Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session K07: Spin Liquids and Ices 
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Sponsoring Units: DCMP GMAG Chair: Randy Fishman, Oak Ridge National Laboratory Room: BCEC 109B 
Wednesday, March 6, 2019 8:00AM  8:12AM 
K07.00001: Discovery of chiralspin order in purported Kitaev spinliquids Chandra Varma, Kimberly Modic, Brad Ramshaw, Arkady Shekhter We examine recent magnetic torque measurements in two compounds, $\gamma$Li$_2$IrO$_3$ and RuCl$_3$, which have been discussed as possible realizations of the Kitaev model. The analysis of the reported discontinuity in torque, as an external magnetic field is rotated across the $c$axis in both crystals, suggests that they have a translationallyinvariant chiral spinorder of the from $<{\bf S}_i. \big({\bf S}_j~ \times~ {\bf S}_k\big)> \ne 0$ in the ground state and persisting over a very wide range of magnetic field and temperature. An extraordinary $BB^2$ dependence of the torque for small fields, beside the usual $B^2$ part, is predicted due to the chiral spinorder. Data for small fields is available for $\gamma$Li$_2$IrO$_3$ and is found to be consistent with the prediction upon further analysis. Other experiments such as inelastic scattering and thermal Hall effect and several questions raised by the discovery of chiral spinorder, including its topological consequences are discussed. 
Wednesday, March 6, 2019 8:12AM  8:24AM 
K07.00002: Characteristics of the intermediatefield ordered phase of alpha RuCl3 revealed by magnetic diffraction Arnab Banerjee, Christian Balz, Paige LampenKelley, Jiaqiang Yan, David George Mandrus, Yaohua Liu, Stephen Nagler There has been great interest in alphaRuCl3 as a material that manifests Kitaev terms in the spin Hamiltonian and a possible fieldinduced quantum spin liquid (QSL) phase by 8 T [1]. Bulk Measurements show that in the inplane magnetic field causes a transition to an intermediate ordered phase around 6 Tesla [1, 2], preceding the total suppression of magnetic order around 7.3 T. Here we use timeofflight neutron diffraction to elucidate the nature of intermediate phase and its magnetic order. Our results provide new insights into the dimensionality of the magnetic order and the effective spinHamiltonian of the material. 
Wednesday, March 6, 2019 8:24AM  8:36AM 
K07.00003: Magnon thermal Hall effect in the candidate Kitaev spinliquid αRuCl_{3} Jonathan Cookmeyer, Joel Moore Motivated by recent experiments measuring the thermal Hall conductivity of αRuCl_{3} and using spinwave theory and model effective Hamiltonians from the literature, the magnon Thermal Hall coefficient in the magnetically ordered phase is computed. Comparing directly with the experiment, no previously proposed model well matches the data. A model is determined which can explain the data demonstrating that the magnon Berry curvature could be responsible. The discrepancy could also be explained by phonons but not by Kitaevlike excitations. 
Wednesday, March 6, 2019 8:36AM  8:48AM 
K07.00004: Effective dimension, level statistics, and integrability of SachdevYeKitaevlike models Eiki Iyoda, Hosho Katsura, Takahiro Sagawa Recently, quantum manybody systems exhibiting fast scrambling (delocalization of quantum information) have attracted much attention. A wellknown example is the SachdevYeKitaev (SYK) model, describing random alltoall fourbody interactions among fermions. Here, we introduce a variant of the SYK model, which we refer to as the Wishart SYK model [1]. The model includes the original SYK model without quenched disorder as a special case. 
Wednesday, March 6, 2019 8:48AM  9:00AM 
K07.00005: A candidate Theory for the "Strange Metal" phase based on the SachdevYeKitaev physics Xiaochuan Wu, Xiao Chen, ChaoMing Jian, Yizhuang You, Cenke Xu We propose a lattice model for strongly interacting electrons with the potential to explain the main phenomenology of the strange metal phase in the cuprate hightemperature superconductors. Our model is motivated by the recently developed “tetrahedron” rank3 tensor model that mimics much of the physics of the betterknown SachdevYeKitaev (SYK) model. Our electron model has the following advantageous properties: (1) it needs only one orbital per site on the square lattice. (2) It does not require any quenched random interaction. (3) It has local interactions and respects all the symmetries of the system. (4) The soluble limit of this model has a longitudinal dc resistivity that scales linearly with temperature within a finite temperature window. (5) Again, the soluble limit of this model has a fermion pairing instability in the infrared, which can lead to either superconductivity or a “pseudogap” phase. The linearT longitudinal resistivity and the pairing instability originate from the generic scaling feature of the SYK model and the tetrahedron tensor model. 
Wednesday, March 6, 2019 9:00AM  9:12AM 
K07.00006: Charge transport in graphenebased mesoscopic realizations of SachdevYeKitaev models Oguzhan Can, Emilian Nica, Marcel Franz We address transport properties of a mesoscopic realization of the SachdevYeKitaev (SYK) model which is an exactly solvable system of interacting spinless fermions connected to the black hole physics through the holographic principle. Starting with a recent proposal for simulating the SYK model in a graphene flake in an external magnetic field and extending it by considering leads attached to it, we model a realistic transport experiment and calculate directly measurable quantities featuring nonFermi liquid signatures of the SYK physics. We show that the graphene flake realization is robust in the presence of leads and that measuring the tunneling current across the leads one can experimentally observe a nonFermi liquid  Fermi liquid transition by tuning the external magnetic field threading the flake. After establishing the transport signatures of the SYK model near equilibrium using linear response framework, we then derive a formula to extend our results for tunneling current using Keldysh formalism to explore the effects of finite bias voltage across the leads, going beyond equilibrium. 
Wednesday, March 6, 2019 9:12AM  9:24AM 
K07.00007: Characterization of quantum manybody chaos with quantum Lyapunov exponents and by twopoint correlations: application to a generalized SachdevYeKitaev model Hrant Gharibyan, Masanori Hanada, Brian Swingle, Masaki Tezuka We propose two quantities for characterization of quantum manybody chaos. Firstly, we define a simple quantum generalization of the spectrum of finitetime classical Lyapunov exponents. [1] We study its statistical features for the SYK model and find random matrix behavior, which is lost when the model is deformed away from chaos towards integrability [2] by a random twofermion term. Secondly, we propose that twopoint correlation functions can also characterize quantum manybody chaos, with numerical evidences for the SYK model as well as for the XXZ spin chain with random field, and discuss the plausibility of laboratory experiments. 
Wednesday, March 6, 2019 9:24AM  9:36AM 
K07.00008: Chiral spin liquid phase of the triangular lattice Hubbard model Aaron Szasz, Johannes Motruk, Michael Zaletel, Joel Moore

Wednesday, March 6, 2019 9:36AM  9:48AM 
K07.00009: Monopoles in Dirac spin liquids I: from spinon band topology to monopole quantum numbers Xueyang Song, Chong Wang, YinChen He, Ashvin Vishwanath We explore a lowenergy theory for 2D quantum magnets, the Dirac spin liquid (DSL), a version of Quantum Electrodynamics ( QED$_3$) with four flavors of Dirac fermions coupled to photons. We study the spatial/timereversal symmetry properties of the magnetic monopoles, an important class of excitations that drive confinement. We show that the underlying band topology of spinon insulators, e.g., quantum spin hall phase/wannier insulator protected by rotation provides crucial information on the tricky Berry phase of monopole (under timereversal / rotations, respectively). We also prove the existence of a trivial monopole on bipartite lattices by invoking its ascension to a QCD with SU(2) gauge structure. 
Wednesday, March 6, 2019 9:48AM  10:00AM 
K07.00010: Monopoles in Dirac spin liquids II: towards a unifying description of 2D quantum magnetism Chong Wang, Xueyang Song, YinChen He, Ashvin Vishwanath The understanding of monopole quantum numbers in Dirac spin liquids provides a unified description of 2D quantum magnetism. In particular, it allows us to naturally account for various orders on both bipartite lattices such as the square and honeycomb lattice as well as the nonbipartite triangular and Kagome lattices. A dichotomy in behavior between the bipartite and non bipartite lattices is traced to the difference in monopole symmetry properties on these two lattices. We characterize universal signatures of the Dirac spin liquid state, including those that result from monopole excitations, which serve as a guide to numerics and to experiments on existing materials. Even when unstable, the Dirac spin liquid unifies multiple seemingly unrelated ordered states, which could help organize the plethora of phases observed in strongly correlated two dimensional materials. 
Wednesday, March 6, 2019 10:00AM  10:12AM 
K07.00011: Interplay of uniform U (1) quantum spin liquid and magnetic phases in rare earth pyrochlore magnets: a fermionic parton approach Sambuddha Sanyal, Kusum Dhochak, Subhro Bhattacharjee We study the uniform time reversal invariant quantum spin liquid (QSL) with low energy fermionic quasiparticles for rare earth pyrochlore magnets and explore its magnetic instability employing an augmented fermionic parton mean field theory approach. Self consistent calculations stabilise an uniform QSL with both gapped and gapless parton excitations as well as fractionalised magnetically ordered phases in an experimentally relevant part of the phase diagram near the classical phase boundaries of the magnetically ordered phases. The gapped QSL has a bandstructure with a nonzero topological invariant. The fractionalised magnetic ordered phases bears signature of both QSL through fermionic excitations as well as magnetic order. Thus this provides a possible way to understand the unconventional diffuse neutron scattering in rareearth pyrochlores such as Yb Ti O , Er Sn O and Er Pt O at low/zero external magnetic fields. We calculate the dynamic spin structure factor to understand the nature of the diffuse twoparticle continuum. 
Wednesday, March 6, 2019 10:12AM  10:24AM 
K07.00012: Microscopic model for Tbbased pyrochlore magnets Wen Jin, Daniel Wong, Michel J P Gingras There have been extensive experimental investigations of the magnetic structures and excitations of Tbbased pyrochlores, Tb2B2O7 (B=Ti, Sn, Ge). Tb2Ti2O7 is a candidate for quantum spin ice, while Tb2Sn2O7 and Tb2Ge2O7 have longrange ordered spin ice state coexistent with strong spin fluctuations. However, a microscopic model of these materials is still lacking. In this talk, I will discuss our efforts to derive a pseudo spin1/2 model of Tbbased pyrochlores via a projection operator approach. We find the virtual crystal field excitations between the two lower doublets result in significant renormalization of the classical Ising interactions and generate symmetryallowed threebody interactions that play a crucial role in the selection of ground state. We find the electric quadrupolequadrupole interactions also give rise significant quantum fluctuations. 
Wednesday, March 6, 2019 10:24AM  10:36AM 
K07.00013: Magnetic critical dynamics and monopole clustering of pyrochlore spin ice Puhan Zhang, Jing Luo, GiaWei Chern Spinice materials such as Ho2Ti2O7 and Dy2Ti2O7 are a class of geometrically frustrated ferromagnets that retain an extensive residual entropy even at very low temperatures. Importantly, the elementary dipole excitations fractionalize into magnetic monopoles in these compounds. Indeed, the fieldinduced firstorder transition in spinice compounds can be understood as a monopole liquidgas transition. Despite extensive studies on magnetic monopole dynamics at low magnetic field, the kinetics of the liquidgas transition and the particular role played by monopoles have not been thoroughly investigated. Here we present extensive numerical simulations to investigate the structural as well as dynamical properties of monopoles in the vicinity of the fieldinduced firstorder transition. Our results relate the critical magnetic dynamics to the percolation of magnetic monopoles in the monopole gas phase. Interestingly, we show that similar percolating clusters consisting of bound monopole hole pairs also appear in the dense monopole liquid phase. 
Wednesday, March 6, 2019 10:36AM  10:48AM 
K07.00014: Quantum Anomalous Hall Phase Stabilized via Realistic Interactions on a Kagome Lattice Yafei Ren, Tiansheng Zeng, Wei Zhu, Donna Sheng Spontaneous topological phases driven by interactions have been proposed in various lattice models, which, however, have not been observed in experiments. In this work, we report an experimentally feasible scheme of realizing spontaneous quantum anomalous Hall effect (QAHE) driven by spatially decaying interactions between spinless Fermions on a topologically trivial kagome lattice with a quadratic band touching Fermi point. In the presence of weak first and second nearestneighbor repulsive interactions (V_{1} and V_{2}), the presence of QAHE is demonstrated by employing exact diagonalization and densitymatrix renormalization group methods. The timereversal symmetry is broken spontaneously by forming loop currents with longrange correlation. Quantized Hall conductance is obtained by measuring the pumped charge through inserting flux in a cylinder geometry. We find that the topologically nontrivial energy gap can be enhanced remarkably by a moderate V_{2}<V_{1} via calculating the spectrum and charge excitation gaps. 
Wednesday, March 6, 2019 10:48AM  11:00AM 
K07.00015: Probing The SpinSpin Correlator Of Kagome Lattice Heisenberg Antiferromagnets Through Nonlinear Optical Harmonic Generation Nicholas Laurita, Alon Ron, Jeong Woo Han, Jongseok Lee, John P Sheckelton, Rebecca W. Smaha, Wei He, Jiajia Wen, Young Sang Lee, Michael Norman, David Hsieh Antiferromagnetically coupled quantum spins on the Kagome lattice are proposed to realize a spinliquid ground state where shortrange spin correlations exist but symmetry breaking is avoided through frustration and quantum fluctuations. The temperature dependence of the shortrange spinspin correlator is important for understanding the underlying spin Hamiltonian of candidate materials but is typically difficult to measure. Here we show that the nonlinear optical response embeds the spinspin correlator in spinliquid candidates by performing high harmonic generation experiments on Herbertsmithite and ZnBarlowite. We find the temperature evolution of the spin correlations of these materials to be in very good agreement with the theoretical prediction for the S = 1/2 Heisenberg Kagome antiferromagnet. These results provide a new route to probing the governing spin Hamiltonian of spinliquid candidates. 
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