Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session D18: Focus Session: Low D/Frustrated Magnetism - Kagome Lattices |
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Sponsoring Units: GMAG DMP Chair: Joel Helton, NIST Center for Neutron Research Room: D172 |
Monday, March 21, 2011 2:30PM - 2:42PM |
D18.00001: Highly frustrated magnets: a class of emergent gauge systems Michael Lawler Condensed matter exhibit a wide variety of exotic emergent phenomena, such as the topological order in the fractional quantum Hall effect, and the ``cooperative paramagnetic'' response of geometrically frustrated magnets. The classical and quantum dynamics of spins exploring the large configuration space associated with the latter are not well understood analytically. I consider the constrained classical Hamiltonian dynamics of spins exploring such a configuration space as a starting point from which a complete classical and semi-classical description may be reached. The method I employ, introduced by Dirac [1] and now forms the basis of gauge theory, applies to any frustrated system constrained to a continuous set of configurations. Remarkably, in the kagome lattice model I consider as an example, these dynamics are similar to the ``topological'' (Chern-Simons) dynamics of electrons in the fractional quantum Hall effect and have non-locally entangled edge modes as the only degrees of freedom. In principle, these edge states may be found in any kagome-like Heisenberg antiferromagnets such as Herbertsmithite, the Jarosites, SrCr$_{8-x}$G$_{4+x}$O$_{19}$ and Na$_4$Ir$_3$O$_8$. \\[4pt] [1] Dirac, P. A. M. {\it Generalized hamiltonian dynamics}. Can. J. of Math. {\bf 2}, 129-148 (1950) [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D18.00002: Predictions for the ARPES spectral function of kagome antiferromagnetic insulators Sumiran Pujari, Michael J. Lawler There are now a number of spin liquid candidate materials possibly with exotic spin-1/2 ``spinon'' excitations. Motivation by these discoveries, we consider the scaling properties of the hole spectral function for the frustrated Kagome Heisenberg antiferromagnet assuming Dirac Spin Liquid(DSL) ground state proposed for Herbertsmithite $[2]$. We predict a sublinear in energy power law dependence of the ARPES spectral function at certain wave vectors. Using Renormalization group techniques, we show how (gauge) fluctuations of the DSL mean field give an anomalous exponent to spinons$[2]$ and no anomalous exponent to holons thereby leading to the sublinear power law. If this behavior is observed in experiments, they would provide strong evidence for the existence of spinons in highly frustrated magnets.\footnote{Y. Ran et al, Phys. Rev. Lett. 98, 117205 (2007)} [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D18.00003: Exact Chiral Spin Liquid with Stable Spin Fermi Surface on the Kagome Lattice Victor Chua, Hong Yao, Gregory Fiete We study an exactly solvable quantum spin model of Kitaev type on the kagome lattice. We find a rich phase diagram which includes a topological (gapped) chiral spin liquid with gapless chiral edge states, and a gapless chiral spin liquid phase with a spin Fermi surface. The ground state of the current model contains an odd number of electrons per unit cell which qualitatively distinguishes it from previously studied exactly solvable models with a spin Fermi surface. Moreover, we show that the spin Fermi surface is stable against weak perturbations. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D18.00004: Quantum criticality in the kagome staircase system Co$_3$V$_2$O$_8$ in transverse magnetic field K. Fritsch, K.C. Rule, K.A. Ross, Y. Qiu, J.R.D. Copley, K. Kiefer, K. Habicht, H.A. Dabkowska, B.D. Gaulin Co$_3$V$_2$O$_8$ (CVO) belongs to the kagome staircase family of orthorhombic materials in which Ising-like Co$^{2+}$, S=3/2 magnetic moments decorate a stacked and buckled version of the two-dimensional kagome lattice. In zero applied magnetic field, this material displays a complex series of five different magnetically ordered phases below $\sim$11 K which culminate in a simple ferromagnetic state below Tc$\sim$6 K. Previous inelastic neutron scattering work[1] on this quasi-two-dimensional system showed that the exchange interactions within the kagome planes are rather weak (J$\sim$1.25 meV), making this system an ideal candidate for the study of transverse field-induced quantum critical phenomena as have been observed in LiHoF$_4$ or recently in CoNb$_2$O$_6$. We have investigated the phase diagram of CVO with the transverse field applied along the stacking direction using magnetization as well as single crystal neutron scattering techniques. We will discuss how the ground state magnetic structure and spin dynamics of CVO evolve upon tuning the transverse magnetic field through the quantum critical point near Hc$\sim$6 T. [1] M. Ramazanoglu et al., PRB 79, 024417 (2009). [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D18.00005: Herbertsmithite: a slightly less than ideal kagom\'e antiferromagnet Yuan Wan, Zhihao Hao, Oleg Tchernyshyov Herbertsmithite, one of the best realizations of the Heisenberg antiferromagnet on kagom\'e, shows no signs of magnetic order down to the lowest accessible temperatures and likely possesses a quantum-disordered ground state. A recent site-specific X-ray diffraction experiment [1] shows deviations from the ideal model, most notably in the form of excess copper spins residing outside of kagom\'e planes. We study the influence of these impurities on the magnetic properties of herbertsmithite, focusing on locally induced Jahn-Teller distortions in kagom\'e planes and on exchange interactions between the excess spins.\\[4pt][1] Danna E. Freedman \textit{et al}, J. Am. Chem. Soc. {\bf132}, 16185 (2010). [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D18.00006: Magnetic studies of S=1/2 kagom\'{e} lattice single crystals Tianheng Han, Joel Helton, Andrea Prodi, Claudio Mazzoli, Peter Muller, Deepak Singh, Jose Rodriguez, Collin Broholm, Daniel Nocera, Shaoyan Chu, Young Lee The Zn-paratacamite mineral family, Zn$_{x}$Cu$_{4-x}$(OH)$_{6}$Cl$_{2}$, presents a promising system for studies of frustrated magnetism on a S=1/2 kagom\'{e} lattice. Here we report a new synthesis method, by which high quality single crystals of Zn-paratacamite can be produced. The x = 1 mineral herbertsmithite is a spin-liquid candidate. This compound displays a magnetic susceptibility that is anisotropic at high temperatures. A small anisotropy is observed in specific heat measurements with magnetic field applied in-plane and normal-to-plane. Inelastic neutron scattering has been performed and we will discuss the observed structure factor in the context of various theoretical expectations. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D18.00007: Terahertz conductivity of a metal-organic hybrid Kagom\'e lattice: A candidate spin liquid Daniel Pilon, Alex Frenzel, Danna Freedman, Daniel Nocera, Nuh Gedik Recent theoretical studies predict that the optical conductivity of a spin liquid should exhibit power law behavior in frequency at low temperatures. Materials with the Kagom\'e structure are the most promising candidates for observing spin liquid behavior due to their high degree of magnetic frustration. We have measured the optical conductivity of Cu(1, 3-bdc), a spin-1/2 Kagom\'e lattice material, in the range 0.5 - 2 THz. We compare these results to the theoretical predictions and comment on the implications for the existence of a spin liquid state in this material. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D18.00008: Valence-bond crystal in the extended Kagom\'e spin-$1/2$ quantum Heisenberg antiferromagnet: A variational Monte Carlo approach Federico Becca, Yasir Iqbal, Didier Poilblanc The highly-frustrated spin-$1/2$ quantum Heisenberg model with both nearest ($J_1$) and next-nearest ($J_2$) neighbor exchange interactions is revisited by using an extended variational space of projected wave functions that are optimized with state-of-the-art methods. Competition between modulated valence-bond crystals (VBC) proposed in the literature and the Dirac spin liquid (DSL) is investigated. We find that the addition of a {\it small} ferromagnetic next-nearest-neighbor exchange coupling $|J_2|>0.09 J_1$ leads to stabilization of a 36-site unit cell VBC, although the DSL remains a local minimum of the variational parameter landscape. This implies that the VBC is not trivially connected to the DSL: instead it possesses a non-trivial flux pattern and large dimerization. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D18.00009: Magnetization ramp of the Kagome lattice antiferromagnet Toru Sakai, Hiroki Nakano Magnetization process of the S=1/2 isotropic Heisenberg antiferromagnet on the Kagome lattice is studied. Data from numerical-diagonalization method up to 39-spin systems, are reexamined from the viewpoint of the derivative of the magnetization with respect to the magnetic field. We find that the behavior of the derivative around the 1/3 height of the magnetization saturation is quite different from the cases of typical magnetization plateaux. The magnetization process of the Kagome-lattice antiferromagnet reveals a new phenomena, which we call the ``magnetization ramp.'' We also compare it with the 1/3 magnetization plateau of the triangular antiferromagnet. \\[4pt] [1] H. Nakano and T. Sakai: J. Phys. Soc. Jpn. 79 (2010) 053707, arXiv:1004.2528. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:54PM |
D18.00010: Spin dynamics in the hyperkagome compound Gd$_{3}$Ga$_{5}$O$_{12}$ Invited Speaker: We present the first neutron inelastic scattering results on the magnetic state of the frustrated hyperkagome compound $\rm Gd_3Ga_5O_{12}$ (GGG) at low temperatures and in applied magnetic field. Our neutron scattering studies reveal a remarkable range of timescales. Short-range spatial correlations appear static within the instrumental resolution (50~$\mu$eV). Three distinct inelastic modes are found at 0.04(1), 0.12(2) an 0.58(3)~meV at 0.06~K. The application of a magnetic field up to 2.5~tesla reveals disparate behavior of the magnetic excitations. In zero applied field, the lowest and highest energy excitations show spatial dependencies indicative of dimerized short-range antiferromagnetic correlations that survive to high temperatures, comparable to the nearest neighbor exchange interactions. Our results suggest that the ground state of a three dimensional hyperkagome compound differs distinctly from its frustrated counterparts on a pyrochlore lattice and reveal a juxtaposition of cooperative paramagnetism and strong dimerized coupling. These results are surprising since GGG is often classified as a strongly frustrated system with a manifold of connected states for which one would expect a continuum of gapless excitations. \\[4pt] In collaboration with: Pascale Deen, Institut Laue Langevin; G. Balakrishnan, Department of Physics, University of Warwick; B.D. Rainford, Department of Physics and Astronomy, Southampton University; C. Ritter, Institut Laue-Langevin; L. Capogna, Istituto Officina dei Materiali, IOM-CNR; H. Mutka and T. Fennell, Institut Laue-Langevin. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D18.00011: Effects of doping on the geometrically frustrated Heisenberg antiferromagnet Gadolinium Gallium Garnet D.M. Silevitch, M.A. Schmidt, S. Ghosh, G. Aeppli, T.F. Rosenbaum Geometric frustration in the Heisenberg antiferromagnet Gadolinium Gallium Garnet (GGG) gives rise to a set of quantum protectorates where clusters of spins decouple from the overall spin liquid state. At 110 mK, there is a partial transition to an ordered AF state. Here, we examine the effect of lightly doping GGG with Nd ions, which partially alleviates the underlying frustration. We examine the size and binding energy of the spin clusters as a function of doping and temperature, and also characterize the suppression of the Neel temperature as the dopant concentration is increased. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D18.00012: Electron magnetic resonance studies of the Pr$_{3}$Ga$_{5}$SiO$_{14}$ and Nd$_{3}$Ga$_{5}$SiO$_{14}$ kag\'{o}me systems Sanhita Ghosh, Saiti Datta, Haidong Zhou, Michael Hoch, Stephen Hill In recent years, there has been considerable interest in materials exhibiting magnetic frustration due to their novel ground state properties. Pr$_{3}$Ga$_{5}$SiO$_{14}$ (PGS) and Nd$_{3}$Ga$_{5}$SiO$_{14}$ (NGS) have trigonal crystal structures in which the rare earth ions are arranged in corner sharing triangles to form a distorted kagom\'{e} lattice. We report high frequency electron magnetic resonance (EMR) measurements on single crystals of NGS and PGS in order to ascertain the nature of their ground states. Both compounds exhibit extremely rich EMR spectra at low temperatures, with a large number of sharp peaks. For each frequency investigated, the peak positions display a strong, systematic dependence on the temperature. However, the usual paramagnetic resonance frequency/field variation is not observed, with the pattern of peaks varying dramatically from one frequency to the next. We thus conclude that the observed spectra correspond to collective excitations associated with finite size ordered clusters that persist on EMR time scales. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D18.00013: Valence Bond Crystal on the Hyperkagome Antiferromagnet Emil Bergholtz, Andreas Lauchli, Roderich Moessner We describe our recent work that indicates that the ground state of the antiferromagnetic spin-1/2 Heisenberg model on the highly frustrated, three-dimensional, hyper-kagome lattice is a valence bond crystal (VBC). Performing a series expansion around an arbitrary dimer covering on the hyper-kagome we find that a ground state with a huge (72 site) unit cell is selected by the quantum fluctuations. The regularity and favorable energetics of our series expansion establishes the VBC as a serious contender to the earlier spin liquid proposals. We find that the ground state supports many, very low lying, excitations in the singlet sector and that the low energy spinful excitations (spinons and triplons) are effectively confined to various emergent lower-dimensional structures. If applicable to the recently studied sodium iridate compound, Na$_4$Ir$_3$O$_8$, this scenario has interesting observable implications, such as spatially anisotropic neutron scattering spectra and possibly multiple finite temperature signatures in the magnetic specific heat due to a multi-step breaking of discrete symmetries. Most saliently, here---as for several proposed states for analogous kagome and pyrochlore magnets---one might expect a clearly resolved Ising transition at relatively high temperature. \\[4pt] Ref: E.J. Bergholtz, A.M. L\"auchli and R. Moessner, Phys. Rev. Lett., in press (2010) [arXiv:1010.1345] [Preview Abstract] |
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