Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session R46: Triangular Lattice IIFocus

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Sponsoring Units: GMAG DMP Chair: Oleg Starykh, University of Utah Room: 708 
Thursday, March 5, 2020 8:00AM  8:12AM 
R46.00001: Characterizing the Spin½ Heisenberg model on the triangular lattice through the Dynamical Spin Structure Ta Tang, Thomas Devereaux, Brian Moritz, YiFan Jiang The ground state phase of the spin1/2 Heisenberg model on the triangular lattice with nearestneighbor exchange J_{1} can be frustrated by the next nearest neighbor coupling J_{2} and turns into a stripe phase for large J_{2}. While both small and large values of J_{2}/J_{1} lead to magnetically ordered phases, a spin liquid phase with short correlation length can be established at intermediate values. However, the nature of this spin liquid phase remains under debate, with finite size DMRG calculations suggesting a gapped Z_{2} spin liquid phase, but also showing possible gapless spinon excitations. Here, we use exact diagonalization on a series of finitesize clusters to analyze the character of the ground state and lowestlying the excited states of the model for various values of J_{2}. We further characterize the evolution of the model as a function of J_{2} by analyzing the dynamical spin structure factor S(q,w) to provide key information on the changes of the underlying magnetic structure. 
Thursday, March 5, 2020 8:12AM  8:24AM 
R46.00002: Generalized SU(3) Spin Wave Theory on Effective S=1 Triangular Lattice Magnet FeI_{2} Hao Zhang, ShangShun Zhang, Xiaojian Bai, Zhiling Dun, Matthew Stone, Alexander Kolesnikov, Feng Ye, Martin Mourigal, Cristian Batista Magnetic and quadrupolar excitations can coexist in magnetically ordered S=1 spin systems because the local Hilbert space has dimension three. By using a generalized SU(3) spin wave theory (GSWT), we uncover the hybridized nature of the lowenergy modes revealed by inelastic neutron scattering (INS) measurements of the effective S=1 triangular antiferromagnet FeI_{2}. Band crossing between dipolar and quadrupolar modes enables a large hybridization between them that is induced by symmetric exchange anisotropy. Because of this hybridization, modes with dominant quadrupolar character can be observed with INS. These modes can also be obtained from a generalized LandauLifshitz dynamics of SU(3) spins (3 dipolar + 5 quadrupolar components), which turns out to be the adequate dynamics for describing the lowtemperature dynamical properties of FeI_{2}. 
Thursday, March 5, 2020 8:24AM  8:36AM 
R46.00003: Magnetic Excitations of the Frustrated Triangular Ising Magnet FeI2 Xiaojian Bai, ShangShun Zhang, Zhiling Dun, Hao Zhang, William Adam Phelan, Haidong Zhou, Matthew Stone, Alexander Kolesnikov, Feng Ye, Cristian Batista, Martin Mourigal We present a detailed investigation of the spin dynamics in singlecrystals of the layered spinone triangularlattice compound FeI2. Previous thermomagnetic measurements revealed a strong Ising singleion anisotropy for the Fe2+ ions in FeI2 and a magnetically longrange ordered state below 9.3K, which can be understood from the competition between nearest neighbor ferromagnetic interactions and a complex set of furtherneighbor interactions. Early neutron scattering, farinfrared and ESR measurements, revealed the emergence of a twomagnon bound state (TMBS) as the lowest energy mode from this ordered state. The TMBS carries an apparent gfactor that is doubled compared to that of single magnon excitations, which can be explained by a change of 2 units in spin angular momentum, at odds with the dipole selection rule. We revisit the spin excitations of FeI2 using modern neutronscattering instrumentation and map out the magnetic structure, diffuse scattering and lowenergy magnetic excitation spectrum. We extract a model Hamiltonian for FeI2 and elucidate a novel hybridization mechanism that quantitatively explains current and previous spectroscopic experiments on this enigmatic compound. 
Thursday, March 5, 2020 8:36AM  9:12AM 
R46.00004: Anisotropicexchange magnets: triangular lattice and all Invited Speaker: Alexander Chernyshev I will describe our efforts to understand the phase diagram of a model that combines paradigmatic geometrical frustration of spins on a triangular lattice with strong spinorbitinduced interactions. This model is relevant to a growing family of rareearthbased magnets and other related materials and our work sets up a consistent interpretation of the current and future experiments in them. I will also attept to review our recent incursion to the honeycomblattice territory and attempts to shed light on the properties of the Kitaev materials such as αRuCl_{3}. 
Thursday, March 5, 2020 9:12AM  9:24AM 
R46.00005: Interplay between magnetic structures, domain physics, and transport properties in a family of triangular layered materials Chunxiao Liu, Leon Balents The triangular layered magnet PdCrO2 has attracted interest due to the interplay between its antiferromagnetic order and high inplane conductivity, widely believed to be the key to understanding an unconventional anomalous Hall effect (AHE) in the material. More recent experiments reveal that the domain physics in the material may also play an important role on its electronic transport. Motivated by these experimental findings, we study the interplay between magnetic structures, domain physics, and transport properties of a layered triangular lattice with alternating layers of hopping electrons and localized spins. We analyze the possible magnetic orders of the most general symmetry allowed spin Hamiltonian at nearest neighbor level. Assuming appropriate intralayer exchange parameters and treating the interlayer exchange perturbatively, we show that threefold rotation symmetry breaking domains can emerge, in accordance with the experiments. We further show that, under certain conditions the Kondo lattice Hamiltonian possesses a particular symmetry that forbids contribution from the electron band topology to the Hall conductance. Finally we study various other effects including magnetic breakdown and spinorbit coupling and comment on their contribution to the unconventional AHE. 
Thursday, March 5, 2020 9:24AM  9:36AM 
R46.00006: Triangular antiferromagnet NiGa2S4: a lowdimensional analog of the pyrochlores? Haoyu Wang, Oleg Tchernyshyov NiGa2S4 is a frustrated magnet on a triangular lattice with a strong antiferromagnetic thirdneighbor exchange J3 and a weak ferromagnet firstneighbor exchange J1. Although the system has been studied for more than a decade [1], its behavior, particularly the lack of longrange order, remains enigmatic. Exotic scenarios have been proposed for its explanation, including the formation of a quadrupolar spin order that cannot be seen by the usual probes like neutrons [2]. We propose a different scenario based on a simple observation: the 4sublattice structure resulting from a strong J3 makes the system similar to a pyrochlore antiferromagnet. The geometrical frustration of the pyrochlore antiferromagnet can be relieved by a lattice distortion [3]. A similar tendency may exist in NiGa2S4. The spinPeierls phase transition may be preempted by structural disorder involving random strains. 
Thursday, March 5, 2020 9:36AM  9:48AM 
R46.00007: Complex magnetic behavior of the Co^{2+} triangular (sawtooth) 1D chains in CsCo_{2}(MoO_{4})_{2}(OH) Duminda Sanjeewa, Vasile O Garlea, Jie Xing, Athena S. Sefat Lowdimensional magnetic materials have drawn continued attention in condensedmatter physics for their electronic and magnetic anisotropic properties. In particular, the oxyanionbased systems, with transitionmetal (M) oxide sublattices that are magnetically isolated by closedshell nonmagnetic oxyanions (SiO_{4}^{−4}, PO_{3}^{−4}, AsO_{3}^{−4}), show great potential for diverse magnetic phenomena. We synthesize CsCo_{2}(MoO_{4})_{2}(OH) single crystals, find the crystal structure, and characterize thermodynamic properties. CsCo(MoO_{4})_{2}(OH) belongs to a rare class of compounds named deltachain (or sawtooth) type structure, with cornersharing isosceles triangles whose vertices consist of one Co(1) and two Co(2) atoms. The magnetic susceptibility result reveals longrange ordering below 5 K with a strong anisotropy. The isothermal magnetization data shown a stepwise magnetization when the magnetic field is along the CoOCo (sawtooth chain) direction, with two metamagnetic transitions at 0.3 kOe and 40 kOe. The magnetic structure consists of ferrimagnetic chains which are antiferromagnetically coupled with each other, seen in neutron diffraction. 
Thursday, March 5, 2020 9:48AM  10:00AM 
R46.00008: Synthesis and Characterization of RareEarth Based Triangular Antiferromagnet Borates Matthew Ennis, Rabindranath Bag, Sachith Dissanayake, Zhenzhong Shi, Alexander Kolesnikov, Jose RodriguezRivera, Hui Wu, Craig Brown, David E Graf, Sara Haravifard Recently, triangular antiferromagnetic materials have attracted attention because competing interactions on the lattice can give rise to exotic phenomena, such as Quantum Spin Liquids. We have initiated systematic efforts to synthesize single crystal samples of a family of rareearth based triangular antiferromagnet borates. We have used xray diffraction as well as thermal and magnetic measurements to characterize these compounds. Additionally we have carried out neutron scattering experiments to probe the static and dynamic properties of these systems. In this talk, we will present the results of our experimental efforts. 
Thursday, March 5, 2020 10:00AM  10:12AM 
R46.00009: Magnetic shortrange correlations and KosterlitzThouless transitions in TmMgGaO4 Marcus Daum, Zhiling Dun, Haidong Zhou, Martin Mourigal Transverse Ising model on a triangular lattice is known to host two KosterlitzThouless (KT) transitions at low temperatures. Triangular lattice antiferromagnet TmMgGaO4 is a promising candidate to realize such physics due to the interplay between geometric frustration, spin orbit coupling, and crystal electric fields. In TmMgGaO4 shortrange magnetic correlations are observed over a broad temperature range. However, the nature of this shortrange magnetic order is under debate. Recent numerical work has predicted a KosterlitzThouless transition at 4 K, which however lacks any experimental evidence thus far. Using AC susceptibility and neutron scattering measurements, we unveil this transition and discuss its implications in understanding the rich physics of TmMgGaO4. 
Thursday, March 5, 2020 10:12AM  10:24AM 
R46.00010: Studying the Dynamic Magnetic Properties of the Triangular Antiferromagnet YbZnGaO4 in Applied Field William Steinhardt, Sachith Dissanayake, Zhenzhong Shi, Nicholas Butch, David E Graf, Andrey Podlesnyak, Yaohua Liu, Yang Zhao, Guangyong Xu, Jeffrey Lynn, Casey Marjerrison, Sara Haravifard In the last few years Ybbased triangular antiferromagnets emerged as promising quantum spin liquid candidates, and subsequently their ground states have been the subject of ardent debates. Though many experimental and theoretical studies have been devoted to investigating the magnetic properties of these systems at very low temperatures, and exploring a range of possible explanations for the observed spin liquidlike phenomena, a definitive description remains elusive mainly due to thermal fluctuations and chemical disorder. In this talk we discuss neutron scattering experiments in applied magnetic field to probe features of the “partially magnetized” state in YbZnGaO4, which generated great excitement in YbMgGaO4 as a possible signature in support of spinon Fermi surface hypothesis [1]. 
Thursday, March 5, 2020 10:24AM  10:36AM 
R46.00011: A quantitative study of shortrange correlations in triangular Ising antiferromagnet TmMgGaO_{4} via magnetic pair distribution function analysis Raju Baral, Benjamin Frandsen, Henry Edward Fischer, Qing Huang, Haidong Zhou, Marcus Daum, Zhiling Dun, Martin Mourigal Interacting Ising spins decorating a triangular lattice are expected to show rich magnetic behavior, including a possible KosterlitzThouless (KT) transition at low temperature. Recently, TmMgGaO_{4} has been found to exhibit a perfect triangular lattice of Isinglike Tm^{3+} magnetic moments, providing a promising platform for investigating potential KT physics in a solidstate magnetic system. Initial studies have revealed significant diffuse magnetic scattering at low temperature, indicating the presence of shortrange magnetic correlations that may hold the key to understanding the magnetic ground state of this system. Here, we utilize the magnetic pair distribution function (mPDF) technique to probe these correlations in real space via Fourier transformation of the diffuse scattering pattern. We present a magnetic model that reproduces the observed mPDF pattern with quantitative accuracy and discuss the significance of these results in the context of existing work on TmMgGaO_{4}. 
Thursday, March 5, 2020 10:36AM  10:48AM 
R46.00012: Chiral correlation in triangular lattice Hubbard model BinBin Chen, Wei Li, Andreas Weichselbaum The fermionic triangular lattice Hubbard (TLH) model constitutes a playground for intriguing emergent quantum phenomena and novel exotic states in strongly correlated system. In a recent paper [1], by using mixed real and momentumspace basis in iDMRG, a chiral spin liquid is found for intermediate onsite Coulomb interaction U between the smallU metallic phase and largeU threesublattice magnetic phase, against the conclusion in ref. [2]. 
Thursday, March 5, 2020 10:48AM  11:00AM 
R46.00013: Topological frustration: Quantum statistics of vortices from a dual theory of the XY ferromagnet Sayak Dasgupta, Shu Zhang, Ibrahima Bah, Oleg Tchernyshyov The familiar concept of geometrical frustration can be extended to frustration induced by the topology of spin textures. For instance, in a magnetic vortex the spins on the periphery rotate in the easy plane, leaving the spins at the vortex core frustrated. To reduce the exchange energy, the core spins come out of the easy plane and point along the hard axis. There is a degeneracy resulting from the associated Z_{2} symmetry (polarization) which has wellknown important consequences for the dynamics of a vortex. Here we show that topological frustration at the vortex core may induce a nontrivial quantum statistics of vortices. We extend the wellknown mapping between the easyplane ferromagnet and electrostatics in d = 2 spatial dimensions, introduced by Kosterlitz [1], to dynamical and quantum phenomena in a d = 2+1 spacetime. Ferromagnetic vortices behave like quantum particles with an electric charge equal to the vortex number and a magnetic flux equal to the transverse spin of the vortex core. Vortices with halfinteger core spin exhibit fermionic statistics. 
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