Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session A37: Kitaev MagnetismFocus

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Sponsoring Units: GMAG DCMP DMP Chair: ZhengXin Liu, Renmin University of China Room: BCEC 206A 
Monday, March 4, 2019 8:00AM  8:36AM 
A37.00001: Gapless Visons and Emergent U(1) Spin Liquid in Kitaev's Honeycomb Model Invited Speaker: Ciarán Hickey In the field of quantum magnetism, the exactly solvable Kitaev honeycomb model serves as a paradigm for the fractionalization of spin degrees of freedom and the formation of Ζ_{2} spin liquid ground states. An intense experimental search has led to the discovery of a number of spinorbit entangled Mott insulators that realize its characteristic bonddirectional spin interactions and, in the presence of strong magnetic fields, exhibit no indications of longrange magnetic order. Here, we map out the complete phase diagram of the Kitaev model in tilted magnetic fields and report the emergence of a distinct gapless quantum spin liquid at intermediate field strengths. Analyzing a number of static, dynamical, and finite temperature quantities using numerical exact diagonalization techniques, we find strong evidence that this phase exhibits gapless fermions coupled to a massless gauge field resulting in a dense continuum of lowenergy states. Such a phase can be naturally understood within the framework of Abrikosov fermionic partons as a U(1) quantum spin liquid with a spinon Fermi surface, emerging via a superconductormetal transition. Finally, we discuss its stability in the presence of perturbations, Heisenberg and offdiagonal symmetric exchange interactions, that naturally arise in spinorbit entangled Mott insulators alongside Kitaev interactions. 
Monday, March 4, 2019 8:36AM  8:48AM 
A37.00002: Ground State of the Spin1/2 Honeycomb Γ Model: Zigzag Magnetic Order HaiJun Liao, RuiZhen Huang, YiBin Guo, ZhiYuan Xie, Bruce Normand, Tao Xiang The offdiagonal symmetric interaction, Γ ( S_{i}^{α}^{ }S^{β}_{i+γ }+ S_{i}^{β }S^{α}_{i+γ }), has sprung to prominence 
Monday, March 4, 2019 8:48AM  9:00AM 
A37.00003: Nonlinear magnetic susceptibility in the Kitaev model Yoshitomo Kamiya, Junki Yoshitake, Yasuyuki Kato, Joji Nasu, Yukitoshi Motome We study the nonlinear spin susceptibility in the Kitaev model [1]. The model has been serving as a paradigmatic model for studying a quantum spin liquid and extensive experimental efforts are currently undertaken. While the JackeliKhaliullin mechanism [2] predicts the ferromagnetic Kitaev model, recent theoretical studies suggest that the antiferromagnetic Kitaev model may stabilize distinct spin liquids in a magnetic field. Since the determination of the sign of the Kitaev coupling can be experimentally problematic, we propose a convenient complementary experimental signature to distinguish the two cases. Here, in the gapped spin liquid phase, we derive an analytical expression in perturbation theory and find that the nonlinear spin susceptibility exhibits a characteristic sign change at finite temperature in the ferromagnetic Kitaev model. We also present results based on numerical simulations (exact diagonalization and quantum Monte Carlo simulations), with which we show that the characteristic sign change also appears in the gapless spin liquid phase with a ferromagnetic coupling [3]. 
Monday, March 4, 2019 9:00AM  9:12AM 
A37.00004: Groundstate phase diagram of the extended KitaevHeisenberg model on a honeycomb lattice Seiichiro Suga, Takafumi Suzuki, Takuto Yamada Recent studies on 4d and 5d transitionmetal compounds on honeycomb lattices have unveiled possibility of realizing the Kitaev spin liquid. In this study, we consider the KitaevHeisenbergΓ model on a honeycomb lattice, where symmetricanisotropic interactions (Γ) exist in addition to the Kitaev and Heisenberg interactions. According to abinitio calculations, this model describes the magnetism in spinorbitcoupled honeycomblattice Mott insulators. We calculate the ground–state energy using series expansions, and obtain the phase diagram. This method has an advantage that can deal with infinitesize systems. We discuss our results by comparing them with the results so far obtained with various numerical methods [13]. [1] J. G. Rau, et al., Phys. Rev. Lett. 112, 077204 (2014). [2] A. Catuneanu, et al., npj Quantum Materials 3, 23 (2018). [3] M. Gohlke, et al., Phys. Rev. B 97, 075126 (2018). 
Monday, March 4, 2019 9:12AM  9:48AM 
A37.00005: Dynamical and Topological Signatures of the KitaevModel in a [111] Magnetic Field Invited Speaker: Matthias Gohlke Quantum spinliquids represent exotic phases of matter that host emergent fractionalized excitations. The Kitaev model [1] is a twodimensional model system in this context and relevant for recent experiments on putative quantum spinliquid materials. Here, we present results for the Kitaev model coupled to a magnetic field along the [111] axis. Using infinite DMRG, we confirm three phases with vastly different transition fields depending on the sign of the Kitaev exchange [2]: A topological phase hosting nonabelian anyons at low fields, an intermediate regime only existing for antiferromagnetic Kitaev exchange, and a fieldpolarized phase hosting topological magnons [3]. 
Monday, March 4, 2019 9:48AM  10:00AM 
A37.00006: Fieldinduced neutral Fermi surface and QCD3 quantum criticalities in Kitaev materials Liujun Zou, YinChen He We perform both numerical and theoretical studies on the phase diagram of the Kitaev materials in the presence of a magnetic field. We find that a new quantum spin liquid state with neutral Fermi surfaces emerges at intermediate field strengths, between the regimes for the nonAbelian chiral spin liquid state and for the trivial polarized state. We discuss the exotic fieldinduced quantum phase transitions from this new state with neutral Fermi surfaces to its nearby phases. We also theoretically study the fieldinduced quantum phase transitions from the nonAbelian chiral spin liquid to the symmetrybroken zigzag phase and to the trivial polarized state.Utilizing the recently developed dualities of gauge theories, we find these transitions can be described by critical bosons or gapless fermions coupled to emergent nonAbelian gauge fields, and the critical theories are of the type of a QCD$_3$ChernSimons theory. We propose that all these exotic quantum phase transitions can be direct and continuous in the Kitaev materials. Therefore, besides being systems with intriguing quantum magnetism, Kitaev materials may also serve as tabletop experimental platforms to study the interesting dynamics of emergent strongly interacting quarks and gluons in $2+1$ dimensions. 
Monday, March 4, 2019 10:00AM  10:12AM 
A37.00007: Formation of magnetic order in the KitaevHeisenberg model ShangShun Zhang, Gabor Halasz, Wei Zhu, Cristian Batista We compute the lowenergy excitation spectrum and the dynamical magnetic spin structure factor of the KitaevHeisenberg model using a variational approach, that becomes exact at the exactly solvable Kitaev points. This approach reveals the physical origin of the asymmetry in the stability range of Kitaev spin liquid phases around the ferromagnetic and antiferromagnetic Kitaev points. We also show that bound states of fractionalized excitations appear in the proximity of a quantum phase transition between the ferrmagnetic Kitaev spin spin liquid and the magnetically ordered states induced by ferro and antiferromagnetic Heisenberg interactions. 
Monday, March 4, 2019 10:12AM  10:24AM 
A37.00008: Quantum phases in spin1 honeycomb antiferromagnets: application to Ni_{2}Mo_{3}O_{8} Shuyi Li, Vaideesh Loganathan, Wenjun Hu, Andriy Nevidomskyy Ni_{2}Mo_{3}O_{8} is a recently synthesized material containing spin1 moments on a honeycomb lattice [1]. Such systems are of interest due to their potential to exhibit topological magnons. According to the recent neutron scattering experiment [1], the two sublattices making up the bipartite honeycomb lattice each display a zigzag antiferromagnetic order. Moreover, the order is noncoplanar with a nontrivial angle between adjacent spins due to competing interactions. In this work, we attempt to explain this spin ordering by the means of meanfield theory and Density Matrix Renormalization Group (DMRG) calculations. We use ab initio Density Functional Theory (DFT) calculations to extract the spinexchange coefficients in the effective lowenergy model. We propose that the DzyaloshinskiiMoriya interaction is the most natural way to explain the observed magnetic ordering. 
Monday, March 4, 2019 10:24AM  10:36AM 
A37.00009: WITHDRAWN ABSTRACT

Monday, March 4, 2019 10:36AM  10:48AM 
A37.00010: Nonlocal String Order Parameter in the S=1/2 KitaevHeisenberg Ladder Erik Sorensen, HaeYoung Kee, Andrei Catuneanu We study the S=1/2 KitaevHeisenberg (KJ) model in a twoleg ladder. Without a Heisenberg interaction, the Kitaev phase in the ladder model has Majorana fermions with local Z2 gauge fields and is usually described as a disordered phase without any order parameter. Here we prove the existence of a nonlocal string order parameter (SOP) in the Kitaev phase which survives with a finite Heisenberg interaction. The SOP is obtained by relating the Kitaev ladder, through a nonlocal unitary transformation, to a onedimensional XY chain with an Ising coupling to a dangling spin at every site. This differentiates the Kitaev phases from other nearby phases including a rung singlet. Two phases with nonzero SOP corresponding to ferromagnetic and antiferromagnetic Kitaev interactions are identified. The full phase diagram of the KJ model is determined using exact diagonalization and density matrix renormalization group methods, which shows a striking similarity to the KJ model on a twodimensional honeycomb lattice. 
Monday, March 4, 2019 10:48AM  11:00AM 
A37.00011: Groundstate phase diagram of the KitaevHeisenberg model on a kagome lattice Katsuhiro Morita, Masanori Kishimoto, Takami Tohyama The KitaevHeisenberg model on the honeycomb lattice has been studied for the purpose of finding exotic states such as quantum spin liquid and topological orders. On the kagome lattice, in spite of a spinliquid ground state in the Heisenberg model, the stability of the spinliquid state has hardly been studied in the presence of the Kitaev interaction. Therefore, we investigate the ground state of the classical and quantum spin systems of the kagome KitaevHeisenberg model. In the classical system, we obtain an exact phase diagram that has an eightfold degenerated canted ferromagnetic phase and a subextensive degenerated Kitaev antiferromagnetic phase. In the quantum system, using the Lanczostype exact diagnalization and cluster meanfield methods, we obtain two quantum spinliquid phases, an eightfold degenerated canted ferromagnetic phase similar to the classical spin system, and an eightfold degenerated q=0, 120^{o} ordered phase induced by quantum fluctuation. 
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