a diamagnetic metallic system with low density of states at the Fermi level. The high magnetic moment, xray absorption nearedge spectroscopy as well as theoretical DFT calculations indicate localized U 5f^{2} states in UBeGe. 
Wednesday, March 7, 2018 3:18PM  3:30PM 
P23.00005: Structural complexity and magnetic phase transformations: 4f electroninterplay Arjun Pathak, Y Mudryk, Vitalij Pecharsky Compounds that respond to varying magnetic field (H), pressure (p), temperature (T), or composition (x) by exhibiting structural or/and magnetic phase transformations are an intriguing subject matter for materials science, chemistry, and physics communities. Here we show that unlike the parent binaries – DyCo_{2} and ErCo_{2} – which exhibit lone distortions at their respective Curie temperatures, T_{C}, a mixed pseudobinary Er_{0.75}Dy_{0.25}Co_{2} demonstrates two successive magnetostructural transitions: a first order at T_{C}, followed by a second order below T_{C}. Surprisingly, the ground state phase adopts Imma symmetry instead of the commonly observed Fddd. We show that the unusual sequence of structurally and magnetically distinct phases in Er_{1x}Dy_{x}Co_{2} can be judiciously controlled via 4f electrons, where Er and Dy – two next near neighbor lanthanides – each play a distinct role in defining phase stability (PRB 96, (2017) 064412). 
Wednesday, March 7, 2018 3:30PM  3:42PM 
P23.00006: New Theoretical Characterizations for HalfMetallic Ferromagnets Pastor Chura, Kevin Bedell In the current literature HalfMetallic Ferromagnets (HMFs) have been theoretically characterized mainly by using the Band Theory approach. As a matter of fact, a first standard characterization of HMFs states that a HMF is a ferromagnet in which spinup electrons have metallic band structure whereas spindown electrons have insulating or semiconducting band structure. A second standard characterization says that a HMF is a ferromagnet in which at T=0K the density of states for the spindown component at the Fermi level is zero whereas the density of states for the spinup component at the Fermi level is different from zero. In this work, we propose new ways of theoretically characterizing HMFs by using an approach based on Fermi Liquid Theory for Spin Polarized Systems and the concept of exchange splitting, These new characterizations have proved to be useful in describing the thermodynamics and spin dynamics of HMFs in a way consistent with experiment. 
Wednesday, March 7, 2018 3:42PM  3:54PM 
P23.00007: Magnetic and orbital order in 2D electron gases at (111) oxide surfaces and interfaces Nazim Boudjada, Gideon Wachtel, Arun Paramekanti Recent experiments have explored twodimensional electron gases (2DEGs) at oxide (111) surfaces and interfaces. We discuss manybody instabilities of such (111) 2DEGs, incorporating multiorbital interactions in the t_{2g} manifold which can induce diverse magnetic and orbital orders. Such broken symmetries may lead to electronic nematicity in transport, which could cooperate or compete with phonon mechanisms. We present an effective field theory for the interplay of magnetism and nematic charge order, and discuss implications of the nematicity for transport in (111) 2DEGs. 
Wednesday, March 7, 2018 3:54PM  4:06PM 
P23.00008: Quantum Phase Transitions in a Multiorbital Hubbard Model with SpinOrbit Coupling Nitin Kaushal, Jacek Herbrych, Alberto Nocera, Gonzalo Alvarez, Adriana Moreo, Fernando Reboredo, Elbio Dagotto We used the density matrix renormalization group technique to perform a detailed study of the effect of spinorbit coupling on a threeorbital Hubbard model in one dimension [1]. Fixing the Hund coupling to a robust value compatible with some multiorbital materials, we present the phase diagram varying the Hubbard U and spinorbit coupling λ, at zero temperature for n=4 [2]. We also present the doping and spinorbit coupling λ phase diagram, for intermediate and strong Hubbard U [3]. Among many results, we observe an interesting transition from an orbitalselective Mott phase to an excitonic insulator with increasing λ at intermediate U. In the strong U coupling limit, we find a nonmagnetic insulator with an effective angular momentum〈(Jeff)^{2}〉≠0 near the excitonic phase, smoothly connected to the 〈(Jeff)^{2}〉= 0 regime. In the same model we also found a novel and robust excitonic insulator phase with Block spin ordering, at various dopings and in both the intermediate and strong coupling region. 
Wednesday, March 7, 2018 4:06PM  4:18PM 
P23.00009: Ultrafast optically induced magnetic phase transition in GdTiO_{3} from firstprinciples Guru Bahadur Khalsa, Nicole Benedek Recent development of intense ultrashort mid and far infrared laser sources has created an opportunity for functional materials based on direct excitation of infrared active phonons. Strong excitation of infrared active phonons can produce large unidirectional distortions of the crystal structure through special nonlinear coupling^{14}. Complex oxides provide an important testground for this experimental approach due to their diversity, strong coupling to optical fields, and demonstrated connection between subtle structural changes and fundamental properties. Early experiments in complex oxides are intriguing  suggesting that this mechanism can transiently induce insulatormetal phase transitions^{1} and enhance superconductivity^{2}. Hidden Atype antiferromagnetism (AAFM) has been predicted in rareearth titanates (between SmTiO_{3} and GdTiO_{3}) but not found using static techniques like solid solution or strain. We show how optical excitation of infrared phonons gives dynamical access to the AAFM phase in GdTiO_{3} through JahnTeller and Gd distortions that are challenging to access with static techniques. 
Wednesday, March 7, 2018 4:18PM  4:30PM 
P23.00010: SU(6) Symmetry in a Model of Lattice Electrons: The Case of NaTiSi_{2}O_{6} Weiguo Yin, Alexei Tsvelik, Emil Bozin Dimerization of Ti^{3+} ions in NaTiSi_{2}O_{6}, a quasionedimensional Mott insulator, has attracted considerable attention due to its possibility as a rare inorganic spinPeierls material. Recently based on twoorbital models, it was suggested that this material has an orbitalassisted Peierls ground state [1]. Here we point out that in an ideal case, a remarkable property of NaTiSi_{2}O_{6} is that its electronic band is exactly 3 times degenerate—with the lattice period of six Ti ions along the chain structure—despite the crystal field splitting of the three t2g orbitals into a local doublet and a higher energy singlet. We thus study NaTiSi_{2}O_{6} in proximity to this case using densityfunctional theory and the SU(6)symmetric Sutherland Hamiltonian. The perturbation from electronlattice interactions breaks the SU(6) symmetry, leading to a spontaneous dimerization accompanied by a structural distortion having the characteristic length of about six TiTi bonds. 
(Author Not Attending)

P23.00011: NMR in an electric field: a bulk probe of the hidden spin and orbital polarizations Ion Garate, Samuel Boutin, Jorge RamirezRuiz Recent theoretical work has established the existence of hidden spin and orbital textures in nonmagnetic materials with inversion symmetry. Here, we propose that these textures can be detected by nuclear magnetic resonance (NMR) measurements carried out in the presence of an electric field. In crystals with hidden polarizations, a uniform electric field produces a staggered magnetic field that points to opposite directions at atomic sites related by spatial inversion. As a result, the NMR resonance peak corresponding to inversion partner nuclei is split into two peaks. The magnitude of the splitting is proportional to the electric field and depends on the orientation of the electric field with respect to the crystallographic axes and the external magnetic field. As a case study, we will present the theory of electricfieldinduced splitting of ^{209}Bi peaks in Bi_{2}Se_{3}. We will also mention other potentially more promising candidate materials, whose crystal symmetry enables strategies to suppress the linewidth produced by the Oersted field. 
Wednesday, March 7, 2018 4:42PM  4:54PM 
P23.00012: Phase transitions in the Hubbard model studied by a cluster slave spin method WeiCheng Lee, TingKuo Lee The cluster slavespin method is introduced to systematically investigate the solutions of the Hubbard model including the symmetrybroken phases. In this method, the electron operator is factorized into a fermionic spinon describing the physical spin and a slavespin describing the charge fluctuations. We show that the selfconsistent equations to explore various symmetrybroken density wave states can be constructed in general with a cluster of multiple slavespin sites, and we employ this method to study the antiferromagnetic (AFM) state in the single band Hubbard model with the two and foursite clusters of slave spins. We find that the cluster slavespin method can capture correct behaviors in both weak and strong coupling limit within a unified framework. In addition, the holondoublon correlator as functions of U and doping x is also computed, which exhibits a strong tendency toward the holondoublon binding in the strong coupling regime. We further show that the results are progressively improved as the cluster is enlarged from a single site to four sites. Our results demonstrate that the cluster slavespin method can be a powerful tool to systematically investigate the strongly correlated system. 
Wednesday, March 7, 2018 4:54PM  5:06PM 
P23.00013: Shining Light on SpinOrbital Separation Tamas Palmai, Andrew James, Robert Konik We consider shining light on a quasionedimensional Mott insulator exhibiting spinorbital separation. The lowenergy sector of such a compound can be described by a spin chain with additional orbital degrees of freedom. Starting from an initial state with a highly polarized orbital sector, depending on the direction and polarization of the light, different orbitals can be activated or deactivated to take part in the time evolution. By employing a combination of Floquet theory and numerical methods, we find that orbital polarization can be controlled by a chirped laser. We discuss our results in connection to pumpprobe experiments on the one dimensional spinorbital chain compound Sr2CuO3. 
Wednesday, March 7, 2018 5:06PM  5:18PM 
P23.00014: Z_{2} topological order near the Neel state of the square lattice antiferromagnet Shubhayu Chatterjee, Subir Sachdev, Mathias Scheurer We classify quantum states proximate to the Neel state of the spin S=1/2 square lattice antiferromagnet with twospin nearneighbor and fourspin ring exchange interactions. Motivated by recent experiments on the cuprates and the iridates, we examine states with Z_{2 }topological order, an order which is not present in the semiclassical limit. Some of the states break one or more of reflection, timereversal, and lattice rotation symmetries, and can account for the observations. Such states can also be generalized to metals at finite doping, and may have implications for the pseudogap phase of the cuprates. 
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