Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Y29: Focus Session: Spin Liquids |
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Sponsoring Units: GMAG Chair: Tsuyoshi Kimura, Osaka University Room: 333 |
Friday, March 20, 2009 8:00AM - 8:12AM |
Y29.00001: Topological pairing of skyrmions and symmetry breaking in low-dimensional SU(N) antiferromagnets Alexei Kolezhuk I study what happens to a so-called SU(N) antiferromagnet when the high SU(N) symmetry getss explicitly broken. Physically, such SU(N) antiferromagnets can be realized in cold atom systems in optical lattices (particularly, the N=3 case corresponds to spin-1 bosons), and similar models might be possibly relevant for some magnetic materials like $\rm NiGa_2S_4$. I consider two perturbations breaking the SU(N) symmetry down to O(N) and SU(N-1), respectively, and study the phase diagram of the system. Breaking symmetry has a twofold effect: except favoring a certain type of order (spin-nematic or antiferromagnetic), it also affects the topological (Berry) phases. It is shown that the physically interesting case N=3 is very special: the effect of ``topological pairing'' of skyrmions leads to a change in the degeneracy of the disordered phase in case of SU(N) to O(N) perturbation, and the SU(N) to SU(N-1) perturbation brings the system into a critical phase. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y29.00002: Cascade of quantum phase transitions in the spin-1/2 triangular-lattice antiferromagnet Cs$_{2}$CuBr$_{4}$ Nathanael Fortune, Adrienne Wilson-Muenchow, Scott Hannahs , Yasu Takano, Yasuo Yoshida, Todd Sherline, Toshio Ono, Hidekazu Tanaka In classical magnetic spin systems, geometric frustration leads to a large number of states of identical energy. We report here calorimetric and magnetocaloric evidence that in Cs$_{2}$CuBr$_{4}$ --- a geometrically frustrated Heisenberg S= 1/2 triangular antiferromagnet --- quantum fluctuations stabilize a series of gapped collinear spin states bounded by first-order transitions at simple increasing fractions of the saturation magnetization for fields directed along the c axis. Only the first of these quantum phase transitions has been theoretically predicted. We discuss how the higher fraction quantum states might arise and propose model spin arrangements. The Dzyaloshinskii-Moriya interaction breaks the symmetry when the magnetic field is directed along the triangular layers, providing one possible explanation for the directional dependence and the 1st order nature of the transitions. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y29.00003: Dzyaloshinskii-Moriya interactions and non-magnetic impurities in the s=1/2 kagome antiferromagnet Ioannis Rousochatzakis, Salvatore Manmana, Andreas Laeuchli, Bruce Normand, Frederic Mila Motivated by recent NMR experiments[1] on ZnCu$_3$(OH)$_6$Cl$_2$, we present an exact diagonalization study of the combined effect of non-magnetic impurities and Dzyaloshinskii-Moriya (DM) interactions in the $s=1/2$ Kagom\'e antiferromagnet. The magnetization response and the correlation matrix data reveal that the dimer freezing which occurs around the vacancy for $D=0$ [2,3] ($D$ is the magnitude of the DM vectors) persists up to $D/J \simeq 0.07$, above which a phase transition to the $Q=0$ semiclassical 120$^{\circ}$ state[4] takes place. Surprisingly however, the dimers next to the vacancy remain strong up to $D/J \sim 2-3$, i.e. well above the critical point. Implications for ZnCu$_3$(OH)$_6$Cl$_2$ will be discussed. 1. A. Olariu, {\it et al.}, Phys. Rev. Lett. {\bf 100}, 087202 (2008). 2. S. Dommange, {\it et al.}, Phys. Rev. B {\bf 68}, 224416 (2003). 3. A. L\"auchli, {\it et al.}, Phys. Rev. B {\bf 76}, 144113 (2007). 4. O. C\'epas, {\it et al.}, Phys. Rev. B {\bf 78}, 140405 (2008). [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y29.00004: Neel ordering and the stability of the spin 1/2 kagome lattice antiferromagnet in Zn-Paratacamite Michael J. Lawler, Erik S. Sorensen, Yong Baek Kim Zn-Paratacamite is a rare spin 1/2 antiferromagnetic insulator with an ideal kagome lattice structure in part of its phase diagram. As a function of Zn doping, this material undergoes a structural distortion which relieves the frustration and introduces magnetic order in the ground state, though the precise nature of the order is not clear at this point. In this talk, I will present strong evidence for Neel ordering in the distorted phase of Zn-Paratacamite through the application of quantum monte-carlo and exact diagonalization methods to the appropriate Heisenberg model. These numerical results strongly support a recent Schwinger-boson mean field theory of Zn-Paratacamite. Furthermore, our results indicate a large basin of stability of the ideal kagome lattice ground state in the presence of this type of distortion. This suggests that the ideal kagome ground state may be stable towards weak doping-induced distortion, though further studies of local effects may be necessary to make a firm conclusion. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y29.00005: Magnetization steps on the spin liquid ground state of the $S $= 1/2 kagome-like antiferromagnet Cu$_{3}$V$_{2}$O$_{7}$(OH)$_{2}$2H$_{2}$O Hiroyuki Yoshida, Yoshihiko Okamoto, Takashi Tayama, Toshiro Sakakibara, Masashi Tokunaga, Akira Matsuo, Yasuo Narumi, Koichi Kindo, Makoto Yoshida, Masashi Takigawa, Zenji Hiroi The ground state of the $S$ = 1/2 kagome antiferromagnet (KAFM) is expected to be a spin liquid with a finite spin gap \textit{$\Delta $ }$\sim \quad J$ / 20. Here, we report the magnetic properties of $S$ = 1/2 KAFM Cu$_{3}$V$_{2}$O$_{7}$(OH)$_{2}$$\cdot$2H$_{2}$O studied by magnetization, specific heat and V NMR measurements. Neither magnetic long- range order nor a spin gap has been detected down to 60 mK, in spite of a large antiferromagnetic interaction $J$ = 86 K, suggesting a gapless spin liquid. Surprisingly, we observed three step-like increases in magnetization at $H_{S1}$ = 4.3, $H_{S2}$ = 25.5, and $H_{S3}$ = 46 T, which implies that there exist at least four kinds of spin liquid or other quantum state under magnetic fields. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y29.00006: $^{69,71}$Ga NMR Probe of the Spin Dynamics in the Rare-Earth Kagom\'{e} Pr$_3$Ga$_5$SiO$_{14}$ Lloyd Lumata, K.-Y. Choi, T. Besara, M. J. R. Hoch, H. D. Zhou, J. S. Brooks, P. L. Kuhns, A. P. Reyes, N. S. Dalal, C. R. Wiebe We report $^{69,71}$Ga nuclear magnetic resonance investigation of the spin dynamics in the rare-earth kagom\'{e} system Pr$_3$Ga$_5$SiO$_{14}$. We find that the spin-lattice relaxation rate $^{69}1/T_1$ exhibits a peak around 30 K, below which the Pr$^{3+}$ spin correlation time $\tau$ shows novel field-dependent behavior consistent with a field-dependent gap in the excitation spectrum. The spin-spin relaxation rate $^{69}1/T_{2}$ exhibits a maximum at a lower temperature (10 K) below which field-dependent power-law behavior close to $T^{2}$ is observed. These results point to the interplay of single-ion anisotropy and field-induced formation of nanoscale magnetic clusters consistent with recent neutron scattering measurements. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:48AM |
Y29.00007: Ba$_3$Cr$_2$O$_8$, a new non-Cu based quantum s=1/2 spin singlet system Invited Speaker: Field-induced condensation of magnons has been experimentally observed in several weakly coupled quantum ($s = 1/2$) dimer systems that are based on Cu$^{2+}$ ions, such as TlCuCl$_3$ and BaCuSi$_2$O$_6$, and it has been adequately described by the Bose-Einstein condensation (BEC) theory. However, the robustness of such descriptions can only be truly evaluated with investigation into complementary materials, in particular materials that are based on non-Cu$^{2+}$ ions. Recently, a new spin dimer system, Ba3Cr2O8 has been found, where Cr$^{5+}$ ($s=1/2$) ion with the unusual 5+ electronic valence forms quantum dimers along the $c$-axis and a frustrating triangular lattice in the $ab$-plane. Using elastic and inelastic neutron scattering measurements on single crystals and a powder sample, we have characterized the magnetic interactions to show that Ba$_3$Cr$_2$O$_8$ is indeed an excellent model system of weakly coupled quantum dimers [1]. We have also investigated the field-induced condensation of magnons in this compound, using specific heat, bulk magnetization, and elastic neutron scattering measurements under an external magnetic field. The experimental results and comparison to the theories will be discussed. *This work is in collaboration with J.-H. Kim, S. Ji, S.-H. Lee (University of Virginia), H. Ueda, Y. Ueda (ISSP, University of Tokyo), H. Nojiri (IMR, Tohoku University), B. Lake, K. Rule (Helmholtz Centre Berlin). \\[4pt] [1] M. Kofu {\it et al}., cond-mat/0809.5069 (2008). [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y29.00008: BEC of triplon in the complex quantum spin liquid BaCuSi$_2$O$_6$ Raivo Stern, Steffen Kraemer, Mladen Horvatic, Ivo Heinmaa, Enno Joon, Claude Berthier, Tsuyoshi Kimura, Joel Mesot We present a $^{63,65}$Cu and $^{29}$Si NMR study of the quasi-2D coupled spin 1/2 dimer compound BaCuSi$_2$O$_6$ in the magnetic field range 13-26 T and at temperatures as low as 50~mK. NMR data and neutron scattering experiments in the gapped phase reveal that below 90~K different intra-dimer exchange couplings and different gaps ($\Delta_{\rm{B}}/\Delta_{\rm{A}}$ = 1.16) exist in every second plane along the $c$-axis, in addition to a planar incommensurate (IC) modulation. $^{29}$Si spectra in the field induced magnetic ordered phase reveal that close to the quantum critical point at $H_{\rm{c1}}$ = 23.35 T the average boson density $\overline{n}$ of the Bose-Einstein condensate (BEC) is strongly modulated along the $c$-axis with a density ratio for every second plane $\overline{n}_{\rm{A}}/\overline{n}_{\rm{B}} \simeq 5$. An IC modulation of the local density is also present in each plane. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y29.00009: High-Frequency EPR Studies of the Antiferromagnet Spin Dimer Compound Ba$_{3}$Mn$_{2}$O$_{8}$ Changhyun Koo, Stephen Hill, Eric C. Samulon, Ian R. Fisher Ba$_{3}$Mn$_{2}$O$_{8}$ is a triangular lattice antiferromagnetic spin-dimer system based on pairs of spin $S$ = 1 Mn$^{5+}$ ions. In zero-field, Ba$_{3}$Mn$_{2}$O$_{8}$ possesses a singlet ground state. Meanwhile, application of a large magnetic field induces several ordered phases associated with the closing of gaps to the excited triplet and quintet states. Field orientation dependent high frequency electron paramagnetic resonance (HFEPR) measurements for a single crystal of Ba$_{3}$Mn$_{2}$O$_{8}$ have been carried out in fields up to 45~T using the hybrid magnet at the National High Magnetic Field Laboratory. Broad low-frequency transitions are observed in the field ranges close to $\sim $9~T and $\sim $32~T, suggesting inter-spin multiplet excitations. Sharper resonances are observed at higher frequencies, which persist to 42~T. Meanwhile, low-field studies provide information on the magnetic anisotropy of the system, which is of the easy-plane type with $D$ = +0.375 K. We attempt to account for the observed HFEPR spectra in the context of a model recently developed to explain the magnetic properties of this compound [E. C. Samulon et al., Phys. Rev. B \textbf{77}, 214441 (2008)]. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y29.00010: $^{135,137}$Ba NMR study of condensed phase and critical behavior in \textit{S}=1 spin dimer system Ba$_{3}$Mn$_{2}$O$_{8}$ Steve Suh, W.G. Clark, S.E. Brown, E.C. Samulon, I.R. Fisher, C.D. Batista, A.P. Reyes, P. Kuhns, L.L. Lumata, J.S. Brooks Ba$_{3}$Mn$_{2}$O$_{8}$ is a trigonal $S=$1 spin dimer system, in which we have used $^{135,137}$Ba NMR spectroscopy and relaxation to study the static and dynamic properties of the field-induced phases and phase transitions to temperatures as low as 20mK, and with emphasis on magnetic fields up to 12T. Specifically, we report on the nature and variation of the order parameter in the condensed phases as well as the form of the spin lattice relaxation in the condensed phases, and compare to behavior anticipated for BEC and 3DXY universality classes. The temperature dependence of magnetization and the relaxation rate at the quantum critical point at H$_{c}\sim $9T is also reported. For the case of the magnetization, a 3D to 2D crossover is evident, and shown consistent with calculations based on the magnon dispersion relations (M.B. Stone et. al, PRL 100, 237201 (2008)). [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y29.00011: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y29.00012: The symmetry of the spin Hamiltonian in herbertsmithite, a spin-1/2 kagom\'e lattice Oren Ofer, Amit Keren We present magnetization measurements on oriented powder of ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ along and perpendicular to the orienting field. We find a dramatic difference in the magnetization between the two directions. It is biggest at low measurement fields $H$ or high temperatures. We show that the difference at high temperatures must emerge from Ising-like exchange anisotropy. This allows us to explain muon spin rotation data at $T\rightarrow 0$ in terms of an exotic ferromagnetic ground state. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y29.00013: Magnetic Excitations in the Stacked Quantum Magnets NaNiO$_2$ and LiNiO$_2$ J.P. Clancy, B.D. Gaulin, J.P.C. Ruff, K.A. Ross, G.J. Van Gastel, D.L. Abernathy, M.B. Stone NaNiO$_2$ and LiNiO$_2$ are isostructural stacked triangular lattice quantum magnets, in which magnetism is conventionally thought to arise due to spin 1/2 moments carried by Ni$^{3+}$ ions. Surprisingly, while NaNiO$_2$ undergoes a cooperative Jahn-Teller transition at 480K and magnetically orders below T$_N$ $\sim$ 23K, LiNiO$_2$ undergoes a glass transition at T$_g$ $\sim$ 9K and remains disordered down to the lowest measured temperatures. The absence of long-range magnetic order in LiNiO$_2$ has been attributed to either geometric frustration caused by mixing of the Li and Ni sublattices, or orbital degeneracy due to the absence of a coherent Jahn-Teller distortion. We have performed time of flight neutron scattering measurements on polycrystalline samples of NaNiO$_2$ and LiNiO$_2$ using the wide Angular-Range Chopper Spectrometer (ARCS) at the SNS. Our measurements reveal previously unobserved magnetic excitations at relatively high energy transfers, which we associate with ferromagnetic spin waves mediated by in-plane interactions. We also find evidence of critical scattering in NaNiO$_2$ near the magnetic phase transition at T$_N$. These results will be compared with previous measurements collected using the DCS at NIST. [Preview Abstract] |
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