Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session Y27: Focus Session: Kagome Magnets |
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Sponsoring Units: GMAG Chair: Ying Chen, National Institute of Standards and Technology Room: Morial Convention Center 219 |
Friday, March 14, 2008 11:15AM - 11:27AM |
Y27.00001: Magnetic Diffuse Scattering in the Frustrated Kagome Antiferromagnet YBaCo$_{4}$O$_{7}$ Pascal Manuel, Laurent Chapon, Paolo Radaelli, John Mitchell, Hong Zheng Cobalt oxides of composition RBaCo$_{4}$O$_{7}$ (R=Y, Tb-Lu) crystallize with a lattice structure topologically related to that of the pyrochlore. Considering only the magnetic transition metal sublattice, R-114 appears as Kagome sheets linked by triangular layers and is therefore expected to provide a new materials class for exploring geometric magnetic frustration. We have recently shown that stoichiometric R-114 compound orders antiferromagnetically into a long-range ordered (LRO) structure with features common to the $\sqrt 3 \ast \sqrt 3$ negative chirality spin arrangements often found in Kagome net systems. In contrast, small excesses of O added to the system, as little as 0.1, destroys this LRO state. To explore the nature of the frustrated magnetism in this novel system, we have measured magnetic diffuse scattering on YBaCo$_{4}$O$_{7}$ and YBaCo$_{4}$O$_{7.1}$ single crystals at the ISIS facility. Large maps of reciprocal space in several planes have been recorded showing a very structured diffuse scattering. The data compared to models obtained by the Monte-Carlo method using the metropolis algorithm, reveal the exact nature of the disordered ground state in this new class of frustrated magnets. [Preview Abstract] |
Friday, March 14, 2008 11:27AM - 11:39AM |
Y27.00002: $^{63}$Cu, $^{35}$Cl, and $^{1}$H NMR in the S=1/2 Kagom\'{e} Lattice ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ Takashi Imai, E.A. Nytkoc, B.M. Bartlett, M.P. Shores, D.G. Nocera ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ (S=1/2) is a promising new candidate for an ideal Kagom\'{e} Heisenberg antiferromagnet, because there is no magnetic phase transition down to $\sim $50 mK. We investigated its local magnetic and lattice environments with NMR techniques (ArXiv:cond-mat/0703141). From $^{35}$Cl Knight shift data, we demonstrate that the intrinsic spin susceptibility follows a Curie-Weiss law down to $\sim $0.2J, then decreases toward T = 0. Comparison of $^{1}$H and $^{35}$Cl spin-lattice relaxation rate 1/T$_{1}$ evidences for slow freezing of the lattice near $\sim $50 K, presumably associated with OH bonds. Spin dynamics near T = 0 obey a power-law behavior in the presence of high magnetic fields. [Preview Abstract] |
Friday, March 14, 2008 11:39AM - 11:51AM |
Y27.00003: Theory of Collinear ordering on the Kagome Lattice of Zn-paratacamite Lars Fritz, Michael Lawler, Yong-Baek Kim, Subir Sachdev We present a theory of the collinearly ordered phase discovered in a recent neutron scattering experiment [1] on Zn-paratacamite, Zn$_x$Cu$_ {4-x}$(OH)$_6$Cl$_2$, at small $x$. Zn-paratacamite has been considered as an excellent model system for the kagome lattice of spin-1/2 moments with antiferromagnetic exchange interactions. We studied both the classical and quantum Heisenberg models on the distorted kagome lattice appropriate for Zn-paratacamite with small x. Our theory naturally explains the emergence of a N\'eel phase. A theory for the Valence-Bond Solid phase also discovered in this experiment is presented in a related contributed talk.\newline \newline [1] S.-H. Lee et al., Nature Materials, 6, 853 [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y27.00004: Theory of the Valence-Bond-Solid Phase on the Kagome Lattice of Zn-paratacamite Michael Lawler, Lars Fritz, Yong Baek Kim, Subir Sachdev We present a theory of the valence-bond-solid phase discovered in a recent neutron scattering experiment [1] on Zn-paratacamite, Zn$_x$Cu$_{4-x}$(OH)$_6$Cl$_2$, at small $x$. A theory of the Neel phase also discovered in this experiment will be presented in a related contributed talk. Zn-paratacamite has been considered as an excellent model system for the kagome lattice of spin-1/2 moments with antiferromagnetic exchange interactions. We study both the classical and quantum Heisenberg models on the distorted kagome lattice appropriate for Zn-paratacamite with small x. Our theory naturally explains the emergence of the valence bond solid phase. We suggest future inelastic neutron and elastic X-ray scattering experiments that can test our predictions. [1] S.-H. Lee et al., Nature Materials, 6, 853 [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y27.00005: New natural spin-1/2 kagom\'{e} systems --- kapellasite Cu$_3$Zn(OH)$_6$Cl$_2$ and haydeeite Cu$_3$Mg(OH)$_6$Cl$_2$ Oleg Janson, Helge Rosner New natural spin-1/2 systems with kagom\'{e} layers --- kapellasite Cu$_3$Zn(OH)$_6$Cl$_2$ and haydeeite Cu$_3$Mg(OH)$_6$Cl$_2$ --- are studied by full potential density functional calculations using the fplo6.00-24 code. The band structure, obtained by a paramagnetic calculation, was used to solve a tight-binding model. The transfer integrals were mapped subsequently to a Hubbard model and to a Heisenberg model, giving an estimate for the antiferromagnetic (AF) exchange. The total exchange, containing AF and ferromagnetic (FM) parts, was derived from LSDA + \textsl{U} supercell calculations. As the main result, we find that in both compounds only two exchange integrals are relevant: the nearest neighbour exchange \textsl{J}$_1$ and the interaction \textsl{J}$_{\mathrm{d}}$ along the diagonals of the Cu$^{2+}$ hexagons. Surprisingly, the size of these integrals depends strongly on the O---H bond length which was therefor optimized with respect to the total energy, resulting in about 1 \AA for both compounds. Using the optimized O---H bond length, we find \textsl{J}$_{1}>\sl{J}_{\mathrm{d}}$ in kapellasite and \textsl{J}$_{1}\sim\textsl{J}_{\mathrm{d}}$ in haydeeite. According to our results, kapellasite can be described as a modified kagom\'{e} lattice, while interpenetrating chains should be considered for haydeeite. Our results should encourage new experimental studies of these interesting materials. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y27.00006: Classical antiferromagnet on a hyper-kagome lattice John Hopkinson, Sergei Isakov, Hae-Young Kee, Yong Baek Kim Motivated by recent experiments on Na$_4$Ir$_3$O$_8$ [Y. Okamoto {\it{et al.}}, Phys. Rev. Lett. 99, 167402 (2007)], we study the classical antiferromagnet on a frustrated three-dimensional lattice obtained by selectively removing one of four sites in each tetrahedron of the pyrochlore lattice. This ``hyper-kagome'' lattice consists of corner-sharing triangles. We present (J. Hopkinson {\it{et al.}}, Phys. Rev. Lett. 99, 037201 (2007)) the results of large-$N$ mean field theory and Monte Carlo computations on $O(N)$ classical spin models. We find the classical ground states to be highly degenerate. Nonetheless, at low temperatures, nematic order emerges via ``order by disorder'' in the Heisenberg model ($N$=3), representing the dominance of coplanar spin configurations. Above this transition, the spin-spin correlations show a dipolar form which can be understood to arise from a generalized ``Gauss' law'' constraint. The relevance of these results to ongoing neutron scattering measurements will be discussed. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y27.00007: Tuning of the spin liquid ground state in the kagome system Pr$_3$Ga$_5$XO$_{14}$ C. R. Wiebe, H. D. Zhou, Y.-J. Jo, M. A. Castellano, L. Balicas, M. J. Case, Y. Qiu, J. R. D. Copley, V. Ramachandran, N. S. Dalal, J. S. Gardner We report on the single crystal growth of the series of kagome oxides Pr$_3$Ga$_5$XO$_{14}$ (X = Si, Ti, Ge, and Sn). The material Pr$_3$Ga$_5$SiO$_{14}$ has near neighbor antiferromagnetic interactions between the Pr spins ($\theta$ = -2.3 K), but there is no long range order down to 0.035 mK (f $\sim$ 66). The presence of 2D low energy spin excitations results in a strong T$^2$ component to the specific heat typical of other kagome systems such as SCGO. By tuning the size of the lattice through substitution on the Si site, one can adjust the exchange between the spins in a regular fashion. Our data shows a systematic decrease in the amplitude of the T$^2$ component of the specific heat as the magnetic exchange becomes weaker through Ti and Ge substitution. In the case of Sn doping, the system orders as the dipolar interactions dominate over the weak antiferromagnetic exchange. To our knowledge, this is the first example of a tunable spin liquid kagome system. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y27.00008: Thermodynamics of Ising Spins on the Triangular Kagome Lattice Yen Lee Loh, Daoxin Yao, Erica W. Carlson In the compounds $\mbox{Cu}_{9}\mbox{X}_2(\mbox{cpa})_{6}\cdot x\mbox{H}_2\mbox{O}$ (cpa=2-carboxypentonic acid; X=F,Cl,Br), the Cu spins form a fascinating and unique pattern called a triangular kagome lattice (TKL). We present a detailed study of Ising spins on such a lattice using exact methods and Monte Carlo simulation. We calculate the free energy, internal energy, specific heat, entropy, sublattice magnetizations, and susceptibility, and we find a rich phase diagram as a function of coupling constants, temperature, and applied magnetic field. In the frustrated regime at $T=0$, the system effectively decouples into independent degrees of freedom, giving residual entropy $s_0=\frac{1}{9} \ln 72$ per spin and correlation length $\xi=0$ -- an interesting contrast with the triangular and kagome lattice Ising models. Applying a field induces a critical phase (related to the honeycomb lattice dimer model) that has irrational entropy $0.0359$ per spin and $1/r^2$ correlations that should be detectable by neutron scattering. [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y27.00009: Spin Triplet Excitations of the Heisenberg Antiferromagnet on the Kagome Lattice Kwon Park, Bohm-Jung Yang, Yong Baek Kim, Jaejun Yu The Kagome lattice Heisenberg antiferromagnet is one of the most frustrated spin systems in two dimension, which has generated various theoretical proposals for the ground state. While recent experiments strongly suggest that the ground state is not magnetically ordered, identification of the true ground state remains highly controversial. Possible candidate phases include various spin liquids and a valence bond solid, particularly with a 36-site unit cell. It is therefore important to theoretically explore decisive properties of the candidate ground states which can be directly compared with experiments. To this end, we investigate the low-energy spin triplet excitations of the valence bond solid state with a 36-site unit cell, which are gapped in contrast to spin singlet excitations. Implications to future experiments are discussed. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y27.00010: Multiple magnetic phases in the frustrated spin-dimer compound Ba$_{3}$Mn$_{2}$O$_{8}$ E. C. Samulon, Y.-J. Jo, P. Sengupta, G. M. Schmiedeshoff, C. D. Batista, M. Jaime, L. Balicas, I. R. Fisher Ba$_{3}$Mn$_{2}$O$_{8}$ is a spin-dimer compound based on S=1 3d$^{2}$ Mn$^{5+}$ ions on a triangular lattice. Antiferromagnetic intradimer exchange leads to a singlet ground state in zero-field. Here we present the first results of thermodynamic measurements for single crystals probing the high-field ordered states of this material. Specific heat, magnetocaloric effect, torque magnetometry and magnetostriction measurements were performed in magnetic fields up to 32T and temperatures down to 20 mK. These measurements reveal the presence of multiple ordered states for fields above H$_{c1}\sim $8.7T. Both single-ion anisotropy and geometric frustration play crucial roles in determining the phase diagram. [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y27.00011: $^{135,137}$Ba NMR study of Ba$_{3}$Mn$_{2}$O$_{8}$ Steve Suh, W.G. Clark, Guoqing Wu, S.E. Brown, E.C. Samulon, I.R. Fisher, C.D. Batista, A.P. Reyes, P. Kuhns, L.L. Lumata We report results from $^{135,137}$Ba NMR spectroscopy and relaxation rate (1/$T_{1})$ measurements in single crystal Ba$_{3}$Mn$_{2}$O$_{8}$, an $S$ =1 dimer system with a singlet ground state. Thermodynamic measurements have shown it has multiple field-induced phase transitions for fields exceeding a critical field $H_{c1}\simeq $90 kOe and varying with field orientation. We have evaluated the hyperfine couplings and electric field gradients in the normal phase for one of the two inequivalent Ba sites, and find a significant anisotropic component to the hyperfine coupling. Measurements of 1/$T_{1}$ made at fixed fields down to temperatures $T<$0.4 K are consistent with critical behavior in the vicinity of $H_{c1}$. However, lower temperatures are needed to clarify the universality class. Goals for upcoming experiments include a determination of the spectrum in the low-symmetry phases and an evaluation of 1/$T_{1}$ for $T<$0.4 K. This work is supported at UCLA by NSF Grants 0520552 (SEB), DMR-00334869 (WGC), Stanford by DMR-0134613 (IRF), and NHMFL by 0084173 and the State of Florida. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y27.00012: Ground State Phase Diagram of the Heisenberg Model on Anisotropic Triangular Lattice. Tommaso Pardini, Rajiv R.P. Singh We study the spin-half and spin-one Heisenberg models on the anisotropic triangular lattice with interactions $J_1$ and $J_2$. The model interpolates between the limits of square lattice ($J_1=0$), triangular lattice ($J_1=J_2$) and decoupled one dimensional linear chains ($J_2=0$). Results are obtained by means of linked-cluster series expansions around the colinear antiferromagnetic phase (CAF) and the non colinear antiferromagnetic phase (NCAF), also known as the spiral phase. For the spin-half model, both phases can be stabilized within our calculations for small $J_2$. However, the NCAF phase always appears to have a lower energy. The pitch of the spiral is substantially renormalized from the classical values. For the spin-one model, we find a transition from the Haldane gap phase to the NCAF phase as a function of $J_1/J_2$. Interchain coupling required for this transition is more than a factor of $30$ larger than when the chains are coupled in an unfrustrated square-lattice geometry. The CAF phase does not appeared to be stabilized for any value of $J_1/J_2$ for the spin-one model. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y27.00013: Spin order and excitations of a model triangular antiferromagnet Wei Bao, Yiming Qiu, Yingxia Wang, Kuo Li, Jianghua Lin, Ross Erwin The triangular antiferromagnet is a model system situating close to the boundary between a three-sublattice order and a quantum-liquid state, due to delicate balance among magnetic interaction, quantum fluctuations and geometrical frustration. The unique topology of the non-collinear three-sublattice order has profound consequences in finite-temperature phase-transitions and spin excitations, which are not yet fully understood. Experimental investigation on such issues has been impeded by imperfect materials which fail to represent the theoretical model at low temperatures. Here we show by neutron scattering that the three-sublattice order in the exceptional new material La$_2$Ca$_2$MnO$_7$ remains two-dimensional down to 40 mK. The order parameter and critical spin fluctuations suggest a phase transition at 3.8 K, but the in-plane correlation length becomes resolution-limited only below 1.8 K. While the spin-wave cone at low energy and the softening of high-energy modes in current theories are supported by our observations, measured spectral intensity above the upper energy limit of spin-waves distribution and a pseudogap developing below 1.8 K are not anticipated. [Preview Abstract] |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y27.00014: Spin Waves in the Ferromagnetic Ground State of the Kagome Staircase System Co$_{3}$V$_{2}$O$_{8}$ Mehmet Ramazanoglu, J. Pat Clancy, A. John Berlinsky, Bruce D. Gaulin, Carl Adams, Zahra Yamani, Ritta Szymczak, Henryk Szymczak, Jan Fink-Finowicki We have performed inelastic neutron scattering measurements on a single crystal sample of cobalt vanadate, Co$_{3}$V$_{2}$O$_{8}$ (CVO). The magnetic Co ions in CVO decorate a stacked, Kagome staircase crystal structure, and the system displays a complex sequence of ordered magnetic phases, culminating in a ferromagnetic phase at low $T$ [1,2]. We studied the spin wave dispersion relations and intensities within the Kagome planes and in the low temperature ferromagnetic phase which the system enters below $T_{c}\sim $6 K. Linear spin wave theory can model the qualitative features and much quantitative detail of these inelastic neutron measurements. These results show strongly anisotropic exchange interactions within this plane, and interesting finite lifetimes to the spin waves well below $T_{c}$. \newline [1] R. Szymczak et. al. PRB 73, 094425, (2006) \newline [2] Y. Chen et. al. PRB 74,014430, (2006) [Preview Abstract] |
Friday, March 14, 2008 2:03PM - 2:15PM |
Y27.00015: Oxygen Sublattice Tuning of Magnetic Order in the Kagom\'{e} Antiferromagnet YBaCo$_{4}$O$_{7+\delta}$ J.F. Mitchell, O. Chmaissem, H. Zheng, A. Huq, P. Stephens YBaCo$_{4}$O$_{7+\delta}$ (Y-114) is a relatively new compound that contains the Kagom\'{e} net motif and is structurally related to the pyrochlore lattice, differing only in the stacking of the triangular layers that link successive Kagom\'{e} planes. We have previously shown that the stoichiometric compound orders antiferromagnetically (AFM) at T$_{N}$ = 108 K into a structure that compromises a collinear arrangement along the $c$-axis and a 120$^{o}$ structure (akin to the well-known $\sqrt{3} \mbox{x} \sqrt{3}$ supercell) in the Kagom\'{e} planes. This ordered state is considered to result from a symmetry-breaking structural distortion that lifts the geometric frustration. Here we show from neutron diffraction the effect of added oxygen on the structure and magnetism of the parent compound. By controlling this parameter, we can tune the system from the AFM ordered ground state into a disordered state. We discuss two possible mechanisms for this evolution: (1) suppressed structural distortion leading to geometric frustration, and (2) the formation of S=0 Co$^{3+}$ centers that can break magnetic exchange pathways. [Preview Abstract] |
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