Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session B4: Focus Session: Antiferromagnets on a Triangular Lattice |
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Sponsoring Units: GMAG Chair: Oleg Starykh, University of Utah Room: 112/110 |
Monday, March 3, 2014 11:15AM - 11:51AM |
B4.00001: Molecular Quantum Magnetism in LiZn$_2$Mo$_3$O$_8$ Invited Speaker: Martin Mourigal Considerable theoretical and experimental efforts are devoted to understanding frustrated two-dimensional antiferromagnets, searching for quantum spin-liquid states hosting deconfined fractional spin excitations. To make quantitative comparisons with theory, the sensitivity to defects and site mixing inherent to magnetic transition metal oxides is a significant challenge. Spin degrees of freedom delocalized on stable organic molecules or inorganic clusters offer an interesting alternative. The layered insulating material LiZn$_2$Mo$_3$O$_8$ is such a compound. It comprises spin-1/2 Mo$_3$O$_{13}$ molecules organized on the triangular lattice [1]. Its thermo-magnetic properties suggest it hosts collective magnetic phenomena with hints of a possible valence-bond condensation and absence of long-range spin order. Inelastic neutron scattering from powder specimen of LiZn$_2$Mo$_3$O$_8$ reveal the presence of gapless collective magnetic excitations at low-energy that are surprisingly broad in momentum space and involve at most a third of the spins. The corresponding structure factor is consistent with the presence of valence-bonds involving nearest-neighbor and next-nearest-neighbor spins [2]. No magnetic signal is apparent at higher energies, suggesting that the remaining spins contribute as a broad continuum rather than as a well defined resonance. LiZn$_2$Mo$_3$O$_8$ thus offers an example of molecular based spin-liquid material with collective excitations consistent with a disordered or dynamic ground-state.\\[4pt] [1] J. P. Sheckelton, J. R. Neilson, D. G. Soltan, and T. M. McQueen, Nature Mater. {\bf 11}, 493496 (2012).\\[0pt] [2] M. Mourigal, W. T. Fuhrman, J. P. Sheckelton, A. Wartelle, J. A. Rodriguez-Rivera, D. L. Abernathy, T. M. McQueen, and C. L. Broholm, arXiv:1309.1165 [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B4.00002: Local-scale magnetic studies of the condensed valence-bond state in LiZn$_{2}$Mo$_{3}$O$_{8}$ John Sheckelton, Francesca Foronda, LiDong Pan, Camilla Moir, Ross McDonald, Tom Lancaster, Peter Baker, N. Peter Armitage, Takashi Imai, Stephen Blundell, Tyrel McQueen The reduced molybdenum oxide LiZn$_{2}$Mo$_{3}$O$_{8}$ is an insulating material composed of layers of Mo$_{3}$O$_{13}$ clusters, with the clusters arranged on a triangular lattice and non-magnetic Li/Zn inter-layers. A formal electron count results in each molybdenum cluster acting as S$=$1/2 magnetic unit. Superexchange between clusters is mediated through Mo-O-Mo oxo bridges that lead to a frustrated magnetic state. Local-scale magnetic measurements indicate the existence of a gapless spin excitation spectrum that persists down to the lowest temperatures measured. In addition, these local probe measurements indicate local magnetic behavior that corresponds to bulk measurements. The data presented are consistent with expected magnetic responses of a condensed valence-bond state. Structural and measured magnetic properties and ongoing research will be discussed. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B4.00003: Dynamical structure factor of the triangular-lattice antiferromagnet Alexander Chernyshev, Martin Mourigal, Wesley Fuhrman, Michael Zhitomirsky We have elucidated the role of magnon interaction and spontaneous decays in the spin dynamics of the triangular-lattice Heisenberg antiferromagnet by calculating its dynamical structure factor within the spin-wave theory. Explicit theoretical results for neutron-scattering intensity will be shown for spins $S = 1/2$ and $S = 3/2$. The dynamical structure factor exhibits unconventional features such as quasiparticle peaks broadened by decays, non-Lorentzian lineshapes, and significant spectral weight redistribution to the two-magnon continuum. This rich excitation spectrum illustrates the complexity of the triangular-lattice antiferromagnet and provides distinctive qualitative and quantitative fingerprints for experimental observation of decay-induced magnon dynamics. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B4.00004: Possible Structural Distortions in NiGa$_{2}$ S$_{4}$ indicated by T-dependent Raman Modes Michael Valentine, Satoru Nakatsuji, Tomoya Higo, Collin Broholm, Natalia Drichko NiGa$_{2}$S$_{4}$ contains two dimensional sheets of spin-1 Ni$^{2+}$ ions arranged in a triangular lattice where ferromagnetic nearest neighbor interactions and antiferromagnetic third nearest neighbor interactions lead to magnetic frustration which suppresses three dimensional magnetic ordering above 1.5K [1]. We studied structural distortions in NiGa$_{2}$S$_{4}$ by Raman spectroscopy on single crystals in the energy range of 150 cm$^{-1}$ to 500 cm$^{1}$. For temperatures below 300K the 446 cm$^{-1}$ A$_{\mathrm{1g}}$ mode splits and additional E$_{\mathrm{g}}$ modes are observed between 250 cm$^{-1}$ and 450 cm$^{-1}$. These high energy features are associated with sulfur vibrations but are not predicted to occur on the basis of the point group symmetry inferred from x-ray diffraction. We discuss possible lattice distortions due to magneto-elastic coupling and their potential effects on low temperature frustrated magnetism. \\[4pt] [1] C. Stock et al., Phys. Rev. Lett. 105, 037402 (2010) [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B4.00005: Ba NMR studies of the triangular lattice antiferromagnets Georgios Koutroulakis, Tong Zhou, Cristian Batista, Yoshitomo Kamiya, Joe Thompson, Stuart Brown, Haidong Zhou Ba$_3$MSb$_2$O$_9$, with $M$=Co, Ni are triangular lattice magnetic systems with near-neighbor antiferromagnetic exchange. Previous studies have shown that Ba$_3$CoSb$_2$O$_9$ has a stabilized up-up-down spin configuration with in-plane field and the resultant one-third magnetization plateau has been observed. On the other hand, for the $M$=Ni system with 6H-B structure there has been no evidence of a magnetic ordered phase and thus it is being seen as a candidate spin-liquid material. Existing NMR data show a very broad Ba line comprised of signals from three different Ba sites, and the relaxation rate show a very weak temperature dependece, which is similar to the Co compound in the high symmetry phase. Here we report on Ba nuclear magnetic resonance (NMR) spectroscopy and spin-lattice relaxation measurements for both compounds. For the Co system, we will report data revealing the magnetization process up to 30T and present a detailed picture of the phase diagram. For the Ni compound, we are reporting the temperature evolution of the spectra and the temperature dependence of the relaxation rate for both Ba and Sb. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B4.00006: Successive magnetic phase transitions and multiferroicity in quasi-two-dimensional triangular lattice Heisenberg antiferromagnets Ba$_{3}$CoNb$_{2}$O$_{9}$ and Ba$_{3}$MnNb$_{2}$O$_{9}$ M. Lee, J. Hwang, E.S. Choi, J. Ma, C.R. Dela Cruz, M. Zhu, X. Ke, Z.L Dun, H.D. Zhou We have measured magnetic, dielectric and thermodynamic properties of quasi-two-dimensional triangular lattice antiferromagnet (TLAF), Ba$_{3}$CoNb$_{2}$O$_{9}$ (S$=$ 1/2) and Ba$_{3}$MnNb$_{2}$O$_{9}$ (S$=$ 3/2). At zero magnetic field, Ba$_{3}$CoNb$_{2}$O$_{9}$ undergoes a two-step transition at 1.36 K and 1.10 K and enters a 120 degree ordered state. By applying magnetic fields, a series of magnetic phases with fractional saturation magnetization (1/3, 1/2, 2/3 (or $\sqrt 3 $/3Ms) are observed. The collinear spin phase with 1/3 Ms becomes more robust at lower temperatures due to quantum fluctuations. For Ba$_{3}$MnNb$_{2}$O$_{9}$, the 120 degree ordered state is stabilized below 3.10 K at zero field. Under the magnetic field, successive magnetic phase transitions are observed with fractional magnetization 1/3 and 1/2 Ms. The 1/3 Ms phase becomes more stable at higher temperatures due to thermal fluctuations. The ferroelectricity emerges in all spin states in both compounds regardless of the spin chirality. Therefore, Ba$_{3}$CoNb$_{2}$O$_{9}$ and Ba$_{3}$MnNb$_{2}$O$_{9}$ are unique TLAFs exhibiting not only a series of magnetic phase transitions but also multiferroicity. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B4.00007: Calorimetric determination of magnetic phase diagram of Ba$_{3}$CoSb$_{2}$O$_{9}$ Nathanael Fortune, Scott Hannahs, Ju-Hyun Park, Haidong Zhou, Christopher Aoyama, Yasumasa Takano We report heat capacity measurements as a function of temperature, magnetic field, and field angle for the spin 1/2 triangular antiferromagnet Ba$_{3}$CoSb$_{2}$O$_{9}$, for fields up to the saturation field of 32.5 T and temperatures down to 0.1 K. The system behaves as a frustrated Heisenberg antiferromagnet with weak xy anisotropy. For magnetic fields applied along the a axis, 3 main magnetic phases are expected [1], each corresponding to a different spin arrangement: a low field ``Y'' phase, an intermediate ``up - up - down'' phase, and a high field 'V' phase. We find instead 5 distinct magnetic phase transitions with increasing field, suggesting that the ``Y' and ``V'' phases split into alternating and non-alternating co-planar subphases. \\[4pt] [1] A.V. Chubukov \& D.I. Golosov, J.Phys.Cond.Mat 3, 69 (1991). [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B4.00008: The Angular Dependent Magnetic Phase Diagram of Cs$_{2}$CuCl$_{4}$ Scott T. Hannahs, Nathanael Fortune, Yasumasa Takano, Toshio Ono, Hidekazu Tanaka We present a determination of the phase diagram of the $S = {1\over2}$ quasi-2D triangular Heisenberg quantum antiferromagnet Cs$_{2}$CuCl$_{4}$ at temperatures to down 100mK and fields up to the saturation field of 9 tesla. We have determined the low temperature phase boundaries as a function of angle to the magnetic field using a unique low temperature rotatable calorimeter. Measurements at several angles intermediate to the in-plane and perpendicular directions elucidate the evolution of the complex phase diagram between these two principal axis. For fields directed along the a-axis (perpendicular to the plane of the triangular lattice), we observe a series of magnetic phases. New, unexpected phases arise at intermediate angles as we rotate the magnetic field away from the a-axis, into the bc plane. Variation of phase boundaries with field angle within the bc plane reflect interactions due to the Dzyaloshinskii-Moriya mechanism. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B4.00009: Exact Diagonalization Study of the Heisenberg Anisotropic Triangular Model Using Twisted Boundary Conditions Mischa Thesberg, Erik S. Sorensen The anistropic triangular model, which is believed to describe materials such as Cs$_2$CuCl$_4$ and Cs$_2$CuBr$_4$, is dominated by incommensurate spiral physics and is thus extremely resistant to numerical analysis on small system sizes. In this talk we will discuss new exact diagonalization work using twisted boundary conditions to study the phase diagram of this model. With these boundary conditions we are able to extract the inter- and intra-chain ordering $q$-vectors for the $\frac{J'}{J} < 1$ region, as well as identify a phase transitions at $\frac{J'}{J} \sim 0.50/0.88$ (depending on finite system geometry). We also explore the nature of the two phases. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B4.00010: Spin-Current Order in Anisotropic Triangular Antiferromagnets O.A. Starykh, A.V. Chubukov We analyze instabilities of the collinear up-up-down state of a two-dimensional quantum spin-$S$ spatially anisotropic triangular lattice antiferromagnet in a magnetic field. We find, within the large-$S$ approximation, that near the end point of the plateau, the collinear state becomes unstable due to the condensation of two-magnon bound pairs rather than single magnons. The two-magnon instability leads to a novel two-dimensional vector chiral phase with alternating spin currents but no magnetic order in the direction transverse to the field. This phase breaks a discrete $Z_2$ symmetry but preserves a continuous $U(1)$ one of rotations about the field axis. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B4.00011: Phase Diagram of Spin-1/2 Triangular-Lattice Antiferromagnets with Exchange Anisotropy and a Magnetic Field Daisuke Yamamoto, Giacomo Marmorini, Ippei Danshita A triangular-lattice spin system is a fundamental model of geometric frustration. Recent experimental developments in magnetic materials synthesis and in frustrated optical lattices of ultracold atoms have renewed interest in studying magnetic properties of ideal two-dimensional frustrated systems over wide range of external field and anisotropy. We study the spin structures of $S=$1/2 antiferromagnets on a triangular lattice using a large-size cluster mean-field method combined with a scaling scheme. We determine the ground-state phase diagram of the spin model in the plane of magnetic field and XXZ anisotropy, and compare it with the classical counterpart in order to discuss the quantum effects. We find that a nontrivial continuous degeneracy existing in the classical model is broken up into two first-order phase transitions between which a non-classical phase emerges as a result of the selection by quantum fluctuations. We also use the dilute Bose gas expansion in the vicinity of the saturation field and interpret one of the first-order transitions as the 0-$\pi $ transition of the relative phase between two magnon Bose-Einstein condensates. We suggest that the quantum phase transitions can be observed in current or near-future experiments. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B4.00012: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 2:03PM - 2:15PM |
B4.00013: Topological defects in a spin-nematic phase on the triangular lattice Hiroaki Ueda, Nic Shannon Topological defects play an important role in the theory of nematic phases in liquid crystals. However, relatively little is known about their role in quantum spin nematics[1,2,3]. Here we consider the topological defects which could arise in such a state. The model we consider is the spin-1 bilinear biquadratic model on the triangular lattice, tuned to an SU(3) point[4,5,6]. We classify defects by homotopy theory, and explore how they evolve into the neighboring anti-ferroquadrupolar spin-nematic phase. \\[4pt] [1] B. A. Ivanov, R. S. Khymyn, and A. K. Kolezhuk, Phys. Rev. Lett. 100, 047203 (2008).\\[0pt] [2] T. Grover and T. Senthil, Phys. Rev. Lett. 107, 077203 (2011). \\[0pt] [3] C. Xu and A. W. W. Ludwig, Phys. Rev. Lett, 108, 047202 (2012). \\[0pt] [4] A. Lauchil, F. Mila and K. Penc, Phys. Rev. Lett. 97, 087205 (2006).\\[0pt] [5] H. Tsunetsugu and M. Arikawa, J. Phys. Soc. Jpn. 75, 083701 (2006).\\[0pt] [6] A. Smerald and N. Shannon, arXiv:1307.5131. (accepted for publication in PRB) [Preview Abstract] |
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