Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session W51: Triangular SystemsFocus Recordings Available

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Sponsoring Units: GMAG DMP Chair: Sara Haravifard, Duke University Room: McCormick Place W474B 
Thursday, March 17, 2022 3:00PM  3:36PM 
W51.00001: Impact of the lattice on magnetic properties in the S=1 triangular antiferromagnet NiGa_{2}S_{4} Invited Speaker: Natalia Drichko NiGa_{2}S_{4} is a triangular lattice S=1 system with strong two dimensionality of the lattice, and a competition between ferromagnetic nearest neigbor exchange J_{1} and antiferromagnetic third nearest neigbor exchange J_{3}. It was actively discussed as a candidate to host spinnematic order brought about by strong quadrupole coupling. Using Raman scattering spectroscopy we identify a phonon of E_{g} symmetry which can modulate magnetic exchange J_{1} and produce quadrupole coupling. Additionally, our Raman scattering results demonstrate a gradual loss of local inversion symmetry on lowering the temperature of the sample. We associate it with a lattice distortion due to sulfur vacancies. This loss of inversion symmetry will lead to disordered DzyaloshinskiiMoriya interactions, which can prevent longrange magnetic order. Using magnetic Raman scattering response we identify 160 K as a temperature of an upturn of magnetic correlations. The temperature range below 160 K, but above 50 K where antiferromagnetic correlations start to increase, is a candidate for spinnematic regime. 
Thursday, March 17, 2022 3:36PM  3:48PM 
W51.00002: Realization of Quantum Dipoles in Triangular Lattice Crystal Ba_{3}Yb(BO_{3})_{3} Rabindranath Bag, Matthew Ennis, Chunxiao Liu, Sachith E Dissanayake, Zhenzhong Shi, Jue Liu, Leon Balents, Sara Haravifard We investigate the thermodynamic properties of the ytterbiumbased triangular lattice compound Ba_{3}Yb(BO_{3})_{3}. These results demonstrate the absence of any longrange ordering down to 56 mK. The analyses of these results show that Ba_{3}Yb(BO_{3})_{3} may realize an S = ½ quantum dipole lattice, in which the dominant interaction is the long range dipoledipole coupling on the geometrically frustrated triangular lattice, and exchange interactions are subdominant or negligible. 
Thursday, March 17, 2022 3:48PM  4:00PM 
W51.00003: Crystal Growth and Characterization of Rareearth Triangular Lattice Borates Matthew Ennis, Rabindranath Bag, Sachith E Dissanayake, Zhenzhong Shi, Sara Haravifard Triangular antiferromagnetic materials have attracted attention because competing interactions on the lattice can give rise to exotic phenomena, such as quantum spin liquids. Motivated by evidence that Ba_{3}Yb(BO_{3})_{3} may host a spin1/2 quantum dipole lattice, we have synthesized several members of the borate family Ba_{3}RE(BO_{3})_{3} using different rareearth elements to examine the effect of placing different spins in the lattice. We have grown large single crystals of these materials using the optical floating zone technique and performed inhouse thermodynamics characterization measurements, from which we find no evidence of ordering down to the lowest accessible temperatures. In this talk we will present the results of our experimental efforts. 
Thursday, March 17, 2022 4:00PM  4:12PM 
W51.00004: Spinphonon hybridization and its direct effect on thermal transport in the frustrated triangular magnet CsYbSe_{2}. Christopher A Pocs, Minhyea Lee, Michael Hermele, Athena S Sefat, Jie Xing The thermal conductivity of CsYbSe_{2}, and in fact many other frustrated magnets, often exhibits strong, nonmonotonic fielddependence, even when the state of magnetic lattice is disordered and a coherenet itinerant magnetic contribution is absent. We present a novel and highlygeneric way in which phonon heat conduction can acquire unique nontrivial fielddependence through the hybridization of acoustic phonons and spin flip excitations via strainmodulation of the magnetic gtensor. Analytical modelling of the hybridized spectra of this model under applied fields is successful in reproducing the qualitative features of transport data in CsYbSe_{2}, and is potentially widely applicable to many other insulating magnetic systems. 
Thursday, March 17, 2022 4:12PM  4:24PM 
W51.00005: Magnetism in the triangular lattice materials CeCl_{3} and CeBr_{3} Matthew A Cothrine, Adam A Aczel, Matthew B Stone, Stephen E Nagler, David G Mandrus, Colin Sarkis, Luis Stand, Mariya Zhuravleva There is currently great interest in systems with J = 1/2 ground state and Isinglike interactions that may be geometrically frustrated, as in the triangular lattice, or frustrated by competing interactions, like the honeycomb Kitaev model. This interest has spurred our investigation of the triangular lattice rare earth materials CeCl_{3} and CeBr_{3}, where the Ce^{3+} ion is expected to have an effective J = 1/2 ground state. Some magnetic properties of these materials were studied many years ago but prior to this investigation there had been no direct measurements of the magnetic order or excitations using neutron scattering. Here we report neutron scattering investigations of crystal field excitations in CeBr_{3}, confirming the J=1/2 ground state, and neutron diffraction measurements of the magnetic order in CeCl_{3}, showing that the system orders antiferromagnetically at low temperature with an ordering wavevector (1/3 1/3 1/2). 
Thursday, March 17, 2022 4:24PM  4:36PM 
W51.00006: Magnon Pairing, Interactions, and Decay in the Spinorbital Magnet FeI_{2} Xiaojian Bai, Zhiling Dun, ShangShun Zhang, Anaelle Legros, Hao Zhang, Matthew B Stone, Vasile O Garlea, William A Phelan, Peter N Armitage, Cristian Batista, Martin P Mourigal One of the scientific frontiers in quantum magnetism is the discovery and understanding of quantum entangled and topologically ordered states in real bulk materials. At the focal point of the experimental investigation of these quantum spin networks is the identification of fractionalized excitations in transport and spectroscopic measurements. Inelastic neutron scattering has proved a powerful technique to reveal such signatures in a variety of systems ranging from quasi1D magnets to kagome compounds and more. Recent and ongoing developments with neutron scattering instrumentation have allowed the characterization of magnetic excitations in entire volumes of momentumenergy space with high resolution. This has prompted revisiting longoverlooked materials in search of exotic spin dynamics despite seemingly classical magnetically ordered groundstates. In this talk, I will discuss such experiments on a longknown material, FeI2, and show how highfidelity modeling brings new insights into its spin dynamics [1]. I will describe the mechanism that endows low energy quadrupolar fluctuations in FeI2 with large spectral weight and how these can be completely understood using a SU(3) representation of spin degrees of freedom. I will discuss the consequence of having several quasiparticles as the lowenergy degrees of freedom in this system including the formation of heavy boundstates [2] and their mutual decay [3]. 
Thursday, March 17, 2022 4:36PM  4:48PM 
W51.00007: Successive phase transitions and incommensurate magnetic ordering in a triangularlattice antiferromagnet Qing Huang The triangularlattice antiferromagnet (TLAF) has attracted a lot of attention in the past several decades as a prototypical twodimensional (2D) frustrated magnet for which quantum fluctuations may stabilize unconventional groundstates such as the novel upupdown (UUD) ground state. We have conducted a systematic study on a new TLAF compound with perfect equilateral triangular lattice Na2BaMn(PO4)2 with DC and AC susceptibilities and neutron scattering measurements. DC susceptibility measurements show the saturation field to be around 7 T. AC susceptibility measurements on singlecrystal samples have confirmed a longrange magnetic ordering below 1.1 K under zero field and two critical fields at 1.6 T and 2.9 T with the magnetic field applied along the c axis, which also indicates an easyaxis anisotropy. Neutron scattering measurements were conducted on powder sample. The results suggest a novel incommensurate ordering in this compound under zero and finite fields, which is seldom to be observed in TLAF. More work will be done to solve the exact magnetic structure in different magnetic phases. And specific heat measurements will be conducted to map out a complete phase diagram. 
Thursday, March 17, 2022 4:48PM  5:00PM 
W51.00008: Pottsnematic Quantum Phase in Frustrated Heisenberg Antiferromagnets Jeremy Strockoz, Dmitri LaBelle, Jörn W Venderbos In this talk we consider a class of quantum antiferromagnets with frustrated Heisenberg exchange interactions and collinear magnetic ground states which spontaneously break discrete lattice rotation symmetry. In particular, we examine the nature of the quantum phase transition between the disordered paramagnetic state and the magnetically ordered state, and show that this transition can occur via an intermediate paramagnetic phase in which rotation symmetry is broken. To demonstrate this, we investigate a spin1 J_{1}J_{2} model on the triangular lattice with a singleion anisotropy and show that the paramagnet becomes unstable to the formation of nematic twoparticle bound states as the strength of the singleion anisotropy decreases. This indicates the existence of a Pottsnematic quantum paramagnetic phase which lacks magnetic order, but breaks the threefold rotation symmetry of the lattice. We also discuss generalizations to other systems and thermal phase transitions. 
Thursday, March 17, 2022 5:00PM  5:12PM 
W51.00009: Emergent Potts Z3 order in the Heisenberg windmill model with biquadratic exchange interactions AnaMarija Nedic, Victor L Quito, Yuriy Sizyuk, Peter P Orth We design and study a bilinearbiquadratic Heisenberg model with coexisting ferro and antiferromagnetic exchange interactions on the windmill lattice, which consists of coupled triangular and honeycomb lattices. Spins on the triangular layer are coupled antiferromagnetically, while spins on the honeycomb layer interact ferromagnetically. We determine the classical ground state phase diagram, which includes a variety of phases including a noncoplanar canted umbrella state, canted coplanar phases and a fully ferromagnetic state, resembling the finitetemperature finitemagnetic field phase diagram of the classical Heisenberg antiferromagnet on a triangular lattice. We identify a region in the classical ground state phase diagram with a Z3degenerate ground state. Using MonteCarlo simulations and analytical techniques, we determine the finitetemperature phase diagram above this phase and identify a finitetemperature Potts phase transition of an emergent Z3 order parameter. 
Thursday, March 17, 2022 5:12PM  5:24PM 
W51.00010: Commensurate and incommensurate antiferromagnetic orderings in SrMn_{2}As_{2} and CaMn_{2}As_{2 }revealed by NMR Yuji Furukawa, QingPing Ding, N.S. Sangeetha, Abhishek Pandey, David C Johnston Recently much attention has been paid to the Mnbased compounds with AMn_{2}Pn_{2} (A= Sr, Ca, Ba, and Pn = P, As, Sb, Bi) since the systems show a rich variety of magnetic properties with different crystal structures. Different from BaMn_{2}As_{2} which shows the bodycentered tetragonal ThCr_{2}Si_{2}type structure, (Ca,Sr)Mn_{2}Pn_{2} (Pn = P, As, Sb, Bi) crystallize in the trigonal CaAl_{2}Si_{2}type structure where the Mn ions form a triangular lattice bilayer which can be considered as a corrugated Mn honeycomb sublattice. Among the Mn compounds, CaMn_{2}As_{2} and SrMn_{2}As_{2} have been reported to be antiferromagnetic (AFM) insulators with T_{N} = 62 K and 120 K, respectively. In this study, we have carried out ^{75}As nuclear magnetic resonance (NMR) measurements on those systems to investigate the magnetic properties from a microscopic point of view. In the case of SrMn_{2}As_{2}, no obvious distribution of the internal field at the As site was observed in the AFM state. In contrast, broad and complex NMR spectra were observed in CaMn_{2}As_{2} in the AFM state, which clearly shows a distribution of internal field at the As site. Those results indicate that the antiferromagnetic states are commensurate and incommensurate in SrMn_{2}As_{2} and CaMn_{2}As_{2}, respectively. 
Thursday, March 17, 2022 5:24PM  5:36PM 
W51.00011: Suppression of antiferromagnetic order and strong ferromagnetic spin fluctuations in Ca(Co_{1−x}Ni_{x})_{2−y}As_{2} single crystals Santanu Pakhira, Yongbin Lee, Liqin Ke, Volodymyr Smetana, Anjaverena Mudring, Thomas W Heitmann, David Vaknin, David Johnston The highlyfrustrated itinerant magnetic system CaCo_{2y}As_{2 }exhibits Atype antiferromagnetic (AFM) order below T_{N} = 52 K. From the magnetization M versus temperature T, magnetic field H, and heat capacity C_{p}(T, H) measurements it is found that T_{N} decreases to 22 K with only 3% Ni substitution and is completely suppressed for x > 0.16. Magnetic susceptibility χ measurements reveal that strong ferromagnetic (FM) fluctuations develop for 0.11≤ x ≤ 0.52. The FM fluctuations for x = 0.21 and 0.31 are quasi1D in nature, where χ_{c} >> χ_{ab}. The C_{p}(T) measurements further reveal that the FM spin fluctuations are quantum in nature, where a logarithmic Tdependent upturn in C_{p}(T )/T is observed at low T and H = 0, that is suppressed with increasing H. A reentrant spinglass behavior arising out of competing AFM and FM interactions is also observed for x = 0.11 and 0.16 below T_{N}. No signature of any magnetic order or spin fluctuations is observed for x > 0.52 at T ≥ 1.8 K. Density functional theory (DFT) calculations show that the transition to nonmagnetic state occurs via a Stoner transition. 
Thursday, March 17, 2022 5:36PM  5:48PM 
W51.00012: Dynamic and frozen quantum magnetism in the ground states of triangular lattice magnets YbMgGaO_{4}, ErMgGaO_{4} and YbCoGaO_{4} from inelastic neutron scattering HsiaoYuan (Symphony) Huang, Evan M Smith, Edwin Kermarrec, Sylvain Petit, Zachary W Cronkwright, Suvam Bhattacharya, Bruce D Gaulin The putative quantum spin liquid (QSL) state exhibited by YbMgGaO_{4} is largely ascribed to the quasi2D triangular lattice which the magnetic Yb^{3+} moments decorate in concert with anisotropic exchange and disorder in neighbouring disordered Mg^{2+}/Ga^{3+} bilayers [1,2,3]. We present new inelastic neutron scattering (INS) measurements on YbMgGaO_{4} as well as its isostructural sister compounds: ErMgGaO_{4} and YbCoGaO_{4}. Each material was synthesized as a phasepure powder and INS measurements were performed on IN6Sharp with E_{i} = 3.1 meV at the Institut LaueLangevin. We discuss the observed hallmarks of a QSL in YbMgGaO_{4} and contrast these with signatures of frozen spin correlations in each of ErMgGaO_{4} and YbCoGaO_{4} in the INS measurements. The case of YbCoGaO4 is an interesting one as it displays the interplay between a QSLlike state associated with the Yb^{3+} magnetism, interleaved with a twodimensional spin glass state originating from the disordered Co^{2+} magnetism. 
Thursday, March 17, 2022 5:48PM  6:00PM 
W51.00013: First principles study of monolayer magnetic triangular lattice compounds MX_{2} Kira Riedl, Danila Amoroso, Steffen Backes, Aleksandar Razpopov, Thi Nguyen, Kunihiko Yamauchi, Paolo Barone, Stephen Winter, Silvia Picozzi, Roser Valenti In the quest to understand exotic lowdimensional magnetic states, magnetic triangular lattices offer a rich playground. With this study, we model effective magnetic interactions for the monolayer structures of a range of triangular lattice materials MX_{2} (M={V,Mn,Ni}, X={Cl,Br,I}). We also took the opportunity of such a larger study to benchmark two abinitio methods used to extract effective S={3/2,5/2,1} spin models for real materials. Additionally, by means of toy model parameters based on general properties such as filling and hybridization, we shed light on the microscopic mechanism behind the different behaviors and magnetic interactions displayed by these, apparently similar, systems. In particular, the considered relatively light magnetic metal ions have small spinorbit coupling (SOC), which allows to effectively tune the SOC by interchanging the ligand elements. We find that the corresponding SOC matrixelements differ strongly from the atomic limit and that SOC effects manifest only in anisotropic exchange and singleion anisotropy for specific fillings. Noticeably, for monolayer NiI_{2} we do find a sizeable Kitaev coupling, suggested in previous works. 
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