Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session K37: Triangular LatticesFocus

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Sponsoring Units: GMAG DCMP DCOMP Chair: Cristian Batista, University of Tennessee Room: BCEC 206A 
Wednesday, March 6, 2019 8:00AM  8:36AM 
K37.00001: Hierarchy of Exchange Interactions in the TriangularLattice Spin Liquid YbMgGaO_{4} Invited Speaker: Fahad Mahmood The spin1/2 triangular lattice antiferromagnet YbMgGaO_{4} has attracted attention recently as a quantum spinliquid candidate with the possible presence of offdiagonal anisotropic exchange interactions induced by spinorbit coupling. Whether a quantum spin liquid is stabilized or not depends on the interplay of various exchange interactions with chemical disorder that is inherent to the layered structure of the compound. We combine timedomain terahertz spectroscopy and inelastic neutron scattering measurements in the fieldpolarized state of YbMgGaO_{4} to obtain better insight of its exchange interactions. Terahertz spectroscopy in this fashion functions as a highfield electron spin resonance and probes the spinwave excitations at the Brillouin zone center, ideally complementing neutron scattering. A global spinwave fit to all our spectroscopic data at fields over 4 T, informed by the analysis of the terahertz spectroscopy linewidths, yields constraints on the disorderaveraged g factors and exchange interactions. Our results paint YbMgGaO_{4} as an easyplane XXZ antiferromagnet with the combined and necessary presence of subleading nextnearest neighbor and weak anisotropic offdiagonal nearestneighbor interactions. Moreover, the obtained g factors are substantially different from previous reports. This work establishes the hierarchy of exchange interactions in YbMgGaO_{4} from highfield data alone and thus strongly constrains possible mechanisms responsible for the observed spinliquid phenomenology. 
Wednesday, March 6, 2019 8:36AM  8:48AM 
K37.00002: Hidden order and its quantum excitations in the triangularlattice magnet TmMgGaO_{4} Yao Shen, Changle Liu, Yayuan Qin, Shoudong Shen, Yaodong Li, Robert Bewley, Astrid Schneidewind, Gang Chen, Jun Zhao Certain magnetic materials exhibit exotic hiddenorder phases, which are not directly accessible to conventional magnetic measurements due to the lack of internal field. Thus, experimental identification and theoretical understanding of a hidden order are difficult. Here we combine neutron scattering and thermodynamic probes to study the newly discovered rareearth triangularlattice magnet TmMgGaO_{4}. Magnetic Bragg peaks at K points are observed in the elastic neutron diffraction measurements. More interesting, however, is the observation of sharp and highly dispersive spin excitations that cannot be explained by a magnetic dipolar order, but instead is the direct consequence of the preformed multipolar order that arises from a rather weak crystal field splitting and is “hidden” in the conventional magnetic measurements. We demonstrate that the observed unusual spin correlations and thermodynamics can be accurately described by a transverse field Ising model on the triangular lattice with an intertwined dipolar and ferromultipolar order. 
Wednesday, March 6, 2019 8:48AM  9:00AM 
K37.00003: Investigating the Dynamic Magnetic Properties of Triangular Antiferromagnet YbZnGaO4 William Steinhardt, Sachith Dissanayake, Zhenzhong Shi, Nicholas Butch, David E Graf, Yaohua Liu, Hongcheng Lu, Casey Marjerrison, Sara Haravifard Recently the antiferromagnetic triangularlattice family of materials RMM’O4 (where R = rare earth, M and M’ = transition or main group) has attracted much attention, due to their disordered ground state and possible link to quantum spin liquid state. In this talk we present results from our recent fielddependent singlecrystal diffuse and inelastic neutron scattering experiments, as well as thermal and magnetotransport measurements performed on YbZnGaO4. 
Wednesday, March 6, 2019 9:00AM  9:12AM 
K37.00004: Elastic and Inelastic Neutron Scattering Study of the Triangular Lattice Ising Antiferromagnet TmMgGaO4 Marcus Daum, Zhiling Dun, Haidong Zhou, Martin Mourigal The LnMgGaO_{4 }family of materials (with Ln= Yb^{3+}, Er^{3+}, and Tm^{3+}) has been the focus of recent studies in frustrated magnetism due to the combination of anisotropic exchange interactions from strong spin obit coupling, geometrical frustration, and inherent site disorder of Mg^{2+} and Ga^{3+}. In order to further understand the original compound, YbMgGaO4, we have begun to investigate how the nonKramers ion Tm3+ differs from Yb3+ when placed in a similar crystal structure. TmMgGaO_{4} displays a strong Ising behavior and magnetic scattering_{ }at the Kpoint of the triangular Brillouin zone at T= 0.05K. In this talk, I will present recent neutron scattering results on singlecrystals of TmMgGaO_{4} that elucidate the temperature and magnetic field dependence of spin correlations in that material. 
Wednesday, March 6, 2019 9:12AM  9:24AM 
K37.00005: Gapless chiral spin liquid in spin1/2 triangular Heisenberg model Shoushu Gong, Mac Lee, Donna Sheng We study the ground state of spin1/2 triangularlattice antiferromagnet with first (J1), second (J2), and thirdneighbor (J3) interactions, using largescale density matrix renormalization group on cylinder geometry. By increasing the couplings, we establish a quantum phase diagram of the system. Besides the previously identified magnetic phases and J1J2 spin liquid, we find a new gapless chiral spin liquid (CSL) phase with growing J3, which emerges at the neighbor of the J1J2 spin liquid. This CSL spontaneously breaks timereversal symmetry by showing a robust threespin scalar chiral order and the chiral edge mode in the entanglement spectrum. The finitesize scaling of spin triplet gap strongly suggests gapless spin excitations, which is further supported by the logarithmic behavior of the subsystem length dependence of entanglement entropy that leads to a finite central charge. The fitted central charge grows rapidly with increasing system width, which does not support a U(1) Dirac spin liquid but indicates an emergent spinon Fermi surface. This CSL realizes an example of interacting topological phase with chiral edge mode and gapless bulk excitations. 
Wednesday, March 6, 2019 9:24AM  9:36AM 
K37.00006: Anisotropicexchange magnets on a triangular lattice: dual spin liquids Alexander Chernyshev, Pavel Maksimov, Zhenyue Zhu, Steven Robert White We establish a correspondence of the anisotropicexchange model to an extended KitaevHeisenberg model. With the help of Klein dualities, deeper connections between different parts of the parameter space are established. Using these insights, we have performed DMRG studies of the quantum S=1/2 anisotropicexchange model in previously unexplored parts of the phase diagram. The most important implication of the correspondence is that it necessitates an existence of a spin liquid that is Klein dual to the spin liquid found by us earlier. In our present DMRG study, we do confirm existence of this dual spinliquid phase. With the structure factor S(q), we also confirm that the dual spin liquid can be seen as a result of a melting of the dual 120^{○} phase, same way as the earlier spin liquid is a molten 120^{○} phase. The confirmation of the dual spin liquid strengthens our case for both of them. 
Wednesday, March 6, 2019 9:36AM  9:48AM 
K37.00007: Anisotropicexchange magnets on a triangular lattice: spin waves and accidental degeneracies Pavel Maksimov, Zhenyue Zhu, Steven Robert White, Alexander Chernyshev We present an overview of the phase diagram, spinwave excitations, and finitetemperature transitions of the anisotropicexchange magnets on a nearestneighbor triangular lattice. We investigate transitions between five principal classical phases: ferromagnetic, Néel, its dual, and the two stripe phases. Transitions are identified by the spinwave instabilities and by the LuttingerTisza approach. In the stripe phases, quantum fluctuations are mostly negligible, leaving the ordered moment nearly saturated even for S = 1/2. However, for a 2D surface of the full 3D parameter space, the spinwave spectrum in one of the stripe phases exhibits an accidental degeneracy. As a result, the ordering transition temperature in a wide region of the phase diagram is suppressed. This accidental degeneracy is due to a correspondence to an extended KitaevHeisenberg model with emergent symmetries. 
Wednesday, March 6, 2019 9:48AM  10:00AM 
K37.00008: Pressuretuning of the quantum spin Hamiltonian of a triangular lattice antiferromagnet Sergei Zvyagin, David E Graf, Takahiro Sakurai, Shojiro Kimura, Hiroyuki Nojiri, Joachim Wosnitza, Hitoshi Ohta, Toshio Ono, Hidekazu Tanaka Quantum antiferromagnets on a triangular lattice are prototype materials to investigate the phenomena of geometrical frustration in quantum matter. Apart from highly unusual magnetic properties, they possess a very rich phase diagram ranging from a simple unfrustrated square lattice to a quantum spin liquid, yet to be confirmed experimentally. One major obstacle in this area of research is the lack of materials with appropriate (ideally tuned) exchange coupling parameters. Here, we demonstrate an alternative approach where, instead of the chemical composition, the spin Hamiltonian of a triangular lattice antiferromagnet is tuned by hydrostatic pressure. The combination of tunneldiodeoscillator and electron spin resonance techniques allows us to accurately monitor the spinHamiltonian parameters in Cs_{2}CuCl_{4}, revealing a significant increase of its exchange coupling ratio from 0.3 to 0.42 at a pressure of 1.8 GPa. A number of emerging fieldinduced transitions were observed in the highpressure phase. 
Wednesday, March 6, 2019 10:00AM  10:12AM 
K37.00009: Partial Magnetic Order in the Anisotropic Triangular Lattice System CeCd_{3}As_{3} Sarah Dunsiger, Kolawole Akintola, Andre Cote, Alex Fang, Shayan Gheidi, Jeonghun Lee, Shyam Sundar, Eundeok Mun, Jeff Sonier CeCd_{3}As_{3} is one member of a family of rare earth ternary compounds which crystallize into a hexagonal ScAl_{3}C_{3}type structure, where the magnetic Ceions form a quasi two dimensional anisotropic triangular lattice. Such model geometrically frustrated systems have been proposed as candidates for quantum spin liquid behavior [1]. 
Wednesday, March 6, 2019 10:12AM  10:24AM 
K37.00010: Frustrated antiferromagnetic(AFM) order in CeCd_{3}As_{3} Keenan Avers, Roman Movshovich, William Halperin, Joe D Thompson, Priscila Rosa The relatively unexplored compound CeCd_{3}As_{3} crystallizes in the P6_{3}/mmc space group, and has geometrically frustrated triangular arrangements of magnetic cerium atoms in the a/b plane. Magnetization measurements indicate strong out of plane AFM interactions. Our heat capacity, C(T), measurements on a single crystal reveal a sharp peak at T_{N} = 420 mK in zero field, consistent with an out of plane AFM phase transition. Applying a magnetic field within the easyplane raises TN to 490 mK at 2 T demonstrating lifting of frustration. At high fields T_{N} is suppressed to zero, and C(T) no longer resembles that of an AFM. Instead we observed an excess of entropy at the lowest temperatures measured. These results indicate competition between AFM order and frustration at low fields, and a field induced highly degenerate ground state at stronger fields. 
Wednesday, March 6, 2019 10:24AM  10:36AM 
K37.00011: Magnon topology and thermal Hall effect in trimerized triangular lattice antiferromagnet Kyungsu Kim, Ki Lee, Suk Bum Chung, JeGuen Park Magnon band topology and magnon transverse responses have been studied in twodimensional magnetisms for the last few years. 
Wednesday, March 6, 2019 10:36AM  10:48AM 
K37.00012: Selective measurements of intertwined multipolar orders: NonKramers doublets on a triangular lattice Changle Liu, Yaodong Li, Gang Chen Motivated by the rapid experimental progress on the spinorbitcoupled Mott insulators, we propose and study a generic spin model that describes the interaction between the nonKramers doublets on a triangular lattice and is relevant for triangular lattice rareearth magnets. We predict that the system supports both pure quadrupolar orders and intertwined multipolar orders in the phase diagram. We explore the magnetic excitations to reveal the dynamic properties of the systems. Due to the peculiar properties of the nonKramers doublets and the selective coupling to the magnetic field, we further study the magnetization process of the system in the magnetic field. We point out the selective measurements of the static and dynamic properties of the intertwined multipolarness in the neutron scattering, NMR, and μSR probes and predict the experimental consequences. The relevance to the existing materials such as TmMgGaO_{4}, Prbased, and Tbbased magnets, and many ternary chalcogenides is discussed. Our results not only illustrate the rich physics and the promising direction in the interplay between strong spinorbitentangled multipole moments and the geometrical frustration, but also provide a general idea to use noncommutative observables to reveal the dynamics of the hidden orders. 
Wednesday, March 6, 2019 10:48AM  11:00AM 
K37.00013: Physical properties of the trigonal binary compound Nd_{2}O_{3} Gabriele Sala, Matthew Brandon Stone, Binod Rai, Andrew May, Clarina Dela Cruz, Georg Ehlers, Vasile O Garlea, Mark D Lumsden, Hasitha Suriya Arachchige, Victor R. Fanelli, David George Mandrus, Andy Christianson Nd_{2}O_{3} is a structurally and chemically simple material which crystallizes in a trigonal structure, with Nd^{3+} ions surrounded by cages of 7 oxygen anions. Here we study the physical properties of Nd_{2}O_{3} with neutron diffraction, inelastic neutron scattering, heat capacity, and magnetic susceptibility measurements. The inelastic neutron scattering measurements reveal that the crystal field spectrum consists of four excitations spanning the energy range 360 meV. The ground state eigenfunction consists of XYspins in the ab plane. Fits to the magnetic susceptibility data indicate a CurieWeiss temperature of θ_{CW}=23.7(1) K. Neutron diffraction measurements show that long range antiferromagnetic order occurs below T_{N}=550 mK implying a frustration index of θ_{CW} / T_{N} ~43. These results suggest that Nd_{2}O_{3} may be a model system for competing interactions between magnetic moments subject to strong spinorbit coupling. 
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