Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session S4: Focus Session: Kagome Antiferromagnets I |
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Sponsoring Units: GMAG Chair: Oleg Tchernyshyov, Johns Hopkins University Room: 112/110 |
Thursday, March 6, 2014 8:00AM - 8:12AM |
S4.00001: Experimental Studies on Single Crystal Samples of Spin Liquid Materials Tian-Heng Han, Young Lee, John Schlueter, Thomas Rosenbaum, Eric Isaacs Frustrated antiferromagnetism on spin lattices with triangular geometries receives increasing attention due to the promise of RVB spin liquids. I will discuss about recent thermodynamic and scattering studies on highly frustrated magnets, such as triangular kappa-(ET)$_{2}$Cu$_{2}$(CN)$_{3}$ and kagome ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$. It was only until recent years that large crystal samples have been successfully grown for leading spin liquid candidates. Latest experimental studies will be introduced. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S4.00002: Microscopic magnetic modeling for the spin-$\frac12$ kagome compound $[$NH$_4]_2[$C$_7$H$_{14}$N$][$V$_7$O$_6$F$_{18}]$ Oleg Janson, Alexander A. Tsirlin, Ioannis Rousochatzakis, Helge Rosner, Raivo Stern In the recently synthesised compound $[$NH$_4]_2[$C$_7$H$_{14}$N$][$V$_7$O$_6$F$_{18}]$, magnetic $S$=$\frac12$ V$^{4+}$ atoms form an ideal kagome lattice $[$1$]$. Very recent $\mu$SR studies indicate the emergence of a gapless spin liquid state $[$2$]$. Using density functional theory calculations, we address the microscopic magnetic model of this interesting compound. We show that its peculiar symmetry gives rise to two inequivalent nearest-neighbor couplings. The behavior of the resulting spin model is studied using exact diagonalization and compared to the experiments. \\ $[$1$]$ F.H.Aidoudi~\textsl{et~al.}, Nature~Chem.~\textbf{3}, 810 (2011). \\ $[$2$]$ L.Clark~\textsl{et~al.}, Phys.~Rev.~Lett.~\textbf{110}, 207208 (2013). [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S4.00003: Negative quantum renormalization of excitation energies in the distorted kagome lattice antiferromagnet Cs$_2$Cu$_3$SnF$_{12}$ K. Matan, T. Ono, Y. Nambu, T. J. Sato, H. Tanaka Magnetic excitations in the distorted kagome lattice antiferromagnet Cs$_2$Cu$_3$SnF$_{12}$ were studied using neutron scattering. At room temperature, Cs$_2$Cu$_3$SnF$_{12}$ crystalizes in the hexagonal R$\bar{3}m$ space group with the lattice parameters $a = 7.142(4)$ {\AA} and $c=20.381(14)$ {\AA}. The $S=1/2$ Cu$^{2+}$ ions form a perfect kagome lattice. The system undergoes the structural transition at $T_s = 185$ K, doubling the in-plane lattice parameter $a$, and magnetic transition to the N\'{e}el state at $T_N = 20$ K. Spin-wave excitations in the ordered state can be qualitatively described by linear spin-wave theory (LSWT). However, the exchange interactions extracted from the spin-wave data are renormalized by a factor of 0.6 from those calculated by LSWT, almost irrespective of the momentum transfer. This inadequacy of LSWT is attributed to quantum effects and provides evidence of negative quantum renormalization of excitation energies in the kagome magnet. Recent results from a high-intensity pulsed neutron scattering experiment, which show the absence of high-energy spin-wave modes, will also be discussed. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S4.00004: Tensor Renormalization of Quantum Many-Body Systems using Projected Entangled Simplex States T. Xiang, Z.Y. Xie, J. Chen, J.F. Yu, X. Kong, B. Normand We propose a new class of tensor-network states, which we name projected entangled simplex states (PESS), for studying the ground-state properties of quantum lattice models. These states extend the pair-correlation basis of projected entangled pair states (PEPS) to a simplex. PESS are an exact representation of the simplex solid states and provide an efficient trial wave function that satisfies the area law of entanglement entropy. We introduce a simple update method for evaluating the PESS wave function based on imaginary-time evolution and the higher-order singular-value decomposition of tensors. By applying this method to the spin-1/2 antiferromagnetic Heisenberg model on the kagome lattice, we obtain an accurate result for the ground-state energy, $e_0 = - 0.4388(1) J$, which sets the lowest upper bound yet achieved for this quantity. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S4.00005: Neutron Scattering and Thermodynamic Studies of a Flat Mode in an S=$\frac{1}{2}$ Kagome Ferromagnet Robin Chisnell, Danna Freedman, Joel Helton, Deepak Singh, Chris Stock, Franz Demmel, Robert Bewley, Daniel Nocera, Young Lee Systems with flat bands provide macroscopic degeneracy that allows for the emergence of interesting strongly correlated phenomena such as the fractional quantum Hall effect. Hopping models on geometrically frustrated lattices with spin-orbit interactions predict the existence of flat, topologically nontrivial bands. Experimental realizations of these systems have proved challenging, as the flat band is often distorted by additional interactions. Cu(1,3-bdc) is a hybrid organometallic compound featuring S=$\frac{1}{2}$ Cu$^{2+}$ ions on a kagome lattice. The magnetic moments order ferromagnetically below T=1.8K. We present neutron scattering and thermodynamic measurements of Cu(1,3-bdc) and note the emergence of a flat magnon band in the ordered phase. The presence of a small Dzaloshinskii-Moriya(DM) interaction along with an applied magnetic field perpendicular to the kagome plane creates a gap between the flat band and lower energy dispersive band. The DM interaction also gives two of the magnon bands, including the flat band, a non-zero Chern number. We explore possible topological properties of these bands. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S4.00006: Chern-Simons theory for frustrated Heisenberg spins on Kagome Lattice Krishna Kumar, Kai Sun, Eduardo Fradkin There has been a lot of renewed interest in frustrated spin systems on Kagome lattices especially with the discovery of materials like volborthite and herbertsmithite. In the presence of an external magnetic field (or at fractional fillings), these systems can give rise to magnetization plateaus. Numerous studies indicate the existence of a m=1/3 plateau on the Kagome lattice. Here, we look at the problem of anti-ferromagnetic Heisenberg spins using a Jordan-Wigner transformation that maps the spins onto a problem of fermions coupled to a Chern-Simons gauge field. This method has been used successfully to study unfrustrated systems like the square lattice. At a mean-field level the above ideas have also been applied to frustrated systems. However, fluctuations are generally strong in these models and can affect the mean-field physics. We report a method to rigorously extend the Chern-Simon's term to frustrated lattices like the Kagome lattice. We discuss the different phases that arise at the mean-field level from this theory focusing specifically on the case of 1/3-filling, which gives rise to a magnetization plateau and is a topological phase. Finally, we will also comment on the implications of our model in the case of 1/2-filling. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:48AM |
S4.00007: Chiral Spin Liquids Invited Speaker: Laura Messio Frustrated spin lattices are theoretically and experimentally challenging systems in which many fascinating phases exist. On bidimensional lattices, unordered phases (i.e. that are neither N\'eel ordered nor break any other Hamiltonian symmetry) can survive until zero temperature, giving rise to the so-called spin liquid phases. They are related to superconductivity, quantum computing, spintronics... Two such phases were recently identified in experiments on the magnetic compounds Herbertsmithite and Kapellasite. But how to classify all the different spin liquids? How to distinguish several phases without any order parameter? The answer lies in quantities called fluxes, defined on lattice loops. Depending on the Hamiltonian symmetries and on the lattice, only some patterns of fluxes are possible, as was explained by Wen in 2002 with the use of group theory. When only the time reversal symmetry is broken, the phase is a chiral spin-liquid. In that case, new patterns of fluxes are allowed as they can be non trivial (i.e. different from 0 or $\pi$). They are obtained by extending the projective symmetry group approach of Wen. Some spin liquids have a parent classical state, sharing similar flux patterns. This state can be seen as a classical spin liquid. It has specific symmetry properties and is called a regular state. A chiral spin liquid leads to a chiral classical state. Combined with this semi-classical approach, the projective symmetry group theory extended to chiral states has led until now to the identification of two interesting chiral spin liquids. The first one is a new candidate for the kagome antiferromagnet ground state and the second one partially explains the experimental results obtained on Kapellasite. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S4.00008: The nature of the quantum spin-liquid state in Herbertsmithite Matthias Punk, Debanjan Chowdhury, Subir Sachdev Recent neutron scattering experiments on the layered spin-1/2 kagome lattice antiferromagnet Herbertsmithite revealed the first signature of fractionalized excitations in a quantum spin liquid state. The precise nature of this state remains unclear, however. Mean-field models of gapped as well as gapless spin liquids exhibit sharp features in the dynamic structure factor, none of which have been observed in experiment. We are going to show that several of the experimentally observed details can be explained by the presence of topological vortex excitations in a gapped Z2 spin liquid. These so called vison excitations form almost flat bands on the kagome lattice and act as a momentum sink for the spin-carrying excitations probed by neutron scattering. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S4.00009: Symmetry breaking Schwinger Boson Mean Field Theory solutions on Kagome Shivam Ghosh, Christopher L. Henley Schwinger Boson Mean Field theory (SBMFT) is a powerful technique for describing both quantum disordered and symmetry broken phases of Heisenberg spins as a function of spin length $\kappa=2S$. Previous applications of SBMFT have been to study \emph{symmetric} SL's which preserve lattice and time reversal symmetries (TRS). The \emph{assumption} of a symmetric ground state reduces the number of mean field variables simplifying search for SL saddle points. We go beyond the manifold of \emph{symmetric} SL's on the kagome lattice and using an optimization \footnote{G.Misguich, PRB 86, 245132 (2012)} technique search for solutions that may \emph{spontaneously} break lattice and TRS. An exhaustive search for saddle points on a $4\times4$ lattice shows that the lowest energy solutions have zero flux ($[0hex]$) through hexagons in agreement with the Greedy Boson theorem \footnote{O. Tchernyshyov et al. EPL, 73, 278 (2006)} However, amongst the manifold of $[0hex]$ solutions we find a state \emph{lower} in energy than Sachdev's uniform $Q_{1}=-Q_{2}$ state, extending up to $\kappa=0.3$, which \emph{spontaneously} breaks lattice symmetry and differs from uniform solution in flux patterns through length eight loops . We also characterize other (higher in energy) \emph{chiral} saddle points [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S4.00010: Study of vison-spinon bound states on the kagome lattice Junping Shao, Shivam Ghosh, Gil-Young Cho, Michael Lawler We search for low-energy vison-spinon bound states on the kagome lattice. We do this by applying an optimization algorithm to a bosonic spin liquid state with a well separated pair of visons inserted. The resulting wavefunction reveals that the low energy eigen-modes correspond to bound spinon states localized around the visons. We study these modes and their symmetry properties. Our results provide evidence supporting the low energy effective theories of Z2 spin liquids whose bosonic spinons, fermonic spinons and visions are characterized by projective symmetry groups consistent with the expected fusion rules and duality relations. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S4.00011: Phase diagram of an easy-axis Kagome antiferromagnet under a magnetic field Xavier Plat, Fabien Alet, Sylvain Capponi, Pierre Pujol, Keisuke Totsuka We present a quantum Monte-Carlo (QMC) study of a spin-1/2 XXZ model, with second and third-neighbour terms, under a magnetic field on the Kagome lattice. This model, introduced in the zero field case by Balents, Fisher and Girvin [1], exhibits, in the easy-axis limit, a topological gapped Z2 phase with fractional excitation [2-4]. When adding a magnetic field, other gapped incompressible phases are stable for magnetizations 1/3 and 2/3 of its saturation value. Using state-of-the-art measurements, including recently developped tools to compute the topological entropy, we investigate the nature of these ground-states. Finally, we make some connection between these microscopic models and effective constrained models (such as quantum loop model or quantum dimer model respectively), which allow to provide a better understanding of the physical properties. \\[4pt] [1] L. Balents, M. P. A. Fisher, and S. M. Girvin, Phys. Rev. B {\bf 65}, 224412 \\[0pt] [2] S. V. Isakov, Y. B. Kim, and A. Paramekanti, Phys. Rev. Lett. {\bf 97}, 207204 (2006) \\[0pt] [3] S. V. Isakov, M. B. Hastings, and R. G. Melko, Nat. Phys. {\bf 7}, 772 (2011) \\[0pt] [4] S. V. Isakov, R. G. Melko, and M. B. Hastings, Science {\bf 335}, 193 (2012) [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S4.00012: Magnetic Field Driven Phase Transitions in S = $\frac{1}{2}$ Kagome Lattice Antiferromagnet ZnCU$_{3}$(OH)$_{6}$Cl$_{2}$ Lu Li, T. Asaba, T. Han, B.J. Lawson, F. Yu, C. Tinsman, Z. Xiang, G. Li, Y.S. Lee Herbertsmithite ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ is a kagome lattice antiferromagnet with 1/2 spin and has been demonstrated to be a likely candidate of spin liquid by recent neutron scattering measurements. The high magnetic field response of the kagome lattice sample is hard to separate from the magnetic signals from Cu impurities sitting between the kagome planes. To separate these two contributions, we measured the magnetization of a single crystalline ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ using torque magnetometry at temperatures from 20mK to 15K in intense magnetic field as high as 31 T. Below 2 K, several phase transitions are observed in field near 8 T - 16 T, and the transition fields do not show significant dependence on the temperature in the range of 20 mK $\leq T \leq$ 2 K. Moreover, the transition fields are independent of the magnetic field orientation. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S4.00013: $^{17}$O Single Crystal NMR Study on S $=1/2$ Kagome Lattice ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ Mingxuan Fu, Takashi Imai, Tianheng Han, Young. S. Lee Herbersmithite ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ is known to be a promising candidate material hosting a quantum spin liquid ground state. The recent success in single crystal growth of ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ as well as the discovery of a continuum of spinon excitations using inelastic neutron scattering\footnote{T. H. Han \textit{et al}., Nature {\bf 492}, 406(2012)} have opened a new chapter in the study of highly frustrated magnetism. However, the mechanism behind the realization of the non-magnetic ground state in ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ remains controversial, mainly due to the difficulty in understanding the role of defects in its physical properties. Through single-crystal $^{17}$O NMR study, we identified multiple O sites with distinct local magnetic environments. The behavior of local spin susceptibility and spin dynamics observed at these O sites provide invaluable insights into the nature of defects and their potential influence on the kagome spin lattice.\footnote{M. Fu, T. Imai \textit{et al}., in preparation. Also see T. Imai. \textit{et al}., Phys. Rev. B {\bf 84}, 020411(R) (2011); Phys. Rev. Lett. {\bf 100}, 077203 (2008)} [Preview Abstract] |
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