Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session L34: Focus Session: Frustrated and LowD Magnetism  Strongly Frustrated Antiferromagnets in 2D 
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Sponsoring Units: DMP GMAG Chair: Alexander Seidel, Washington University in St. Louis Room: E144 
Tuesday, March 16, 2010 2:30PM  2:42PM 
L34.00001: Successive Magnetic Transitions of the Kagome Plane and FieldDriven Chirality in BaMn$_{2.49}$Ru$_{3.51}$O$_{11}$ Lance De Long , Larysa Shlyk , Sean Parkin The magnetization of singlecrystal BaMn$_{2.49}$Ru$_{3.51}$O$_{11}$ exhibits anomalies at temperatures T$_{1}$ = 183 K, T$_{2}$ = 171 K and T$_{3}$ = 128 K, which signal complex magnetic order induced by competing ferro and antiferromagnetic correlations, and magnetic frustration within the Kagome (hexagonal \textbf{ab}) plane. The T$_{2}$ and T$_{3}$anomalies and unconventional transverse magnetoresistance are observed only for magnetic field \textbf{H} applied in the Kagome plane. We conclude a topological Hall effect (THE) is generated by nonzero scalar chirality K$_{S}$ of spins canted out of the Kagome plane, but is suppressed in a collinear structure induced by only modest inplane fields. BaMn$_{2.49}$Ru$_{3.51}$O$_{11}$ is a unique example of an unusually large and anisotropic THE in a magnetically ordered state. The THE is driven by nonzero scalar spin chirality and can be controlled by unusually modest applied fields $\mu _{o}H \quad <$ 1 T, implying lowfield alteration of scalar spin chirality may provide a new way to control electronic properties in magnetic materials having requisite noncentrosymmetric structure. [Preview Abstract] 
Tuesday, March 16, 2010 2:42PM  2:54PM 
L34.00002: Dynamic Scaling in the susceptibility of the Spin1/2 Kagom\'{e} Lattice Antiferromagnet Herbertsmithite Joel Helton , Kittiwit Matan , Matthew Shores , Bart Bartlett , Emily Nytko , Yiming Qiu , Daniel Nocera , Young Lee The spin$\frac{1}{2}$ kagom\'{e} lattice antiferromagnet herbertsmithite, ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$, appears to display a quantum disordered ground state with effectively gapless excitations. We show that the dynamic susceptibility of this material displays an unusual scaling relation in both the bulk ac susceptibility and the imaginary part of the dynamic susceptibility as measured by inelastic neutron scattering. This behavior is remarkably similar to that seen in certain heavyfermion metals which are doped to be near a quantum critical point. [Preview Abstract] 
Tuesday, March 16, 2010 2:54PM  3:06PM 
L34.00003: Nonmagnetic defects in the classical Kagome bilayer antiferromagnet Arnab Sen , Kedar Damle , Roderich Moessner We consider nonmagnetic substitutions in the quasi twodimensional Kagome bilayer lattice (relevant to experiments on SCGO~\footnote{L.~Limot {\it et. al}, Phys. Rev. B 65, 14447(2002).}) which consists of both triangular and tetrahedral units. Correlated defects, where impurities substitute for all but one spin on a ``defective" triangle or tetrahedron, are particularly interesting in this context. The lone ``orphan spin"~\footnote{P.~Schiffer and R.~Daruka, Phys. Rev. B 56, 13712 (1997), R.~Moessner and A.~J.~Berlinsky, Phys. Rev. Lett. 83, 3293 (1999)} on the simplex acts like a paramagnetic spin as $T \rightarrow 0$ and induces a longranged spin texture around it~\footnote{C.~L.~Henley, Can. J. Phys. 79, 1307 (2001).} at zero temperature. We study such defects at low temperatures and magnetic fields by performing classical MonteCarlo simulations. We also use a simple effective field theory to reasonably capture the features of the induced texture. [Preview Abstract] 

L34.00004: ABSTRACT WITHDRAWN 
Tuesday, March 16, 2010 3:18PM  3:30PM 
L34.00005: A spin1 kagome antiferromagnet Mayra Tovar , Kirill Shtengel , Gil Refael We study a spin1 antiferromagnet on the kagom\'e lattice. We start by constructing a Kleintype SU(2) symmetric Hamiltonian which contains Heisenberg interactions between nearest and nextnearest neighbors as well as threebody terms. Our model Hamiltonian has an extensive degenerate ground state whose manifold is spanned by the AKLTlike valence bond states. We also perturb the parent Hamiltonian by introducing an enhancement to the nearest neighbor antiferromagnetic Heisenberg interactions. By projecting this perturbation onto the basis spanned by the unperturbed ground states, we derive an effective Hamiltonian which is dual to that of the transverse field antiferromagnetic Ising model on the triangular lattice. Based on the parameters of our model, we find it to be in the orderbydisorder phase. The ground state is a valence bond crystal stabilized by quantum fluctuations. We also discuss excitations, both magnetic and nonmagnetic, and address their possible relevance to experiment. [Preview Abstract] 
Tuesday, March 16, 2010 3:30PM  3:42PM 
L34.00006: Dynamical structure factor of spin 1/2 Heisenberg antiferromagnet on Kagome lattice Zhihao Hao , Paula Mellado , Oleg Tchernyshyov The ground state of spin 1/2 antiferromagnet on kagome lattice can be viewed as a collection of small and heavy bound pairs of fermionic spinons [1]. Magnetic excitations of the model correspond to breaking such pairs into their constituents. In the current work, the dynamical structure factor is calcualted for low energy magnetic excitations just above the spin gap. It is observed that the structure factor is similar to the one of an isolated dimer due to the tightness of the bound state. The result is consistent with experiments done in Helton etal [2] and M.A. de Vries etal [3]. \\[4pt] [1] Zhihao Hao and Oleg Tchernyshyov, Phys. Rev. Lett. 103, 187203.\\[0pt] [2] J. S. Helton etal, Phys. Rev. Lett. 98, 107204.\\[0pt] [3] M. A. de Vries etal, arXiv:0902.3194. [Preview Abstract] 
Tuesday, March 16, 2010 3:42PM  3:54PM 
L34.00007: A second species of spinons in the $S=1/2$ Heisenberg antiferromagnet on kagome Paula Mellado , Zhihao Hao , Oleg Tchernyshyov The $S=1/2$ Heisenberg model on kagome can be viewed as an ensemble of spinons, fermionic quasiparticles with $S=1/2$ bound into small, heavy pairs whose binding energy sets the spin gap [1]. The apparent lack of a spin gap in real kagome magnets (e.g. herbertsmithite) may be associated with the DzyaloshinskiiMoriya (DM) term $\mathbf{D} \cdot (\mathbf{S}_i \times \mathbf{S}_j)$ in the Hamiltonian allowed by lattice symmetry. The DM term suppresses the spin gap and eventually induces longrange magnetic order [2]. A recent study [3] hints at the presence of an intermediate gapless phase without magnetic order. We propose that this phase arises as a result of condensation of a second spinon species (kinks). Here we study the motion of a single kink on the Husimi cactus, the analog of kagome in a hyperbolic plane. The kink is localized in the pure Heisenberg model and becomes mobile when $D \neq 0$. We calculate the oneparticle density of states and the bandwidth. [1] Z. Hao and O. Tchernyshyov, Phys. Rev. Lett. \textbf{103,} 187203 (2009). [2] O. C{\'e}pas \textit{et al.}, Phys. Rev. B 78, 140405 (2008). [3] I. Rousochatzakis \textit{et al.}, Phys. Rev. B \textbf{79,} 214415 (2009). [Preview Abstract] 
Tuesday, March 16, 2010 3:54PM  4:06PM 
L34.00008: Valence Bond States on the kagome lattice: Linear Independence and RVBHamiltonian Alexander Seidel A local SU(2)invariant Hamiltonian on the kagome lattice is constructed for which the four SutherlandRokhsarKivelsontype spin1/2 resonating valence bond (RVB) states on the torus are ground states. The uniqueness of these ground states within the subspace of nearestneighbor valence bond states can be demonstrated, as opposed to the case of Kleintype models. The proof depends on the linear independence of the nearestneighbor valence bond states on the (arbitrarily large) kagome lattice. This linear independence property can be demonstrated using tools that are of a piece with those leading to the construction of the RVBHamiltonian. The likely uniqueness of the RVB ground states within the full Hilbert space is also discussed. [Reference: A. Seidel, Phys. Rev. B 80, 165131 (2009).] [Preview Abstract] 
Tuesday, March 16, 2010 4:06PM  4:18PM 
L34.00009: Properties of a Single Hole in a Kagome Antiferromagnet Sumiran Pujari , Michael J. Lawler Various Spin Liquid states have been considered as candidate ground states for the frustrated Kagome Antiferromagnet. Ran \emph{et al} showed that the Dirac Spin Liquid(DSL) state has the lowest ground state energy amongst several proposals \footnote{Y. Ran et al, Phys. Rev. Lett. 98, 117205 (2007)} \footnote{M. Hermele et al, Phys. Rev. B 77, 224413 (2008)}. Assuming the DSL as the ground state, we consider the question of a single hole doped into the antiferromagnet. Our aim is to calculate the hole spectral function 1) at mean field level  where the DSL mean field decomposition of the Heisenberg exchange term forms a background field in presence of which the hole propagates, and 2) beyond mean field  where the question is whether the fluctuations of the DSL mean field renormalizes the hole in a nontrivial way or not. [Preview Abstract] 
Tuesday, March 16, 2010 4:18PM  4:30PM 
L34.00010: Chiralitydriven heavyfermion behavior in kagome Hubbard model Masafumi Udagawa , Yukitoshi Motome Recently, considerable attention has been focused on the heavyfermion behavior of transition metal compounds, such as LiV$_2$O$_4$. As to the rareearth materials, the origin of large quasiparticle mass is attributed to the entropy associated with the localized felectron moments. In contrast, the mechanism of heavyfermion behavior is still unclear for the transition metal compounds because they do not possess an obvious ``entropy reservoir". To explore an alternative scenario in the effect of geometrical frustration under electron correlation, we study the Hubbard model on the kagome lattice at half filling by adopting the cluster dynamical meanfield theory with the continuoustime quantum Monte Carlo method as an impurity solver. We find that the system exhibits a hierarchy of energy scale in charge, spin, and chirality degrees of freedom. The entropy associated with the chirality is released at a much lower temperature than other energy scales for spin and charge fluctuations, leading to a sharp peak in the specific heat and the singleparticle spectrum. These results reveal a new chiralitydriven heavyfermion formation. [Preview Abstract] 
Tuesday, March 16, 2010 4:30PM  4:42PM 
L34.00011: ShastrySutherland tube in a magnetic field Salvatore R. Manmana , JeanDavid Picon , Kai P. Schmidt , Fr\'ed\'eric Mila We study a peculiar quasi1D version of the 2D ShastrySutherland lattice, namely a spin tube made of two coupled orthogonal dimer chains with periodic boundary conditions in the transverse direction. The phase diagram is obtained at zero and at finite magnetic fields combining DMRG and PCUTs approaches. In the limit of small interdimer interactions $J'$ and below half the saturation value an infinite sequence of fractional plateaux is identified with the first few ones located at 1/6, 1/4, 1/3, and 3/8. Above, a pronounced plateau at 1/2 and a smaller one at 3/4 are found. In the limit of large $J'$ the system behaves as a Heisenberg spin1 chain. At intermediate couplings ($0.65 \leq J' \leq 0.7$) a new plateau at 1/5 is realized whose structure cannot be understood in terms of individual triplet coverings of the dimers. We discuss possible implications for the 2D case and ongoing experiments on the plateaumaterial SrCu$_2$(BO$_3$)$_2$. [Preview Abstract] 
Tuesday, March 16, 2010 4:42PM  4:54PM 
L34.00012: Longrange Interaction Effect on Fractional Magnetization Plateaus in the ShastrySutherland Lattice Model Takafumi Suzuki , Yusuke Tomita , Naoki Kawashima We investigate magnetic properties of the S=1/2 Isinglike XXZ model on the hastrySutherland lattices (SSLs) with longrange interactions, using the quantum Monte Carlo method. This model shows magnetization plateau phases at onehalf and onethird of the saturation magnetization when additional couplings are considered. We study the finite temperature transition to onehalf and onethird plateau phases. The obtained results suggest that the former case is of the first order and the latter case is of the second order. We also find that the system undergoes two successive transitions with the 2D Ising model universality, although there is a single phase transition in the Ising limit case[1]. In recent experiments, a large magnetization plateau has been observed in a rareearth SSL compound TmB4[2]. We estimate the coupling ratio to explain the magnetization process. [1] T. Suzuki, Y. Tomita, and N. Kawashima, Phys. Rev. B 80, 180405(R) (2009). [2] K. Siemensmeyer, et al, Phys. Rev. Lett. 101, 037202 (2008). [Preview Abstract] 
Tuesday, March 16, 2010 4:54PM  5:06PM 
L34.00013: Local physics of magnetization plateaux in the ShastrySutherland model Leonid Isaev , Gerardo Ortiz , Jorge Dukelsky We address the physical mechanism responsible for the emergence of magnetization plateaux in the ShastrySutherland model. By using a hierarchical meanfield approach we demonstrate that a plateau is stabilized in a certain {\it spin pattern}, satisfying {\it local} commensurability conditions derived from our formalism. Our results provide evidence in favor of a robust local physics nature of the plateaux states, and are in agreement with recent NMR experiments on ${\rm SrCu_2(BO_3)_2}$. [Preview Abstract] 
Tuesday, March 16, 2010 5:06PM  5:18PM 
L34.00014: Twodimensional atomicscale magnetic skyrmion lattice of Fe/Ir(111) Stefan Heinze , Kirsten von Bergmann , Gustav Bihlmayer , Matthias Menzel , Andre Kubetzka , Roland Wiesendanger , Stefan Bluegel Skyrmions are topologically protected field configurations with particlelike properties which were believed to crystallize only under nonequilibrium conditions. Recently, it has been predicted that they can also become ground states in magnetic systems with broken inversion symmetry [1]. We give theoretical evidence of a twodimensional lattice of atomicscale skyrmions as the magnetic ground state of a monolayer Fe on Ir(111), a system which has been studied by spinpolarized scanning tunneling microscopy [2]. We apply an extended Heisenberg model based on parameters determined from firstprinciples to identify the fourspin interaction as its microscopic origin. This interaction couples degenerate spin spirals and enforces a squaresymmetry on the skyrmionic spin texture decoupled from the underlying hexagonal atomic lattice. [1] U. K. R\"o{\ss}ler et al., Nature 442, 797 (2006). [2] K. von Bergmann et al., PRL 96, 167203 (2006). [Preview Abstract] 
Tuesday, March 16, 2010 5:18PM  5:30PM 
L34.00015: Classification of quantum phases for the starlattice antiferromagnet via a projective symmetry group analysis TingPong Choy , Yong Baek Kim We study possible quantum ground states of the Heisenberg antiferromagnet on the star lattice, which may be realized in the recently discovered polymeric Iron Acetate, Fe$_3$($\mu_3$O)($\mu$OAc)$_6$(H$_2$O)$_3$[Fe$_3$($\mu_3$O)($\mu$OAc)$_{7.5}$]$_2\cdot$ 7H$_2$O. Even though the Fe$^{\rm III}$ moment in this material carries spin5/2 and the system eventually orders magnetically at low temperatures, the magnetic ordering temperature is much lower than the estimated CurieWeiss temperature, revealing the frustrated nature of the spin interactions. Anticipating that a lower spin analog of this material may be synthesized in future, we investigate the effect of quantum fluctuations on the starlattice antiferromagnet using a large$N$ Sp($N$) mean field theory and a projective symmetry group analysis for possible bosonic quantum spin liquid phases. It is found that there exist only two distinct gapped $Z_2$ spin liquid phases with bosonic spinons for nonvanishing nearestneighbor valencebondamplitudes. In particular, the spin liquid phase which has a lower energy in the nearestneighbor exchange model can be stabilized for relatively higher spin magnitudes. [Preview Abstract] 
Tuesday, March 16, 2010 5:30PM  5:42PM 
L34.00016: Firstprinciples study of electronic and magnetic structures of the triangularlattice magnet PdCrO$_2$ Tatsuya Shishidou , Tamio Oguchi Quite recently {\em unconventional\/} anomalous Hall effect has been observed in some geometrically frustrated magnets, where the Hall resistivity does not follow the empirical relation and the noncollinear spin configuration seems to play the key role. PdCrO$_2$[1] is a perfect example of such magnets. It crystallizes in delafossite structure, which is made of alternating stack of triangular layers of Pd and Cr. Its metallic conductivity is attributed to Pd 4d electrons and shows strong twodimensional anisotropy. Cr$^{3+}$ localized moments show antiferromagnetic ordering at 37.5 K, forming 120 degree noncollinear spin structure. At temperatures lower than 20 K, unconventional Hall effect was measured. So far, its microscopic mechanism has not been clarified yet. In this study, we carry out noncollinear LSDA FLAPW calculations for PdCrO$_2$. We will investigate several magnetic structures with and without spinorbit coupling. We will focus on the conducting states at the Fermi level and examine how they are affected by the Cr spin structure. [1] H.~Takatsu H.~Yoshizawa, S.~Yonezawa, and Y.~Maeno, PRB {\bf 79}, 104424 (2009). [Preview Abstract] 
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