Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session P34: Focus Session: Frustrated and Low-D Magnetism -- Antiferromagnets on the Triangular Lattice |
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Sponsoring Units: DMP GMAG Chair: Oleg Starykh, University of Utah Room: E144 |
Wednesday, March 17, 2010 8:00AM - 8:36AM |
P34.00001: Magnetization plateaus in the triangular-lattice antiferromagnet Cs$_{2}$CuBr$_{4}$ Invited Speaker: Studies of antiferromagnets on a triangular lattice date back to the pioneering work of Wannier, Houtappel, and Husimi and Syoji from 1950. Despite the long history, this prototypical frustrated geometry harbors many unsolved problems. In small-spin Heisenberg antiferromagnets on a triangular lattice, the interplay between the geometric frustration and quantum fluctuations leads to non-trivial low-energy excitations and exotic high-field ground states, which are not yet fully understood. We have mapped out the phase diagram of Cs$_{2}$CuBr$_{4}$, one of the best laboratory models for the S=1/2 Heisenberg antiferromagnet on a triangular lattice, in high magnetic fields and have found a cascade of new ordered phases, three of which manifest themselves as magnetization plateaus. Possible spin structures of these phases are strongly limited by the requirement of commensurateness and the absence of Nambu-Goldstone modes for any ordered state that forms a magnetization plateau. This work was in collaboration with N.A. Fortune, S.T. Hannahs, Y. Yoshida, A.A. Wilson-Muenchow, O. Starykh, T. Ono, and H. Tanaka. [Preview Abstract] |
Wednesday, March 17, 2010 8:36AM - 8:48AM |
P34.00002: Fermionic excitations in the spin liquid phase of Cs$_2$CuCl$_4$ as revealed by $^{133}$Cs NMR Vesna F. Mitrovic, Marc-Andre Vachon, Georgios Koutroulakis, Ookie Ma, Brad Marston, Arneil P. Reyes, Philip Kuhns, Radu Coldea, T. Tylczynski Nuclear magnetic resonance measurements of the spin-liquid phase of the spin-1/2 frustrated antiferromagnet Cs$_2$CuCl$_4$ as a function of temperature and applied magnetic field will be presented. Comparison of the magnetization and relaxation rate to the spin-1/2 antiferromagnetic chain $\alpha$-CuNSal and to variational calculations using Gutzwiller-projected mean-field theory implies that the low energy excitations in Cs$_2$CuCl$_4$ are characterized, in the spin liquid phase at non-zero temperature and applied field, by gapless fermionic excitations. Furthermore, interactions in two dimensions are required to reproduce the low energy properties. [Preview Abstract] |
Wednesday, March 17, 2010 8:48AM - 9:00AM |
P34.00003: Finite temperature phase diagram of the classical spatially anisotropic triangular antiferromagnet in magnetic field Shane Head, Christian Griset, Jason Alicea, Oleg Starykh The spatially anisotropic triangular antiferromagnets Cs$_2$CuCl$_4$ and Cs$_2$CuBr$_4$ exhibit rich, and only partially understood, phase diagrams in applied magnetic fields. Motivated by such materials, we employ extensive Monte Carlo simulations to investigate the magnetic field vs. temperature phase diagram of the classical Heisenberg triangular antiferromagnet with anisotropic exchanges $J'$ along weak diagonal bonds and $J$ along horizontal bonds. We find an interesting interplay between spatial anisotropy and entropic (thermal) fluctuations, which favor different types of order and thus compete with one another. For a wide range of magnetic fields and $J'/J$, thermal fluctuations stabilize coplanar spin configurations--including `up-up-down' order near 1/3 magnetization--at the expense of low-temperature non-coplanar `umbrella' structures favored by the spatial anisotropy. The influence of Dzyaloshinskii-Moriya couplings relevant for the above materials will also be discussed. [Preview Abstract] |
Wednesday, March 17, 2010 9:00AM - 9:12AM |
P34.00004: Phase diagram of Cs$_2$CuCl$_4$ in magnetic field Leon Balents, Hosho Katsura, Oleg Starykh The spatially anisotropic spin-1/2 triangular lattice antiferromagnet Cs$_2$CuCl$_4$ exhibits a rich variety of phases and a great sensitivity to the direction of applied magnetic field. We argue that this richness is characteristic of the frustrated quasi-one-dimensional geometry, which leads to an exquisite sensitivity to very weak interactions that are usually neglected. For fields perpendicular to the triangular plane, the ground state is predominantly determined by a Dzyaloshinskii-Moriya (DM) interaction which is only 5\% of the largest Heisenberg exchange. Surprisingly, most of the phases observed for the in-plane direction of the magnetic field are controlled by even weaker inter-plane interactions, which in turn leads to an enhanced effect of yet still weaker in-plane next-nearest-chain exchange interactions between spins. Additional field-direction-dependent subtleties of the phase diagram are explained by accounting for other weak symmetry-allowed asymmetric DM terms. Globally, the various weak interactions lead to a phase diagram which is dramatically different from that of the simple anisotropic triangular lattice J-J' Heisenberg model. [Preview Abstract] |
Wednesday, March 17, 2010 9:12AM - 9:24AM |
P34.00005: Quasiparticles of the spin-1/2 Heisenberg antiferromagnet on an anisotropic triangular lattice in a magnetic field Masanori Kohno We investigate the spectral properties of anisotropic triangular antiferromagnets in a magnetic field using a weak-interchain-coupling approach combined with Bethe-ansatz solutions of a chain. Collective modes induced by interchain interactions behave as quasiparticles (QPs) which show distinctive features in a magnetic field. Different with conventional magnons, they show spin-density-wave-type incommensurate ordering whose momentum strongly depends on the magnetization, high-energy modes originating from 2-string solutions of the Bethe ansatz, and multi-particle crossover in a magnetic field. Their stabilization mechanism is also different from conventional magnons: analogous to collective modes in Fermi liquids, the QPs are induced from liquids of 1D QPs (psinons, antipsinons, and a quasiparticle for a 2-string) by interchain exchange processes, which may thus be regarded as those in a kind of anisotropic-2D spin liquid, distinguished from magnons created from classical orders. In terms of them, various unusual features observed in Cs$_2$CuCl$_4$ are quantitatively explained in a unified manner. [Preview Abstract] |
Wednesday, March 17, 2010 9:24AM - 9:36AM |
P34.00006: Quadrupolar Correlations, Bond Order and Spin Freezing in S=1 Triangular Lattice Antiferromagnets Edwin Miles Stoudenmire, Simon Trebst, Leon Balents Motivated by experiments on the $S=1$ triangular lattice antiferromagnet NiGa$_2$S$_4$ and theoretical predictions that it has a quadrupolar/spin-nematic ground state, we discuss how quadrupolar correlations may actually be more relevant at \emph{finite} temperature, giving rise to an unusual two peak structure in the specific heat. Moreover, dominant third-neighbor Heisenberg exchange in the clean system can lead to a breaking of lattice rotational symmetry at finite temperature, although the sensitivity of the phase to arbitrarily weak non-magnetic disorder could explain the lack of long range order and the slow dynamics observed in experiment. To justify these predictions, we implemented a novel semiclassical approximation that allows $T>0$ quantum effects to be simulated efficiently using classical Monte Carlo. Current efforts include treating quantum effects exactly, reproducing an experimentally observed even-odd spin impurity effect and providing other experimental signatures of the quadrupolar correlations. [Preview Abstract] |
Wednesday, March 17, 2010 9:36AM - 9:48AM |
P34.00007: Variational studies of Triangular Heisenberg Antiferromagnet in magnetic field Tiamhock Tay, Olexei I. Motrunich We present a variational study of the Heisenberg antiferromagnet on the spatially anisotropic triangular lattice in magnetic field. First we construct a simple yet accurate wavefunction for the 1/3-magnetization plateau $uud$ phase on the isotropic lattice. Beginning with this state, we obtain natural extensions to nearby commensurate planar phases on either side of the plateau which are present for low lattice anisotropy. Far away from the 1/3 plateau and for significant anisotropy, incommensurate states have better energetics. In the highly anisotropic regime, our study shows strong signatures of quasi-1d physics. The variational study is supplemented by exact diagonalization calculations which provide a reference for testing the energetics of our trial wavefunctions as well as helping to identify candidate phases. [Preview Abstract] |
Wednesday, March 17, 2010 9:48AM - 10:00AM |
P34.00008: Spin-liquid and magnetic phases in the anisotropic triangular lattice: the case of $\kappa$-(ET)$_2$X Federico Becca, Luca Tocchio, Alberto Parola, Claudius Gros The two-dimensional Hubbard model on the anisotropic triangular lattice, with two different hopping amplitudes $t$ and $t^\prime$, is relevant to describe the low-energy physics of $\kappa$-(ET)$_2$X, a family of organic salts. The ground-state properties of this model are studied by using Monte Carlo techniques, on the basis of a recent definition of backflow correlations for strongly-correlated lattice systems. The results show that there is no magnetic order for reasonably large values of the electron-electron interaction $U$ and frustrating ratio $t^\prime/t = 0.85$, suitable to describe the non-magnetic compound with X=Cu$_2$(CN)$_3$. On the contrary, N\'eel order takes place for weaker frustrations, i.e., $t^\prime/t \sim 0.4 \div 0.6$, suitable for materials with X=Cu$_2$(SCN)$_2$, Cu[N(CN)$_2$]Cl, or Cu[N(CN)$_2$]Br. [Preview Abstract] |
Wednesday, March 17, 2010 10:00AM - 10:12AM |
P34.00009: Slave-Rotor Mean Field Theory of the Extended Hubbard Model on the Anisotropic Triangular Lattice Jeffrey G. Rau, Hae-Young Kee We report on a theoretical investigation of a metal-superconductor-insulator transition on the anisotropic triangular lattice. We apply a self-consistent slave-rotor mean field theory to the extended Hubbard model on this lattice at half-filling. A variety of mean field phases are found in both the insulating and metallic regimes including nematic and smectic spin liquids. Preliminary applications to the strongly correlated organic materials will be discussed. [Preview Abstract] |
Wednesday, March 17, 2010 10:12AM - 10:24AM |
P34.00010: Glassy Spin Freezing and Gapless Spin Dynamics in a Spatially Anisotropic Triangular Antiferromagnet Ag2MnO2 Sungdae Ji, J.-H Kim, S.-H. Lee, Y. Qiu, M. Green, M. Matzuda, Y. Yoshida, Z. Hiroi, T. Ziman We report our elastic and inelastic neutron scattering measurements on a spatially-anisotropic triangular antiferromagnet metal compound of Ag$_2$MnO$_2$. Upon cooling, it undergoes a structural phase transition at 540 K from trigonal to monoclinic due to a ferro-orbital order of the Jahn-Teller active Mn$^3+$ ion resulting in spatially anisotropic magnetic interactions in the triangular plane. Despite the large T$_CW$ $\sim$ 400 K, it does not order down to T$_f$ = 48(6) K below which the Mn spins freeze into a collinear spin state with the frozen moment, M = 2.4(2) $\mu_B$/Mn << gs$\mu_B$/Mn, and the frozen spin order is short ranged indicating extreme two-dimensional magnetic interaction. Moreover, the two-dimensional spin fluctuations have a gapless spectrum with two characteristic relaxation rates, an overall relaxation rate that behave linearly above freezing temperature . We argue that Ag$_2$MnO$_2$ might be an excellent candidate for a gapless spin liquid phase. [Preview Abstract] |
Wednesday, March 17, 2010 10:24AM - 10:36AM |
P34.00011: Thermal-transport measurement in a gapless spin-liquid state of EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ Minoru Yamashita, Norihito Nakata, Yoshinori Senshu, Takasada Shibauchi, Yuji Matsuda, Reizo Kato The quest of the ground state of quantum spins ($S= 1/2$) under a geometrical frustration such as triangular lattice and kagom\'{e} lattice has been attracting tremendous attention for several decades, because a novel quantum state of matter is expected to emerge. One promising method to unveil the ground state is to measure the thermal conductivity because it is very sensitive to delocalized low-lying quasiparticles. We report thermal conductivity ($\kappa_{xx}$) and thermal Hall conductivity ($\kappa_{xy}$) measurements of EtMe$_3$Sb[Pd(dmit) $_2$]$_2$ down to 100 mK. This organic insulator with nearly identical 2D triangular lattice of $S = 1/2$ has been reported to possess a novel spin liquid state down to $\sim J/12,000$ by NMR. We found a clear residual of $\kappa/T$ in the zero- temperature limit, showing a contrasting behavior found in $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ in which a spin gap is formed below $\sim0.5$ K [1]. This residual $\kappa/T$ shows that there are gapless Fermionic excitations, such as spinons. We will discuss results of $\kappa_{xy}$ up to 12 T in terms of recent theoretical suggestion of thermal Hall effects for spinons [2]. [1] M. Yamashita \textit{et al.}, Nature Physics \textbf{5}, 44- 47 (2009). [2] H. Katsura \textit{et al.}, arXiv:0904.3427 [Preview Abstract] |
Wednesday, March 17, 2010 10:36AM - 10:48AM |
P34.00012: Successive phase transitions and phase diagrams of $S$ = 5/2 quasi-two dimensional triangular antiferromagnet Rb$_{4}$Mn(MoO$_{4})_{3}$ Rieko Ishii, Keisuke Onuma, Shu Tanaka, Masashi Tokunaga, Toshiro Sakakibara, Dixie Gautreaux, Julia Chan, Satoru Nakatsuji Our comprehensive experimental studies on $S$ = 5/2 Rb$_4$Mn(MoO$_4$)$_3$ have clarified that this is a model system of a quasi-two dimensional Ising-like triangular Heisenberg antiferromagnet. This material exhibits the successive phase transitions and 1/3 magnetic plateau phase under fields. Furthermore, we have clarified a complete phase diagrams for which quantitative agreement between experiment and Monte Carlo simulation is obtained. We will report experimental results of magnetic susceptibilities, specific heat, and neutron measurements and discuss spin structures under zero/nonzero fields. [Preview Abstract] |
Wednesday, March 17, 2010 10:48AM - 11:00AM |
P34.00013: Chirality and helicity in the frustrated langasite Ba$_{3}$NbFe$_{3}$Si$_{2}$O$_{14}$ Karol Marty, Virginie Simonet, Pierre Bordet, Rafik Ballou, Eric Ressouche Ba$_{3}$NbFe$_{3}$Si$_{2}$O$_{14}$ cristallizes in the trigonal non-centrosymmetric space group P321. The Fe$^{3+}$ carry a spin 5/2 and form a triangular lattice of equilateral triangles. This compound orders magnetically at T$_{N}$=27K, well below the Curie-Weiss temperature of $\theta$=-171K, which is a fingerprint of magnetic frustration. Neutron diffraction experiments allowed solving the magnetic structure. This consists in a ferro-chiral configuration of equal moments lying in the plane perpendicular to the 3- fold c axis, oriented at 120$^{\circ}$ from each other on each triangle of Fe$^{3+}$ (due to the intra-plane geometrical frustration), with the same chirality for all the triangles, the whole helically modulated (due to the inter-plane frustration of interactions) along the 3-fold c axis with a $\approx$ 2$\pi$/7 period. Additional neutron polarimetry experiments revealed that a single helicity is selected and that, in turn, this selects a single chirality, in agreement with group representation analysis. A mean field analysis shows that this unique feature is due to the twist in the spin-spin exchange path from plane to plane. [Preview Abstract] |
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