Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session F48: Frustrated Magnetism: Triangular LatticesFocus

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Sponsoring Units: GMAG DMP Chair: Gregory MacDougall, University of Illinois UrbanaChampaign Room: 395 
Tuesday, March 14, 2017 11:15AM  11:51AM 
F48.00001: Z2 vortex crystals in triangular antiferromagnets with strong spin orbit coupling Invited Speaker: Ioannis Rousochatzakis The triangularlattice Heisenberg antiferromagnet (HAF) is known to carry topological $Z_2$ vortex excitations which form a gas at finite temperatures. Here we show that the spinorbit interaction, introduced via a Kitaev term in the exchange Hamiltonian, condenses these vortices into a triangular $Z_2$ vortex crystal at zero temperature. The cores of the $Z_2$ vortices show abrupt, solitonlike magnetization modulations and arise by a special intertwining of three honeycomb superstructures of ferromagnetic domains, one for each of the three sublattices of the 120$^\circ$ state of the pure HAF. This is a new example of a nucleation transition, analogous to the spontaneous formation of magnetic domains, Abrikosov vortices in typeII syperconductors, blue phases in cholesteric liquid crystals, and skyrmions in chiral helimagnets. As the mechanism relies on the interplay of geometric frustration and spinorbital anisotropies, such vortex mesophases can materialize as a groundstate property in spinorbit coupled correlated systems with nearly hexagonal topology, as in triangular or strongly frustrated honeycomb iridates. [Preview Abstract] 
Tuesday, March 14, 2017 11:51AM  12:03PM 
F48.00002: Low energy electrodynamics of triangular lattice quantum spin liquid candidate YbMgGaO$_{4}$ xinshu zhang, Fahad Mahmood, Nicholas Laurita, Zhiling Dun, Haidong Zhou, Martin Mourigal, Peter Armitage Quantum spin liquids (QSL) are exotic magnetic states where spins do not order down to the lowest temperatures. Spin 1/2 twodimensional triangular antiferromagnet YbMgGaO$_{4}$ is a promising QSL candidate. Here we report an optical measurement on YbMgGaO$_{4}$ using time domain terahertz spectroscopy (TDTS)and Fourier transform spectroscopy (FTIR). We find the real part of inplane dissipative response shows a power law behavior that may arise from spin liquid properties. Applying magnetic field in the Faraday geometry, we approach the spin polarized state from which we can extract gfactors and determine exchange constants quantitatively. We also identify a crystal field excitation in the infrared measurement. Our results provide guidance for the future theoretical study of this QSL. [Preview Abstract] 
Tuesday, March 14, 2017 12:03PM  12:15PM 
F48.00003: Chiral phase near quantum critical point Zhentao Wang, Oleg Starykh, Adrian Feiguin, Andrey Chubukov, Cristian Batista We study the sequence of quantum phase transitions between a quantum paramagnetic state and a magnetically ordered state for a 2D spin one triangular XXZ model with easy plane singleion anisotropy $D S_z^2$. The mean field phase diagram of the model exhibits a direct transition between an XY antiferromagnetic state and a quantum paramagnetic phase (PM) induced by a large enough D value. The two phases are separated by a quantum critical point at $D=D_c$. The Isinglike $J_z$ interaction creates an attraction between quasiparticles of the quantum paramagnet with opposite spin. Upon approaching $D_c$ from the quantum paramagnetic side, we find that the resulting twoparticle bound states condense before the single particle gap closes at $D=D_c$. This twomagnon bound state condensation signals the onset of a chiral liquid, which spontaneously breaks the inversion symmetry, while leaving the $U(1)$ symmetry intact. This leads to an emergent chiral liquid phase, which supports nonzero spin currents (vector spin chirality) without long range magnetic order. In our analytical treatment, the chiral phase appears for arbitrary small value of the Ising interaction. We further show evidence of the chiral phase by means of density matrix renormalization group calculations. [Preview Abstract] 
Tuesday, March 14, 2017 12:15PM  12:27PM 
F48.00004: The origin of exciton mass in a frustrated Mott insulator Na$_2$IrO$_3$ Zhanybek Alpichshev, Edbert Sie, Fahad Mahmood, Gang Cao, Nuh Gedik We use a threepulse ultrafast optical spectroscopy to study the relaxation processes in a frustrated Mott insulator Na$_2$IrO$_3$. By being able to independently produce the outofequilibrium bound states (excitons) of doublons and holons with the first pulse and suppress the underlying antiferromagnetic order with the second one, we elucidate the relaxation mechanism of quasiparticles in this system. By observing the difference in the exciton dynamics in the magnetically ordered and disordered phases, we found that the mass of this quasiparticle is mostly determined by its interaction with the surrounding spins. [Preview Abstract] 
Tuesday, March 14, 2017 12:27PM  12:39PM 
F48.00005: Disorder in NiGa2S4 seen with Raman and infrared spectroscopy M. E. Valentine, T. Higo, S. Nakatsuji, D. Chaudhuri, N. P. Armitage, N. Drichko The crystal structure of NiGa$_{2}$S$_{4}$ contains a two dimensional triangular lattice of Ni$^{2+}$ (S = 1) ions where ferromagnetic nearest neighbor interactions and antiferromagnetic third nearest neighbor interactions lead to magnetic frustration which suppresses magnetic ordering down to at least 1.5K. We studied disorder and structural distortions in NiGa2S4 by Raman and IR spectroscopy on single crystals in the energy range of 10 to 600 cm$^{1}$ which covers the full range of $\Gamma$point optical modes and compared our results to DFT calculations for vibrational modes. In the Raman spectrum for temperatures below 300K, additional modes are observed between 250 and 450 cm$^{1}$. These features have energies that coincide with IR vibrational modes which are Ramanforbidden based on the point group symmetry, suggesting a local loss of inversion symmetry. Also, we observe an increased line width of the phonons involving significant motion of the S atoms adjacent to Ni and responsible for superexchange interactions within the magnetic lattice. This suggests disorder of these atoms which may be the cause of the suppressed magnetic order and spin freezing observed. [Preview Abstract] 
Tuesday, March 14, 2017 12:39PM  12:51PM 
F48.00006: Quantum phase transitions in Heisenberg $J_1J_2$ triangular antiferromagnet in a magnetic field Mengxing Ye, Andrey Chubukov We present the zero temperature phase diagram of a large $S$ Heisenberg antiferromagnet on a frustrated triangular lattice with nearest neighbor ($J_1$) and next nearest neighbor ($J_2$) interactions, in a magnetic field. We show that the classical model has an accidental degeneracy for all $J_2/J_1$ and all fields, but the degeneracy is lifted by quantum fluctuations. We show that at large $S$, for $J_2/J_1 <1/8$, quantum fluctuations select the same sequence of three sublattice coplanar states in a field as for $J_2 =0$, and for $1/8$ $< J_2/J_1$ $<1$ they select the canted stripe state for all nonzero fields. The transition between the two states is first order in all fields, with the hysteresis width set by quantum fluctuations. We also study the model with arbitrary $S$, including $S=1/2$, near the saturation field by exploring the fact that near saturation the density of bosons is small for all $S$. We show that for $S >1$ the transition remains first order, with a finite hystresis width, but for $S=1/2$ and, possibly, $S=1$, there appears a new intermediate phase, likely without a spontaneous longrange order. [Preview Abstract] 
Tuesday, March 14, 2017 12:51PM  1:03PM 
F48.00007: A Schwinger boson mean field study of the $J_1$$J_2$ Heisenberg triangularlattice quantum antiferromagnet DagVidar Bauer, John Ove Fjaerestad We use Schwinger boson mean field theory to study the ground state of the spin$S$ triangularlattice Heisenberg model with nearest ($J_1$) and nextnearest ($J_2$) neighbor antiferromagnetic interactions, treating $\kappa=2S$ as a continuous parameter. We consider two spin liquid Ansatze whose magnetically ordered versions correspond to 120degree order and a a collinear ''stripe" order, respectively. For $\kappa=1$ there is a direct transition between these ordered states as $J_2/J_1$ increases. Motivated by an argument that a smaller $\kappa$ may be more appropriate for describing the $S=1/2$ case qualitatively, we find that as one lowers $\kappa$, a spin liquid region eventually opens up between the ordered phases, in qualitative agreement with various recent numerical studies of the $S=1/2$ model. This picture suggests a symmetric gapped Z$_2$ spin liquid which is the disordered version of the 120degree ordered state. [Preview Abstract] 
Tuesday, March 14, 2017 1:03PM  1:15PM 
F48.00008: Evidence for a spinon Fermi surface in a triangular lattice quantum spin liquid candidate Yao Shen, YaoDong Li, Hongliang Wo, Yuesheng Li, Shoudong Shen, Bingying Pan, Qisi Wang, Helen Walker, Paul Steffens, Martin Boehm, Yiqing Hao, Diana QuinteroCastro, Leland Harriger, Matthias Frontzek, Lijie Hao, Siqin Meng, Qingming Zhang, Gang Chen, Jun Zhao In a quantum spin liquid (QSL), highly entangled spins remain disordered down to zero temperature due to strong frustration and quantum fluctuation. Such exotic quantum state can support fractionalized spin excitations called ‘spinon’. In this talk, we present neutron scattering measurements on a QSL candidate YbMgGaO$_4$, a highly frustrated antiferromagnet in which Yb ions form a quasitwodimensional triangular lattice. Broad spin excitations are revealed covering a wide region of the Brillouin zone which persists from the lowest measured energy to the zone boundary. The observed continuum is a key characteristic for spinon excitations, providing evidences for a QSL state in YbMgGaO$_4$ that has a spin1/2 triangular lattice. [Preview Abstract] 
Tuesday, March 14, 2017 1:15PM  1:27PM 
F48.00009: Variational wave functions for the $S=1/2$ Heisenberg model on the anisotropic triangular lattice: Spin liquids and spiral orders Luca F. Tocchio, Elaheh Ghorbani, Federico Becca By using variational wave functions and quantum Monte Carlo techniques, we investigate the complete phase diagram of the Heisenberg model on the anisotropic triangular lattice, where two out of three bonds have superexchange couplings $J$ and the third one has instead $J'$. This model interpolates between the square lattice and the isotropic triangular one, for $J'/J \le 1$, and between the isotropic triangular lattice and a set of decoupled chains, for $J/J' \le 1$. We consider all the fullysymmetric spin liquids that can be constructed with the fermionic projectivesymmetry group classification [Y. Zhou and X.G. Wen, arXiv:condmat/0210662] and we compare them with the spiral magnetic orders that can be accommodated on finite clusters. Our results show that, for $J'/J \le 1$, the phase diagram is dominated by magnetic orderings, even though a spinliquid state may be possible in a small parameter window, i.e., $0.7 < J'/J < 0.8$. In contrast, for $J/J' \le 1$, a large spinliquid region appears close to the limit of decoupled chains, i.e., for $J/J' < 0.6$, while magnetically ordered phases with spiral order are stabilized close to the isotropic point. [1] E. Ghorbani, L.F. Tocchio, and F. Becca, PRB 93, 085111 (2016). [Preview Abstract] 
Tuesday, March 14, 2017 1:27PM  1:39PM 
F48.00010: Quantum spin liquid states of the triangular HeisenbergKitaev model Matthias Punk, Pavel Kos We study quantum disordered ground states of the two dimensional HeisenbergKitaev model on the triangular lattice using a Schwinger boson approach. Our aim is to identify and characterize potential gapped quantum spin liquid phases that are stabilized by anisotropic Kitaev interactions. For antiferromagnetic Heisenberg and Kitaev couplings we find three different symmetric $Z_2$ spin liquid phases, separated by two continuous quantum phase transitions. Interestingly, the gap remains finite throughout the transitions. We discuss how these spin liquid states are related to known ordered phases in the classical limit. [Preview Abstract] 
Tuesday, March 14, 2017 1:39PM  1:51PM 
F48.00011: Mapping the Phase Diagram of RMM’O4 Compounds in Pursuit of the Coveted Quantum Spin Liquid state William Steinhardt, Casey Marjerrison, Yaohua Liu, Sachith Dissanayake, David Graf, Marcus Daum, Martin Mourigal, Sara Haravifard Recently the rare earth RMM’O4 systems (where R = rare earth, M and M’ = transition or main group) have been suggested as strong spinorbit coupled quantum spin liquid candidates. In these systems, effective spinhalf moments arising from odd numbers of electrons per unit cell lie on a planar, geometrically frustrated triangular lattice. We have synthesized large highquality single crystals of a variety of these compounds using the optical floating zone technique. In this talk we present our recent xray and neutron scattering results at extreme environmental conditions as well as magnetization measurements, and discuss the induced quantum critical phenomena and the phase diagram for this family of materials as a function of site mixing, substitution, magnetic field, pressure, and temperature. [Preview Abstract] 
Tuesday, March 14, 2017 1:51PM  2:03PM 
F48.00012: Fieldinduced decays in XXZ triangularlattice antiferromagnets Pavel Maksimov, Mike Zhitomirsky, Alexander Chernyshev We investigate fieldinduced transformations in the dynamical response of the XXZ model on the triangular lattice that are associated with the anharmonic magnon coupling and decay phenomena. Detailed theoretical predictions are made for a close physical realization of the spin$\frac12$ XXZ model, Ba$_3$CoSb$_2$O$_9$. We demonstrate that dramatic modifications in magnon spectrum must occur in low outofplane fields that are easily achievable for this material. The hallmark of the effect is a coexistence of the clearly distinct welldefined magnon excitations with significantly broadened ones in different regions of the ${\bf k}\!\!\omega$ space. The fieldinduced decays are generic for this class of models and become more prominent at larger anisotropies and in higher fields. [Preview Abstract] 
Tuesday, March 14, 2017 2:03PM  2:15PM 
F48.00013: Hidden Multipolar Orders of DipoleOctupole Doublets on a Triangular Lattice Xiaoqun Wang, Yaodong Li, Gang Chen Motivated by the recent development in strong spinorbitcoupled materials, we consider the dipoleoctupole doublets on the triangular lattice. We propose the most general interaction between these unusual local moments. Due to the spinorbit entanglement and the special form of its wavefunction, the dipoleoctupole doublet has a rather peculiar property under the lattice symmetry operation. As a result, the interaction is highly anisotropic in the pseudospin space, but remarkably, is uniform spatially. We analyze the ground state properties of this generic model and emphasize the hidden multipolar orders that emerge from the dipolar and octupolar interactions. We clarify the quantum mutual modulations between the dipolar and octupolar orders. We predict the experimental consequences of the multipolar orders and propose the rareearth triangular materials as candidate systems for these unusual properties. [Preview Abstract] 
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