Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session U23: Frustration in 2D |
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Sponsoring Units: GMAG Chair: Bruce Gaulin, McMaster University Room: Baltimore Convention Center 320 |
Thursday, March 16, 2006 8:00AM - 8:12AM |
U23.00001: Spin Frustration, Magnetic Susceptibility and Excitations in Spatially Anisotropic Triangular-Lattice Antiferromagnets Rajiv Singh, Weihong Zheng, John Fjaerestad, Ross McKenzie, Radu Coldea We calculate the temperature dependent magnetic susceptibility and excitation spectra for the spatially anisotropic triangular- lattice Heisenberg model. We show that suitably scaled plots of magnetic susceptibility provide a direct measure of frustration in the system and allow one to infer the exchange parameters from the susceptibility data. We find that the organic material $\kappa-(BEDT-TTF)_2Cu_2(CN)_3$ is very well described by the isotropic triangular lattice model. We also find that the excitation spectra of the model shows various anomalies in the Neel and Spiral phases. [Preview Abstract] |
Thursday, March 16, 2006 8:12AM - 8:24AM |
U23.00002: Anomalous renormalization of excitation spectra of the triangular-lattice antiferromagnet John Fjaerestad, Weihong Zheng, Rajiv Singh, Ross McKenzie, Radu Coldea We use series expansions to calculate the excitation spectra of the spin-1/2 triangular-lattice Heisenberg antiferromagnet above the 3-sublattice ordered phase. We find that the spectra are renormalized downwards with respect to linear spin-wave theory. This is in sharp contrast to the square-lattice antiferromagnet, where the spectral frequencies are renormalized upwards due to quantum fluctuations. The triangular-lattice spectra show sharp downward renormalization at special wavevectors, which (a) can be interpreted as evidence of high-energy spinons and (b) provides an explanation for rapid loss of antiferromagnetic correlations with temperature, consistent with earlier high-temperature series expansion studies. [Preview Abstract] |
Thursday, March 16, 2006 8:24AM - 8:36AM |
U23.00003: Pairing of doped holes in the Shastry-Sutherland lattice Jun Liu, Joerg Schmalian, Nandini Trivedi We investigate the magnetic and pairing correlations in a hole doped Shastry-Sutherland lattice. We use a variational wave function that reproduces the valence bond ground state of the un-doped material, relevant to SrCu2(BO3)2. For finite doping we study the disappearance of the valence bond crystal and the emergence of pairing of doped holes. [Preview Abstract] |
Thursday, March 16, 2006 8:36AM - 8:48AM |
U23.00004: In Gap Excitations and Triplet Lifetime Broadening in the Dilute Singlet Ground State System SrCu$_{2-x}$Mg$_{x}$(BO$_{3})_{2}$ S. Haravifard, S.R. Dunsiger, B.D. Gaulin, H.A. Dabkowska, M.T.F. Telling, S. El Shawish, J. Bonca We have carried out high resolution time-of-flight neutron scattering measurements on a new high quality single crystal of SrCu$_{2-x}$Mg$_{x}$(BO$_{3})_{2}$ with $x$ = 0.1. These studies revealed the presence of new excitations within the singlet-triplet gap of this quasi-two dimensional, dilute, singlet ground state system. These new excitations showed little or no shift in energy with increasing applied magnetic field. In addition, we observe substantial broadening of the three triplet excitations in the dilute single crystal, as compared with pure SrCu$_{2}$(BO$_{3})_{2}$.$^{1}$ The triplet excitations in doped SrCu$_{2-x}$Mg$_{x}$(BO$_{3})_{2}$ therefore possess finite lifetimes at low temperatures in the range that can be measured with cold neutron spectroscopy. We have also calculated the dynamical spin structure factor using the zero temperature Lanczos method, and solving a Shastry-Sutherland model at zero and finite doping for different strengths of external magnetic field. This theory reproduces all the qualitative features observed in the experiments on SrCu$_{2-x}$Mg$_{x}$(BO$_{3})_{2}$. $^{1}$ B.D. Gaulin \textit{et al.}, Phys. Rev. Lett., \textbf{93}, 267202, 2004. [Preview Abstract] |
Thursday, March 16, 2006 8:48AM - 9:00AM |
U23.00005: Infrared studies of a quantum magnet SrCu$_2$(BO$_3$)$_2$ T. R{\~o}{\~o}m, D. H{\"u}vonen, U. Nagel, S.V. Dordevic, C.C. Homes, A. Gozar, G. Blumberg, N. Drichko, M.M. Dressel, H. Kageyama We will report results of our infrared studies of SrCu$_2$(BO$_3 $)$_2$, a two-dimensional spin system with a disordered ground state even at very low temperatures, and a spin gap of about 24 cm$^{-1}$ (3 meV). This material has recently attracted attention because of a possibility that doping may lead to a superconductivity mediated by antiferromagnetic fluctuations, possibly similar to high-T$_c$ cuprates. Using polarized light we have probed both crystallographic directions over a broad range of frequencies (from about 30 cm$^{-1}$ to 20,000 cm$^{-1} $) and temperatures (from 4.2 K to 300 K). The results reveal significant differences between the ab-plane and c-axis directions. We will discuss these findings in relation with the resonance effects observed in inelastic light scattering experiments from collective magnetic excitations. [Preview Abstract] |
Thursday, March 16, 2006 9:00AM - 9:12AM |
U23.00006: Magnetization plateaus for Cs$_{2}$CuBr$_{4}$ Shin Miyahara, Kiyoshi Ogino, Masahiro Shimizu, Nobuo Furukawa Cs$_{2}$CuBr$_{4}$ is a new two-dimensional spin-1/2 system, where 1/3- and 2/3-plateaus have been observed in external magnetic fields. The magnetic behaviors of the material are well explained by a two-dimensional antiferromagnetic Heisenberg model on a distorted triangular lattice. In the model, there are two types of interactions $J_{1}$ and $J_{2}$, where $J_{1}$ chains are coupled with inter chain interactions $J_{2}$. Using an exact diagonalization method, we investigated magnetic properties, especially magnetization curve. In the magnetization, 1/3-plateau appears for 0.7 $\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle\sim}\vphantom{_x}}$}} J_{2 }$/$J_{1 }\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle\sim}\vphantom{_x}}$}} $1.3. At the plateau, three-fold degenerate ground state, up-up-down structure, is realized. Our results indicate that the material has a more frustrated character $J_{2 }$/$J_{1 }\approx $0.7 than what has been expected from a classical theory $J_{2 }$/$J_{1 }$= 0.47. The magnetic properties at 2/3-plateau will also be discussed. [Preview Abstract] |
Thursday, March 16, 2006 9:12AM - 9:24AM |
U23.00007: Dynamical properties of the anisotropic triangular quantum antiferromagnet with Dzyaloshinskii-Moriya interaction Rastko Sknepnek, Denis Dalidovich, John Berlinsky, Junhua Zhang, Catherine Kallin We present a detailed study of the anisotropic triangular quantum antiferromagent with Dzyaloshinskii-Moriya (DM) interaction building on earlier work by Veillette, James and Essler [Phys.\ Rev.\ B, {\bf 72}, 134429 (2005)]. The DM interaction generates an easy-plane anisotropy and opens a gap in the spin-wave spectrum at the incommensurate ordering wave vector $\vec Q$. Our calculation utilizes the Holstein-Primakoff representation of spins and goes beyond linear spin wave theory by taking into account magnon-magnon interactions in a $1/S$ expansion. We calculate renormalized dispersion relations for the magnons to order $1/S$ for different values of the DM interaction and pay particular attention to the interesting case of zero DM interaction. The dynamical structure factor is calculated to order $1/S$. It is found that, compared to linear spin wave theory, a significant fraction of the scattering intensity is shifted to higher energies. We compare our findings with the recent neutron scattering data measured on the frustrated quantum antiferromagnet $Cs_2 Cu Cl_4$, [R. Coldea, {\it et. al.}, Phys.\ Rev.\ B, {\bf 68}, 134424, (2003)]. [Preview Abstract] |
Thursday, March 16, 2006 9:24AM - 9:36AM |
U23.00008: Spiral magnetic order on an anisotropic triangular lattice in the presence of Dzyaloshinskii-Moriya interaction Denis Dalidovich, Rastko Sknepnek, A. John Berlinsky, Junhua Zhang, Catherine Kallin We consider the ground state energy, magnetization and the energy spectrum of the two-dimensional antiferromagnets on the triangular lattice in the presence of anisotropy in the exchange couplings, and Dzyaloshinskii-Moriya (DM) interaction $D$. $J$ and $J^{\prime}$ are meant to be the couplings along the chain direction and zigzag bonds respectively. Assuming that in the wide range of those parameters the system has the spiral Neel ordering, we consider the role of quantum fluctuations within the framework of a standard $1/S$ expansion. We show that DM interaction considerably suppresses fluctuations and seems to play an important role in stabilizing the spiral Neel ordering in this frustrated system. We discuss our theoretical results within the context of the recent experimental measurements in the frustrated quantum magnet $Cs_2 Cu Cl_4$, [R. Coldea, et. al., Phys. Rev. B, {\bf 68}, 134424, (2003)], and the recent theoretical structure factor calculations by M. Y. Veillette, et. al., Phys. Rev. B {\bf 72}, 134429 (2005) [Preview Abstract] |
Thursday, March 16, 2006 9:36AM - 9:48AM |
U23.00009: A quasi-1d approach to the triangular antiferromagnet Cs$_2$CuCl$_4$ Leon Balents, Oleg Starykh We discuss the low-temperature properties of a spin-1/2 triangular Heisenberg antiferromagnet in a field, including the effects of Dyaloshinskii-Moriya interactions, as believed appropriate to Cs$_2$CuCl$_4$\footnote{R. Coldea et al., Phys. Rev. Lett. 88, 137203 (2002).}. Our treatment is based upon a view of the problem as a system of weakly-coupled spin chains. Analytic results are obtained through a combination of renormalization group, chain mean-field, and bosonization methods. Using various exactly-known properties of the individual Heisenberg chains, we calculate numerous physical properties, including various phase boundaries in longitudinal and transverse fields. We compare our results to published experimental data. [Preview Abstract] |
Thursday, March 16, 2006 9:48AM - 10:00AM |
U23.00010: Simultaneous breaking of lattice symmetry and spin frustration in triangular lattice antiferromagnet $\rm CuFeO_2$ Y. Ren, F. Ye, Q. Huang, J.A. Fernandez-Baca, Pengcheng Dai, J.W. Lynn, T. Kimura We use high resolution synchrotron X-ray and neutron diffraction to study the geometrically frustrated triangular lattice antiferromagnet (TLA) $\rm CuFeO_2$. We show that the occurrence of the two magnetic transitions, at $14$~K and $11$~K, respectively is accompanied simultaneously by a second-and first- order structural phase transitions from a hexagonal structure to a monoclinic form. This is the first observation of two successive spin-driven structural transitions directly coupled with incommensurate and commensurate magnetic orderings in frustrated TLA systems. \newline \newline This work is supported by the U. S. NSF DMR-0453804 and DOE Nos. DE-FG02-05ER46202 and DE-AC05-00OR22725 with UT/Battelle LLC. Use of the Advanced Photon Source was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. W-31-109-Eng-38. [Preview Abstract] |
Thursday, March 16, 2006 10:00AM - 10:12AM |
U23.00011: Neutron scattering studies on Kagome bilayers -- Spin fluctuations in a broad dynamic range H. Mutka, C. Payen, G. Ehlers, J.R. Stewart, A. Mellergard The kagome bilayer compounds Ba2Sn2ZnCr7xGa10-7xO22 and SrCr9xGa12-9xO19 are localized spin (S=3/2) systems with strong antiferromagnetic exchange (J/k $\approx $ 50 K) that show no long-range order down to mK temperatures. This remarkable behaviour originates from the geometrical frustration. We have examined these systems using polarized neutron diffraction, inelastic neutron scattering (INS) and neutron spin-echo (NSE) spectroscopy for characterizing the magnetic correlations in a very broad energy range. INS data reveals the dynamic nature of the broad diffuse scattering that characterises the quasi-elastic magnetic response above the spin freezing. New aspects on the freezing phenomenon and the ground state properties are revealed using NSE. [Preview Abstract] |
Thursday, March 16, 2006 10:12AM - 10:24AM |
U23.00012: Hall constant on triangular lattice ladder systems Jan Haerter, Michael Peterson, Sriram Shastry To make the transition from a one-dimensional to a planar system one can connect $n$ 1D-chains to yield $n$-leg ladders. For square lattices, this approach has already led to quite interesting and unexpected results (Dagotto et al, Prelov\v sek et al). In this work, frustration is introduced through triangular geometry, achieved by adding additional bonds to the square lattice ladders. We study the response of such $t$-$J$ ladders to an applied magnetic field and investigate the Hall constant $R_H$ as function of temperature, interaction strength, doping, and frequency. These systems complement toroidal or spherical systems since these allow the limit $\partial/\partial B$ to be taken more accurately which leaves the geometric frustration fully preserved. We investigate the crossover from a degenerate Fermi system to a high T regime where $R_H$ grows indefinitely ($R_H\propto T$) with temperature. [Preview Abstract] |
Thursday, March 16, 2006 10:24AM - 10:36AM |
U23.00013: Position-Space Renormalization-Group Treatment of the Triangular Ising Antiferromagnet with Quenched Disorder Michael E. Mihalco, Susan R. McKay We apply the Niemeijer-van Leeuwen cluster approximation [1] to frustrated Ising models on a triangular lattice. The homogeneous Ising antiferromagnet is fully frustrated and shows no ordered phase. Frustration can be relieved via the addition of quenched randomness through either dilution or the introduction of ferromagnetic bonds. The result is a rich phase diagram with different types of ordering depending upon the details of the quenched disorder. [2] Using a binning procedure to retain the full distribution of interactions under rescaling [3], we are able to calculate the phase diagram of this system, with each phase having its own characteristic attractor. This model system provides a two-dimensional example of the impacts of tunable frustration on short- and long-range order. 1. T. Niemeijer and J.M.J. van Leeuwen, Phys. Rev. Lett. \textbf{31}, 1411 (1973); Physica (Utr.) \textbf{71}, 17 (1974). 2. G. S. Grest and E.G. Gabl, Phys. Rev. Lett. \textbf{43}, 1183 (1979); H. Kaya and A.N. Berker, Phys. Rev. E \textbf{62}, 1469 (2000).; M. Robinson, M.S. Thesis, University of Maine (2003). 3. E. Hartford and S. McKay, J. Appl. Phys. \textbf{70}, 6068 (1991); E. Hartford, Ph.D. Thesis, University of Maine (1994); A. Falicov, A.N. Berker, and S.R. McKay, Phys. Rev. B \textbf{51}, 8266 (1995). [Preview Abstract] |
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