Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Q28: Focus Session: Honeycomb Antiferromagnets |
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Sponsoring Units: GMAG DMP Chair: James Analytis, University of California, Berkeley Room: 205 |
Wednesday, March 4, 2015 2:30PM - 3:06PM |
Q28.00001: Dynamical structure factor and Raman scattering of Kitaev spin liquids -- signatures of fractionalization Invited Speaker: Johannes Knolle Topological states of matter present a wide variety of striking new phenomena, most prominently is the fractionalization of electrons. Their detection, however, is fundamentally complicated by the lack of any local order. While there are now several instances of candidate topological spin liquids, their identification remains challenging. We provide a complete and exact theoretical study of the dynamical structure factor and the inelastic Raman scattering response of a two-dimensional quantum spin liquid in Abelian and non-Abelian phases. Our analysis of dynamical properties of the Kitaev model identifies new varieties of the venerable X-ray edge problem and explores connections to the physics of quantum quenches. We discuss the effect of bound states and bond disorder on the response. Overall, we show that there are salient signatures of the Majorana fermions and gauge fluxes emerging in Kitaev's honeycomb model. We make connection to recent experiments and explore more generally the influence of integrability breaking for Kitaev spin liquid response functions. \\[4pt] [1] Dynamics of a two-dimensional quantum spin liquid: signatures of emergent Majorana fermions and fluxes, J. Knolle, D. L. Kovrizhin, J. T. Chalker, R. Moessner, Phys. Rev. Lett. 112, 207203 (2014)\\ [0pt] [2] Raman Scattering Signatures of Kitaev Spin Liquids in A$_2$IrO$_3$ Iridates with A=Na or Li, J. Knolle, Gia-Wei Chern, D. L. Kovrizhin, R, Moessner, N. B. Perkins, Phys. Rev. Lett. 113, 187201 (2014) [Preview Abstract] |
Wednesday, March 4, 2015 3:06PM - 3:18PM |
Q28.00002: Topological spinon semimetals and gapless boundary states in three dimensions Robert Schaffer, Eric Kin-Ho Lee, Yuan-Ming Lu, Yong Baek Kim Recently there has been much effort in understanding topological phases of matter with gapless bulk excitations, which are characterized by topological invariants and protected intrinsic boundary states. Here we show that topological semimetals of Majorana fermions arise in exactly solvable Kitaev spin models on a series of three dimensional lattices. The ground states of these models are quantum spin liquids with gapless nodal spectra of bulk Majorana fermion excitations. It is shown that these phases are topologically stable as long as certain discrete symmetries are protected. The corresponding topological indices and the gapless boundary states are explicitly computed to support these results. The phases discussed in this work are novel examples of gapless topological phases in interacting spin systems. [Preview Abstract] |
Wednesday, March 4, 2015 3:18PM - 3:30PM |
Q28.00003: Unified theory of Kitaev-based spiral magnetism in the harmonic-honeycomb iridates $\alpha,\beta,\gamma$-Li$_2$IrO$_3$ Itamar Kimchi, Radu Coldea, James Analytis, Ashvin Vishwanath We review the recent theoretical developments triggered by the experimental discovery of remarkable 3D polymorphs of Li$_2$IrO$_3$, where J=1/2 moments form two new 3D lattices which generalize the 2D honeycomb lattice. Measurements on both compounds found that they magnetically order into remarkably similar spiral phases, exhibiting a pattern of non-coplanarity and counter-rotation. We examine magnetic Hamiltonians for this family and show how a trio of nearest-neighbor exchanges, which arise from strong spin-orbit coupling, are sufficient for reproducing this spiral order on both lattices. We analyze the origin of this phenomenon by presenting a simple 1D model, transparently demonstrating how the counter-rotating spiral is stabilized by strong Kitaev exchange. We also discuss proximate quantum spin liquid phases which arise from spin-orbit coupling. [Preview Abstract] |
Wednesday, March 4, 2015 3:30PM - 3:42PM |
Q28.00004: Synthesis and doping of nonmagnetic honeycomb iridate single crystals Gilbert Lopez, Nicholas Breznay, Xue Fan, James Analytis The honeycomb iridate Na$_2$IrO$_3$ has been proposed to exhibit many unique properties, including possible spin liquid and topological insulator phases. Although the widely studied layered phase of Na$_2$IrO$_3$ is an antiferromagnetic Mott insulator, I will discuss single-crystal synthesis and electrical and thermodynamic properties of a weakly magnetic Na$_2$Ir$_{1-y}$O$_3$ relative. I will also discuss the effects of chemical doping on the electrical transport and magnetic properties of honeycomb iridate materials. [Preview Abstract] |
Wednesday, March 4, 2015 3:42PM - 3:54PM |
Q28.00005: Thermodynamic Study of 3D ``Harmonic'' Honeycomb Li$_{2}$IrO$_{3}$ Alejandro Ruiz, Toni Helm, Nicholas Breznay, Gilbert Lopez, James Analytis Honeycomb iridates have been the focus of substantial interest due to the strong magnetic frustration that arises from their edge-shared bonding environment, which favors a strongly anisotropic Ising-like exchange between bonds. In materials with edge-shared IrO$_6$ octahedra, spin-anisotropy of the exchange between neighboring effective spin-1/2 states is enhanced by the interference of the two exchange paths across the planar Ir-O$_2$-Ir bond. In the honeycomb lattice, such an interaction couples different orthogonal spin components for the three nearest neighbors; no single exchange direction can be simultaneously satisfied, leading to strong frustration which can be described by the Kitaev-model. We have recently synthesized a new structure that retains the same bonding environment as the honeycomb lattice, and extends this physics to three-dimensions. Previous RMXD experiments on our orthorhombic $^{\mathcal{H}}\langle 1\rangle$-Li$_{2}$IrO$_{3}$ samples revealed an incommensurate, non-coplanar magnetic structure with counter-rotating moments, suggesting that Kitaev exchange is the dominant spin interaction in this system. In this work, we study the thermal properties of our single crystals as a function of temperature and applied magnetic field. [Preview Abstract] |
Wednesday, March 4, 2015 3:54PM - 4:06PM |
Q28.00006: Magnetic Anisotropy of a Three-Dimensional Honeycomb Iridate Kimberly Modic, Ross McDonald, Arkady Shekter, James Analytis, Brad Ramshaw We present the magnetic anisotropy of a 3-dimensional honeycomb iridate, where the large spin-orbit coupling of iridium provides the possibility for exotic magnetic ground states. A complete angular dependence of magnetic torque provides evidence for highly spin-anisotropic exchange interactions at low temperature. An extension of these measurements to high magnetic fields shows that the magnetic anisotropy switches sign at 50 T and becomes five times larger than the anisotropy at low fields. The anisotropy continues to increase up to the largest applied fields suggesting the presence of new magnetically ordered states. [Preview Abstract] |
Wednesday, March 4, 2015 4:06PM - 4:18PM |
Q28.00007: Valence bond solid (AKLT) state from $t_{2g}$ electrons Maciej Koch-Janusz, Daniel Khomskii, Eran Sela The models constructed by Affleck, Kennedy, Lieb, and Tasaki (AKLT) describe gapped quantum spin liquids with fractionalized boundary spin excitations. The AKLT spin-spin interactions consist of projection operators onto the maximal possible spin formed between nearest neighbours, which involves a linear combination of powers of the Heisenberg coupling $(\vec{S}_i \cdot \vec{S}_j)^n$, making these states difficult to realize. Indeed, except for the one dimensional spin-$1$ case, simple antiferromagnetic Heisenberg interactions which are typically found in magnetic insulators, do not stabilize these spin liquid states, but rather generate conventional antiferromagnetically ordered states. We show that this type of interactions can be generated by orbital physics in multiorbital Mott insulators. Motivated by microscopic modeling of spin-orbit entangled Mott insulators such as the layered hexagonal Iridates, we focus on $t_{2g}$ electrons on the honeycomb lattice and propose a physical realization of the spin-$3/2$ AKLT state. Interestingly enough, the valence bond solid (AKLT) state found for the noninteracting electrons survives the increase of the on-site (Hubbard) repulsion, but it changes to the antiferromagnetic Neel state with increase of the Hund's rule coupling. [Preview Abstract] |
Wednesday, March 4, 2015 4:18PM - 4:30PM |
Q28.00008: Competing quantum phases in the $K_1$-$K_2$ Kitaev model on the honeycomb lattice Johannes Reuther, Ronny Thomale, Stephan Rachel The Kitaev-spin model on the honeycomb lattice has attracted enormous interest in recent years as an exactly solvable 2D spin system. Research in this field has been fueled by the possibility to realize the Kitaev model in strongly spin-orbit coupled iridate compounds of the form A$_2$IrO$_3$. In these materials the bond-dependent spin anisotropies of the Kitaev model are provided by spin-orbit entangled Kramers doublets. Experimental as well as theoretical investigations indicate that second neighbor honeycomb interactions are not necessarily small, which particularly applies to the second neighbor Kitaev exchange. We study the $K_1$-$K_2$ Kitaev model on the honeycomb lattice with nearest neighbor and second neighbor Kitaev couplings $K_1$ and $K_2$, respectively. Using a pseudo-fermion functional renormalization group approach for spin systems we map out the entire phase diagram of this model, allowing both couplings to be positive and negative. Aside from Kitaev-spin liquid phases at $K_2 = 0$ we find spin chain-like phases at $K_1 = 0$ where correlations only take place along effective 1D chains. Away from these special points we identify four different types of magnetic phases with collinear orders which retain a clear 1D character in large parts of the phase diagram. [Preview Abstract] |
Wednesday, March 4, 2015 4:30PM - 4:42PM |
Q28.00009: Density matrix renormalization group study of triangular Kitaev-Heisenberg model Shigetoshi Sota, Kazuya Sjinjo, Tomonori Shirakawa, Takami Tohyama, Seiji Yunoki Topological insulator has been one of the most active subjects in the current condensed matter physics. For most of topological insulators electron correlations are considered to be not essential. However, in the case where electron correlations are strong, novel phases such as a spin liquid phase can emerge in competition with a spin-orbit coupling. Here, using the density matrix renormalization group method, we investigate magnetic phase of a triangular Kitaev-Heisenberg (quantum compass) model that contains a spin-orbital interaction and spin frustration in the antiferromagnetic region. The triangular Kitaev-Heisenberg model is regarded as a dual model of the honeycomb Kitaev-Heisenberg model that is usually employed to discuss A$_2$CuO$_3$ (A=Na, K). Systematically calculating ground state energy, entanglement entropy, entanglement spectrum, and spin-spin correlation functions, we discuss the duality between the triangular and the honeycomb Kitaev-Heisenberg model as well as the ground state magnetic phases. [Preview Abstract] |
Wednesday, March 4, 2015 4:42PM - 4:54PM |
Q28.00010: Dynamical properties of honeycomb-lattice magnets Na$_{2}$IrO$_{3}$ Takafumi Suzuki We studied dynamical properties of magnetic effective models for Na$_{2}$IrO$_{3}$. The effective magnetic models have been much discussed to explain the zigzag magnetic ordering of this compound [1-6]. The most characteristic point of the proposed models is that the presence of the Kitaev type anisotropy is expected in addition to the Heisenberg coupling. Although there are several proposals for the coupling magnitude of the effective models, the qualitative and quantitative evaluations have been still lacking from the viewpoint of dynamics. From the above background, we carried out numerical exact diagonalization and calculated the dynamical structure factors for the proposals in pioneering works [2,3,5,6]. The obtained results were compared with inelastic-neutron-scattering measurements [2]. We found that the parameter set proposed in ref. [6] well explains a characteristic boundary structure of the lowest excitation around Y point and an intensity distribution in the low energy region [2]. \\[4pt] [1] F. Ye, et al., Phys. Rev. B 85, 180403(R) (2012).\\[0pt] [2] S. K. Choi, et al., Phys. Rev. Lett. 1008, 127204 (2012).\\[0pt] [3] J. Chaloupka, G. Jackeli and G. Khaliullin, Phys. Rev. Lett. 105, 027204 (2010); Phys. Rev. Lett. 110, 097204 (2013).\\[0pt] [4] I. Kimchi and Y. Z. You, Phys. Rev. B 84, 180407(R) (2011).\\[0pt] [5] V. M. Katukuri, et al., New J. Phys. 16, 013056 (2014).\\[0pt] [6] Y. Yamaji et al., Phys. Rev. Lett. 113, 107201 (2014). [Preview Abstract] |
Wednesday, March 4, 2015 4:54PM - 5:06PM |
Q28.00011: Theory of Magnetic Phases in Hyperhoneycomb and Harmonic-honeycomb Iridates Eric Kin Ho Lee, Yong Baek Kim Motivated by recent experiments, we consider a generic spin model in the $j_{\mathrm{eff}} = 1/2$ basis for the hyperhoneycomb and harmonic-honeycomb iridates. Based on microscopic considerations, the effect of an additional bond-dependent anisotropic spin exchange interaction ($\Gamma$) beyond the Heisenberg-Kitaev model is investigated. We obtain the magnetic phase diagrams of the hyperhoneycomb and harmonic-honeycomb (H-1) lattices via a combination of the Luttinger-Tisza approximation, single-Q variational ansatz, and classical Monte Carlo simulated annealing. The resulting phase diagrams on both systems show the existence of incommensurate, non-coplanar spiral magnetic orders as well as other commensurate magnetic orders. The spiral orders show counter-propagating spiral patterns, which may be favorably compared to recent experimental results on both iridates. The parameter regime of various magnetic orders and ordering wavevectors are quite similar in both systems. We discuss the implications of our work to recent experiments and also compare our results to those of the two dimensional honeycomb iridate systems. [Preview Abstract] |
Wednesday, March 4, 2015 5:06PM - 5:18PM |
Q28.00012: CaMn$_2$Sb$_2$: Spin Waves Near a Tricritical Point of the Antiferromagnetic Honeycomb Lattice Daniel McNally, Jack Simonson, Jed Kistner-Morris, Greg Smith, Julian Hassinger, Lisa DeBeer-Schmidt, Alexander Kolesnikov, Meigan Aronson The classical Heisenberg model for a honeycomb lattice of spins predicts at least three tricritical points, where three different long range ordered magnetic phases co-exist, depending on the relative strength of the nearest and next-nearest exchange interactions J$_{1,2}$. We performed inelastic neutron scattering at T = 5 K << T$_N$ = 85 K on oriented single crystals of the antiferromagnetic insulator CaMn$_2$Sb$_2$, where the Mn spins $\mu$ = 2.8 $\mu$$_B$/Mn form a corrugated honeycomb lattice. Spin wave excitations were observed up to E $\approx$ 24 meV and these data were fit to the spin wave dispersion expected from the classical Heisenberg model to determine the individual exchange interactions SJ$_{1}$ = 8.22 $\pm$ 0.23 meV, SJ$_{2}$ = 1.29 $\pm$ 0.09 meV, SJ$_{c}$ = -0.56 $\pm$ 0.04 meV, where J$_c$ is the exchange interaction between honeycomb planes. The quantum fluctuations resulting from proximity to the tricritical point at J$_2$/J$_1$ = 1/6 are responsible for the relatively low ordering temperature of CaMn$_2$Sb$_2, $T$_N$ = 85 K, much reduced from the mean field ordering temperature T$_{MFT}$ = 2zJ$_1$S(S+1)/3k$_B$ = 560 K. [Preview Abstract] |
Wednesday, March 4, 2015 5:18PM - 5:30PM |
Q28.00013: Exotic magnetism on the FCC lattice of 5$d^n$ double perovskites D.D. Maharaj, E. Kermarrec, C.A. Marjerrison, C.M. Thompson, K. Levin, S. Kroeker, G.E. Granroth, R. Flacau, Z. Yamani, J.E. Greedan, B.D. Gaulin In the search for exotic quantum states, the impact of strong spin-orbit interaction has been recently underlined with the discovery of the $J_{eff} = \frac{1}{2}$ spin-orbital Mott state in the 5$d^5$ layered perovskites iridates. The double perovskite structure can accommodate numerous 5$d$ ions and therefore offers a playground for systematic studies of the exotic ground states stabilized by strong spin-orbit coupling (SOC). Here, we report time-of-flight neutron scattering measurements on the antiferromagnetic (AF), frustrated system, Ba$_2$YOsO$_6$. This 5$d^3$ system undergoes a magnetic transition to a long range ordered AF state below $T_N$ = 70K. Our results reveal a large spin gap $\Delta$ = 18(2)meV, unexpected for an orbitally quenched $d^3$ system. We compare this result to the recent observation of a $\Delta$ = 5meV spin gap in the related 4$d^3$ system, Ba$_2$YRuO$_6$, and conclude to an effect of enhanced SOC. [Preview Abstract] |
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