Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session F51: Honeycomb, SpinOrbit Coupling & AnisotropyFocus Recordings Available

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Sponsoring Units: GMAG DMP Chair: Minseong Lee, Los Alamos Nat'l Lab Room: McCormick Place W474B 
Tuesday, March 15, 2022 8:00AM  8:12AM 
F51.00001: Footprints of the Kitaev spin liquid in the Fano lineshapes of the Raman active optical phonons Natalia B Perkins, Kexin Feng, Swetlana Swarup We develop a theoretical description of the Raman spectroscopy in the spinphonon coupled Kitaev system and show that it can provide intriguing observable signatures of fractionalized excitations characteristic of the underlying spin liquid phase. In particular, we obtain the explicit form of the phonon modes and construct the coupling Hamiltonians based on D3d symmetry. We then systematically compute the Raman intensity and show that the spinphonon coupling renormalizes phonon propagators and generates the salient Fano linshape. We find that the temperature evolution of the Fano lineshape displays two crossovers, and the low temperature crossover shows pronounced magnetic field dependence. We thus identify the observable effect of the Majorana fermions and the Z2 gauge fluxes encoded in the Fano lineshape. Our results explain several phonon Raman scattering experiments in the candidate material alphaRuCl3. 
Tuesday, March 15, 2022 8:12AM  8:24AM 
F51.00002: Resonant xray studies in new generation honeycomb iridates Alberto De La Torre, Benjamin Zager, Faranak Bahrami, Michael DiScala, Juan Chamorro, Mary Upton, Gilberto F Fabbris, Daniel Haskel, Diego M Casa, Tyrel M McQueen, Fazel Tafti, Kemp Plumb The Kitaev Quantum spin liquid model can be realized in materials composed of d5 magnetic atoms in a strong octahedral crystal field and large spinorbit coupling [1]. Of particular interest are Ir 4+ with a network of edgesharing IrO6 octahedra arranged in a honeycomb lattice, for example, αLi_{2}IrO_{3} and NaIrO_{3}, in which strong Kitaev interactions are present. However, the existence of finite Heisenberg and pseudodipolar interactions in these compounds leads to longrange magnetic order at low temperatures [2,3]. Aiming to modify the magnetic interactions in these compounds, new honeycomb iridates, including Ag_{3}LiIr_{2}O_{6} [4] and H_{3}LiIr_{2}O_{6} [5], have been synthesized by introducing interhoneycomblayers Ag and H. However, to date, there is little empirical understanding of the influence of the intercalated atoms on the electronic structure, local Ir environment, and magnetism in this new generation of Kitaev honeycomb iridates. In this talk, I will present a series of resonant xray spectroscopy measurements on a set of honeycomb iridates, including Ag_{3}LiIr_{2}O_{6} [6] and H_{3}LiIr_{2}O_{6}, and discuss the validity of the local j_{eff} = ½ picture in the context of the Kitaev model for these compounds. 
Tuesday, March 15, 2022 8:24AM  8:36AM 
F51.00003: Canted antiferromagnetic order and spin dynamics in the honeycomblattice Tb_{2}Ir_{3}Ga_{9} Zachary Morgan, Feng Ye, Wei Tian, Songxue Chi, Xiaoping Wang, Michael E Manley, David S Parker, Mojammel A Kahn, John F Mitchell, Randy S Fishman Singlecrystal neutron diffraction, inelastic neutron scattering, bulk magnetization measurements are used to investigate the magnetic properties of honeycomb lattice Tb_{2}Ir_{3}Ga_{9}. While the R ln 2 magnetic contribution to the lowtemperature entropy indicates a J_{eff} = 1/2 moment for the lowestenergy crystalfield doublet, the Tb3+ ions form a canted antiferromagnetic structure below 12.5 K. Due to the DzyaloshinskiiMoriya interactions, the Tb moments in the ab plane are slightly canted towards b with a moment of 1.22 μB per formula unit. A minimal xxz spin Hamiltonian is used to simultaneously fit the spinwave frequencies along the highsymmetry directions and the field dependence of the magnetization along the three crystallographic axes. Second nearest neighbors interactions for both inplane and outofplane couplings are needed to account for the observed static and dynamic properties. The z component between Tb moments is larger than the x and y components. The nearestneighbor exchange coupling between moments parallel and perpendicular to the 4f orbitals is negligible. Despite the J_{eff} = 1/2 moments, the spin Hamiltonian is denominated by a large inplane anisotropy K_{z}.The observed diffuse scattering features indicate considerable 3D magnetic correlations along c. 
Tuesday, March 15, 2022 8:36AM  8:48AM 
F51.00004: Mapping the Temperature and Fielddependence of the Thermal Hall Conductivity in RuCl_{3} Peter A Czajka, Arnab Banerjee, Paige LampenKelley, Jiaqiang Yan, David G Mandrus, Stephen E Nagler, N. Phuan Ong RuCl_{3} is thought to exhibit a strong Kitaevtype exchange interaction, but whether this interaction produces a spin liquid state with chiral Majorana edge modes is still unclear. Here, we present new measurements of the material’s thermal Hall conductivity down to He_{3} temperatures. We observe a large, temperaturedependent signal that clearly differs from the temperatureindependent form expected for Majorana fermions. Using the quantitative framework established by Matsumoto and Murakami [1], we show that this temperaturedependence can be fit to a bosonic model and that this fit produces values that are consistent with known parameters for RuCl_{3}. How this result might be reconciled with other thermal transport reports will be discussed. 
Tuesday, March 15, 2022 8:48AM  9:00AM 
F51.00005: NonLoudonFleury Raman scattering in spinorbit coupled Mott insulators Yang Yang, Mengqun LI, Ioannis Rousochatzakis, Natalia B Perkins We revisit the theory of magnetic Raman scattering in Mott insulators with strong spinorbit coupling, with a major focus on Kitaev materials. We show that in order to obtain the correct leading contributions to the Raman vertex operator R one must take into account the precise, photonassisted microscopic hopping processes of the electrons. Hence in systems with multiple hopping paths, R might contain terms beyond those appearing in the traditional LoudonFleury theory. We apply the revised theory to the threedimensional hyperhoneycomb Kitaev material β−Li2IrO3, and the results show a qualitative modification of the polarization dependence, including, e.g., the emergence of a sharp onemagnon peak at low energies, which is not expected in the traditional LoudonFleury theory. This peak is shown to arise from microscopic photonassisted tunneling processes that are of similar type with the ones leading to the symmetric offdiagonal interaction Γ, but take the form of a bonddirectional magnetic dipole term in the Raman vertex. These results are expected to apply across all Kitaev materials and mark a drastic change of paradigm for the understanding of Raman scattering in materials with strong spinorbit coupling and multiple exchange paths. 
Tuesday, March 15, 2022 9:00AM  9:12AM 
F51.00006: Binding of NonAbelian Anyons to Vacancies in Kitaev's Spin Liquid on the Honeycomb Lattice Erik E Aldape, Ehud Altman It was previously found that vacancies in the Z_{2} spin liquid phase of Kitaev's honeycomb lattice model tend to bind a Z_{2} magnetic flux. We find that this phenomenon extends also to the nonabelian spin liquid, where the bound Ising flux is associated with a nonabelian anyon. We show that this phenomenon occurs because a single vacancy is a resonant scatterer of quasiparticles, which therefore induces a sharp resonance peak in the lowenergy local density of states. A bound flux effectively screens the vacancy and suppresses the resonance peak, thereby reducing the ground state energy. Because this result is obtained in the universal lowenergy theory of the spin liquid it is independent of the special features of Kitaev’s exactly solvable model. 
Tuesday, March 15, 2022 9:12AM  9:48AM 
F51.00007: Breakthrough in the study of frustrated ferroantiferromagnets: Rb_{2}Cu_{2}Mo_{3}O_{12} and Cs_{2}Cu_{2}Mo_{3}O_{12} Invited Speaker: Shohei Hayashida Linearchain molybdates A_{2}Cu_{2}Mo_{3}O_{12} (A = Rb, Cs) are among the most intriguing frustrated quasionedimensional quantum ferroantiferromagnets [1]. For over a decade extensive experimental and theoretical work focused on the unique magnetoelectric effect in Rb_{2}Cu_{2}Mo_{3}O_{12} [2,3], and on spinnematic TomonagaLuttinger liquid behavior in Cs_{2}Cu_{2}Mo_{3}O_{12} [4]. Unfortunately, a lack of singlecrystal samples has hampered any further progress. In a breakthrough, our group has succeeded in growing single crystals for both materials. Subsequent measurements revealed that their magnetic properties are even more complex and intriguing than originally thought [57]. The previously reported chiralspinliquid and magnetoelectric hightemperature phase in Rb_{2}Cu_{2}Mo_{3}O_{12} [3] was shown to be spurious and due to impurities. Our comprehensive thermodynamic study mapped out the highly anisotropic lowtemperature phase diagram with quantum critical points of different effective dimensionalities [5]. Dielectric experiments revealed this material to be a quantum multiferroic and enabled the first direct excitationresponse measurements of critical susceptibility at a magnetic BEC quantum critical point [6]. For the Cs system, a host of exotic fieldinduced phases was discovered, including a robust presaturation phase that is likely to be a spinnematic phase [7]. New results pertain to its unconventional magnetoelectric response. 
Tuesday, March 15, 2022 9:48AM  10:00AM 
F51.00008: Fluctuationdriven colossal magnetoresistance without magnetic polarization in the ferrimagnetic insulator Mn_{3}Si_{2}Te_{6} Yifei Ni, Hengdi Zhao, Yu Zhang, Bing Hu, Itamar Kimchi, Gang Cao Magnetic polarization is essential to colossal magnetoresistance (CMR) in almost all CMR materials, except for a few exceptions. Here we report a new type of CMR in the title compound that emerges only when magnetic polarization is avoided. The resistivity drops by 7 orders of magnitude when the magnetic field H is applied along the magnetic hard axis but only by 20% when H is aligned along with the magnetic easy axis, where the magnetization is fully polarized. The anisotropy field separating the easy and hard axes is 13 T, suggesting the strong effect of the spinorbit interactions. Double exchange and JahnTeller distortions that drive the wellstudied holedoped manganites do not exist in Mn_{3}Si_{2}Te_{6}. The phenomena fit no existing models, suggesting a unique, intriguing type of electrical transport. 
Tuesday, March 15, 2022 10:00AM  10:12AM 
F51.00009: Twice Hidden Order in the KitaevGamma Ladder Erik S Sorensen Today, there is a growing class of Kitaev materials where it is believed that bonddependent Kitaev interactions are present. However, other interactions are also often present, among them the socalled Gamma (Γ) term and here we consider the KitaevGamma (KG) model. In order to obtain highly accurate results it is often useful to restrict the analysis to lowdimensions and in this talk we mainly discuss the ladder. Using matrix product states it is possible to determine the phasediagram of this ladder system with very high precision including the effects of a magnetic field in the [111] direction. An abundance of new phases appear. Here we focus on one of them, dominated by the antiferromagnetic Gamma point where it can be shown that twice hidden order is present in the sense that there exists a unitrary mapping to another model which has longrange string order. 
Tuesday, March 15, 2022 10:12AM  10:24AM 
F51.00010: Extent of frustration in the classical KitaevΓ model via bond anisotropy Ahmed Rayyan, Qiang Luo, HaeYoung Kee In the pseudospin1/2 honeycomb Mott insulators with strong spinorbit coupling, there are two types of bonddependent exchange interactions named Kitaev (K) and Gamma (Γ) that lead to strong frustration. While the ground state of the Kitaev model is a quantum spin liquid with fractionalized excitations, the ground state of the Γ model remains controversial. In particular, the phase diagram of the KΓ model with ferromagnetic K and antiferromagnetic Γ interactions has been intensively studied because of its relevance to candidate materials such as αRuCl_{3}. Numerical studies also included the effects of tuning the bond strengths, i.e., zbond strength different from the other bonds. However, no clear consensus on the overall phase diagram has been reached yet. Here we discuss the classical KΓ model with anisotropic bond strengths studied using Monte Carlo simulations to understand the phases that emerge out of the competition between the two frustrated limits. We also address how the anisotropic bond strength affects the phase diagram and strength of quantum fluctuations. 
Tuesday, March 15, 2022 10:24AM  10:36AM 
F51.00011: Thermal Hall effect and spin nernst effect in a frustrated honeycomb magnet without DM interaction Kosuke Fujiwara, Sota Kitamura, Takahiro Morimoto We study thermal responses of magnons in a honeycomb lattice with frustration. While most of the previous studies on transverse thermal responses in magnets consider the effect of DM interaction, we propose that nonzero thermal Hall effect and spin Nernst effect arise without DM interaction once we incorporate frustration with next nearest neighbor coupling and asymmetry between sublattices. We determine spiral spin configurations that minimize the total energy classically and derive a magnon Hamiltonian from the obtained configuration using HolsteinPrimakov transformation. We compute Berry curvature that characterize a nontrivial geometry of the magnon states and discuss thermal Hall effect and spin Nernst effect that originate from such nontrivial geometry. 
Tuesday, March 15, 2022 10:36AM  10:48AM 
F51.00012: Firstorder antiferromagnetic transitions in CaMn_{2}P_{2 }and SrMn_{2}P_{2} single crystals containing corrugatedhoneycomb Mn sublattices N. S. Sangeetha, S. Pakhira, Q.P. Ding, L. Krause, H. Lee, V. Smetana, A. Mudring, B. Brummerstedt Iversen, Y. Furukawa, D.C. Johnston CaMn_{2}P_{2} and SrMn_{2}P_{2} are insulators that crystallize in the trigonal CaAl_{2}Si_{2}type structure containing corrugated honeycomb layers of Mn^{2+} spins 5/2. Magnetic susceptibility χ and heat capacity measurements versus temperature T reveal a strong firstorder antiferromagnetic (AFM) transition in CaMn_{2}P_{2} at T_{N} = 69.8(3) K and a weak firstorder AFM transition in SrMn_{2}P_{2} at T_{N} = 53(1) K. The χ(T) data for both compounds exhibit nearly isotropic and Tindependent behavior below T_{N}. NMR measurements indicate a commensurate AFM structure for CaMn_{2}P_{2} and an incommensurate one for SrMn_{2}P_{2}. CaMn_{2}P_{2} and SrMn_{2}P_{2} are rare examples where an AFM transition in zeromagnetic field is thermodynamically of first order. These firstorder AFM transitions are unique among the (Ca, Sr, Ba)Mn_{2}(P, As, Sb, Bi)_{2} compounds that exhibit secondorder AFM transitions. Determining the mechanism for the firstorder transitions may lead to the development of new materials of technological interest. 
Tuesday, March 15, 2022 10:48AM  11:00AM 
F51.00013: Ground state phase diagram of the KitaevHeisenberg model with general spin S Kiyu Fukui, Yasuyuki Kato, Joji Nasu, Yukitoshi Motome The Kitaev model is an S=1/2 quantum spin model defined on the honeycomb lattice. In spite of strong frustration due to the bonddependent exchange interaction, this model is exactly solvable, and its ground state is a quantum spin liquid state. In recent years, candidates of higher spin Kitaev materials were proposed, while the stability of the Kitaev quantum spin liquid to nonKitaev interactions has not been systematically clarified for general spin S. In this study, we investigate the ground state for the spinS KitaevHeisenberg model by using the pseudofermion functional renormalization group method. We clarify the ground state phase diagram for S=1/2, 1, 3/2, 2, 5/2, and 50. We reveal that quantum spin liquid regions shrink with increasing S, but they remain finite for S=1 and 3/2; the Kitaev quantum spin liquid is extremely fragile against the Heisenberg interaction for S≥2. We also confirm that the result for S=50 well reproduces the phase diagram by the classical Monte Carlo method. Our result will provide the search and design of the candidate materials with high S. 
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