Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session B31: Focus Session: Dimers and Other Frustrated Structures 
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Sponsoring Units: GMAG Chair: Chris Wiebe, Florida State University Room: 335 
Monday, March 16, 2009 11:15AM  11:51AM 
B31.00001: Multiple magnetic phases in the frustrated S=1 spindimer compound Ba$_{3}$Mn$_{2}$O$_{8}$ Invited Speaker: Eric Samulon Ba$_{3}$Mn$_{2}$O$_{8}$ is a spindimer compound based on pairs of S=1 3d$^{2}$ Mn$^{5+}$ ions arranged on a triangular lattice. Antiferromagnetic intradimer exchange leads to a singlet ground state in zerofield. Interactions between dimers broaden the triplet and quintuplet bands such that application of a magnetic field leads to multiple states marked by long range order above characteristic critical fields. Here we present results of magnetization, heat capacity, magnetocaloric effect and torque magnetometry measurements of single crystal samples which reveal a complex phase diagram containing at least three distinct ordered states across the triplet and quintuplet regimes. Much of the phase diagram can be understood in terms of an effective spin $\raise.5ex\hbox{$\scriptstyle 1$}\kern.1em/ \kern.15em\lower.25ex\hbox{$\scriptstyle 2$} $ Hamiltonian containing only the lowest energy states ($\vert $0,0$>$ {\&} $\vert $1,1$>$ and $\vert $1,1$>$ {\&} $\vert $2,2$>$, referred to the dimer states, for the singlettriplet and tripletquintuplet regimes respectively). Two distinct ordered states are observed in the singlettriplet regime, which can be ascribed to the delicate interplay between single ion anisotropy and antiferromagnetic interdimer exchange on the frustrated triangular lattice. [Preview Abstract] 
Monday, March 16, 2009 11:51AM  12:03PM 
B31.00002: Frustrated magnetism in the diamondchain like compound Ba$_{3}$Cu$_{3}$Sc$_{4}$O$_{12}$ A.V. Mahajan, B. Koteswara Rao, J. Bobroff The structure of Ba$_{3}$Cu$_{3}$Sc$_{4}$O$_{12}$, having chains of cornershared square plaquettes, is reminiscent of the ``diamond chain'' compound Cu$_{3}$(CO$_{3})_{2}$(OH)$_{2}$ which has shown novel magnetic properties. We report preparation of polycrystalline samples of Ba$_{3}$Cu$_{3}$Sc$_{4}$O$_{12}$ followed by temperature dependent magnetic susceptibility \textit{$\chi $(T)} and heat capacity $C_{p}(T)$ measurements in applied magnetic fields up to $H$ = 90 kOe. At high$T$, \textit{$\chi $(T)} is fitted by the CurieWeiss law (\textit{$\chi $(T)=C/(T}$\theta _{CW}))$ and is suggestive of ferromagnetic interactions ($\theta _{CW}$ $\sim $ 70 K). However, in lowfields, the \textit{$\chi $(T)} shows a sharp peak at $T_{N}$ $\sim $ 16 K and the variation at lower temperatures is indicative of antiferromagnetic ordering. Clear evidence of the transition at $T_{N}$ is also seen in heat capacity data. The sharp peak in \textit{$\chi $(T)} and $C_{p}(T)$ moves to lower temperatures with increasing $H$. The $T_{N}$ is found to be strongly lowered by an applied field and $T_{N} \quad \sim $ 0 for H $\sim $ 70 kOe. Further work to understand the relative exchange couplings between various Cu atoms is currently in progress. [Preview Abstract] 
Monday, March 16, 2009 12:03PM  12:15PM 
B31.00003: NMR Response in Antiferromagnetic Spin1/2 Chains Jesko Sirker, Nicolas Laflorencie Nonmagnetic impurities break a quantum spin chain into finite segments and induce Friedellike oscillations in the local susceptibility near the edges. The signature of these oscillations has been observed in Knight shift experiments on the hightemperature superconductor YBa$_2$Cu$_3$O$_{6.5}$ and on the spinchain compound Sr$_2$CuO$_3$. Here we analytically calculate NMR spectra, compare with the experimental data, and give a simple criterion to determine the impurity concentration. Our results are based on a parameterfree formula for the local susceptibility of a finite spin chain obtained by bosonization which is checked by comparing with quantum Monte Carlo calculations. [Preview Abstract] 
Monday, March 16, 2009 12:15PM  12:27PM 
B31.00004: Controlling Luttinger Liquid Physics in Spin Ladders under Magnetic Field C. Berthier, M. Klanj\v{s}ek, M. Horvati\'{c}, H. Mayaffre, B. Chiari, O. Piovesana, P. Bouillot, T. Giamarchi, C. Kollath, E. Orignac, R. Citro We present a $^{14}$N nuclear magnetic resonance study of a single crystal of CuBr$_4$(C$_5$H$_{12}$N)$_2$ (BPCB) consisting of weakly coupled spin$1/2$ Heisenberg antiferromagnetic ladders. Treating ladders in the gapless phase as Luttinger liquids, we are able to fully account for (i) the magnetic field dependence of the nuclear spinlattice relaxation rate $T_1^{1}$ at $250$~mK and for (ii) the phase transition to a 3D ordered phase occuring below $110$~mK due to weak interladder exchange coupling. BPCB is thus an excellent model system where the possibility to control Luttinger liquid parameters in a continuous manner is demonstrated and Luttinger liquid model tested in detail over the whole fermion band.\\ M. Klanj\v{s}ek {\em et al.}, Phys. Rev. Lett. {\bf 101} 137207 (2008). [Preview Abstract] 
Monday, March 16, 2009 12:27PM  12:39PM 
B31.00005: Electronic Excitations in NiCl$_{2}$ Hydrates V.C. Long, N.M. Collins, G.R. Rayner, G.C. DeFotis, A.S. Hampton, J.M. Pothen We measured the temperaturedependent optical absorption spectra of pure polycrystalline NiCl$_{2}\cdot $2H$_{2}$O between 4,000 and 35,000 cm$^{1}$ and of powdered NiCl$_{2}\cdot $2H$_{2}$O and NiCl$_{2}\cdot $H$_{2}$O (pressed into KBr pellets) in more limited frequency regions. NiCl$_{2}\cdot $2H$_{2}$O has a known complex phase diagram with distinct antiferromagnetic (AF) phases in successively lower temperature regions. It has a rich low temperature spectrum consisting of parity forbidden spin allowed and spin forbidden $dd$ excitations of the pseudooctahedrally coordinated Ni$^{2+}$ ion with fine structure due to vibronic sidebands, magnetic dipole allowed transitions, and possible electronmagnon coupling. Although most NiCl$_{2}\cdot $2H$_{2}$O bands show a typical gradually increasing definition with decreasing temperature, a narrow absorption at 12,600 cm$^{1}$ is strongly suppressed below the lowest AF transition at 6.3 K. The magnetic properties of NiCl$_{2}\cdot $H$_{2}$O are more poorly understood, involving unconventional low dimensional behavior. Although the pressed pellet spectra reveal a loss of detailed spectral information, a downshift of $\sim $ 300 cm$^{1}$ in the second spin allowed band and associated spin forbidden transition can be discerned for the monohydrate compared to the dihydrate. [Preview Abstract] 
Monday, March 16, 2009 12:39PM  12:51PM 
B31.00006: Spin order of the classical Kagome antiferromagnet: via effective Hamiltonians Christopher L. Henley The classical Heisenberg Kagom\'elattice antiferromagnet (KAF) is only known to have a coplanar ``spin nematic'' (or octupole~\footnote{ M. E. Zhitomirsky, Phys. Rev. B 78, 094423 (2008).}) order, so that lowenergy states are labeled by colorings. Contrary to accepted phenomenology,$^1$ I propose that these colorings develop {\it longrange order}.~\footnote{ C. L. Henley, arxiv:0811.0026.} First, from the spinwave Hamiltonian up to 4th order, most modes are integrated out, leaving an effective quartic Hamiltonian $Q$ for just the ``soft'' (zero at harmonic order) modes. Writing it $Q=Q_0+Q'$, where only $Q'$ depends on the discrete coplanar state, $Q'$ is treated as a perturbation, and its expectation in the $Q_0$ ensemble becomes an effective Hamiltonian $\Phi$ for the colorings. The couplings in $\Phi$ are estimated using ``Coulomb phase'' coarsegrainings.$^2$ Following Huse \& Rutenberg,~\footnote{ D. A. Huse and A. D. Rutenberg, Phys. Rev. B 45, 7536 (1992).} I observe the unweighted coloring model sits at a roughening transition, hence $\Phi$ drives the KAF to longrange order of the $\sqrt{3}\times\sqrt{3}$ type (modulo the inevitable gradual orientation fluctuations of the spin plane). A similar effective Hamiltonian exists for related $d=3$ lattices,~\footnote{ C. L. Henley, arxiv:0809.0079.} but cannot produce order. [Preview Abstract] 
Monday, March 16, 2009 12:51PM  1:03PM 
B31.00007: Quantum stabilization of 1/3 magnetization plateau in Cs$_2$CuCl$_4$. Oleg Starykh, Jason Alicea, Andrey Chubukov We consider the phase diagram of a spatially anisotropic 2D triangular antiferromagnet in a magnetic field. Classically, the ground state is umbrellalike for all fields, but we show that the quantum phase diagram is much richer and contains a 1/3 magnetization plateau, two commensurate planar states, two incommensurate chiral umbrella phases, and, possibly, a spin density wave state separating the two chiral phases. Our analysis sheds light on several recent experimental findings for the spin1/2 system Cs$_2$CuCl$_4$. [Preview Abstract] 
Monday, March 16, 2009 1:03PM  1:15PM 
B31.00008: Magnetic properties of bilayer triangular lattice FeiMing Hu, ShiQuan Su, TianXing Ma, HaiQing Lin We study magnetic properties of the singleband Hubbard model on a coupled bilayer triangular lattice by using the determinant quantum Monte Carlo method. Simulations are focused in the region near the van Hove singularities. We perform investigations on two kinds of double layer triangular lattices, one is a simple triangular structure which has only one nearest neighbor between two layers for every atom and another one is a graphenelike structure which has three nearest neighbors between two layers for every atom. We compare their magnetic properties in the view of the itinerant electron ferromagnetic theory of attribute their behaviors to the density of states on the Fermi surface. [Preview Abstract] 
Monday, March 16, 2009 1:15PM  1:27PM 
B31.00009: DzyaloshinskiiMoriya interactions in valence bond systems I Kirill Shtengel, Kumar Raman, Mayra Tovar We investigate the effect of DzyaloshinskiiMoriya interactions on the low temperature magnetic susceptibility for a system whose low energy physics is dominated by shortrange valence bonds (singlets). Our general perturbative approach is applied to specific models expected to be in this class, including the ShastrySutherland model of the spindimer compound SrCu$_2 $(BO$_3$)$_2$ and the antiferromagnetic Heisenberg model of the recently discovered $S=1/2$ kagome compound ZnCu$_3$(OH)$_6 $Cl$_2$. The central result is that a shortranged valence bond phase, when perturbed with DzyaloshinskiiMoriya interactions, will remain timereversal symmetric in the absence of a magnetic field but the susceptibility will be nonzero in the zero temperature limit. Applied to ZnCu$_3$(OH)$_6$Cl$_2$, this model provides an avenue for reconciling experimental results, such as the lack of magnetic order and lack of any sign of a spin gap, with known theoretical facts about the kagome Heisenberg antiferromagnet. [Preview Abstract] 
Monday, March 16, 2009 1:27PM  1:39PM 
B31.00010: Hierarchical meanfield approach to the $J_1$$J_2$ Heisenberg model on a square lattice Leonid Isaev, Gerardo Ortiz, Jorge Dukelsky We study the quantum phase diagram and excitation spectrum of the frustrated $J_1$$J_2$ spin1/2 Heisenberg Hamiltonian. A hierarchical meanfield approach, at the heart of which lies the idea of identifying {\it relevant} degrees of freedom, is developed. Thus, by performing educated, manifestly symmetry preserving meanfield approximations, we unveil fundamental properties of the system. We then compare various coverings of the square lattice with plaquettes, dimers and other degrees of freedom, and show that only the {\it symmetric plaquette} covering, which reproduces the original Bravais lattice, leads to the known phase diagram. The intermediate quantum paramagnetic phase is shown to be a (singlet) {\it plaquette crystal}, connected with the neighbouring N\'eel phase by a continuous phase transition. We also introduce fluctuations around the hierarchical meanfield solutions, and demonstrate that in the paramagnetic phase the ground and first excited states are separated by a finite gap, which closes in the N\'eel and columnar phases. Our results suggest that the quantum phase transition between N\'eel and paramagnetic phases can be properly described within the GinzburgLandauWilson paradigm. [Preview Abstract] 
Monday, March 16, 2009 1:39PM  1:51PM 
B31.00011: Mixed exchange antiferromagnetic/ferromagnetic S=1/2 Heisenberg rectangular lattices Brian Keith, Tom Valleau, Fan Xiao, Mark Turnbull, Chris Landee The susceptibilities of mixed antiferromagnetic/ferromagnetic rectangular Heisenberg lattices of $S = 1/2$ have been simulated using Quantum Monte Carlo techniques. These simulations include lattices in which the stronger interaction is ferromagnetic or antiferromagnetic along with the isotropically mixed lattice. The two exchange strengths, $J$ and $J'$, are related by $J'= \alpha J$, where $\alpha$ is the aspect ratio which ranges from $0 \le \alpha \le 1$. These simulations were done for $0 \le \alpha \le 1$ in .05 increments. The results are discussed and the models are used to fit suspected mixed antiferromagnetic/ferromagnetic rectangles such as $ Cu(pyz)(NO_3)(HCO_2) $ and $ Cu(pyz)(N_3)_2 $. [Preview Abstract] 

B31.00012: ABSTRACT WITHDRAWN 
Monday, March 16, 2009 2:03PM  2:15PM 
B31.00013: Ce$_{2}$Pt$_{2}$Pb : Frustrated heavy fermion system with the ShastrySutherland lattice Moo Sung Kim, Meigan Aronson We have synthesized single crystals of Ce$_{2}$Pt$_{2}$Pb which has the ShastrySutherland lattice network of Ceions on the crystallographic cplane, that can induce magnetic frustration. The specific heat, magnetic susceptibility, and resistivity were measured for asgrown crystals. Above 30 K, CurieWeiss behavior is found in the magnetic susceptibility, with a Ce moment of 2.33 $\mu _{B}$/Ce and a Weiss temperature of 15 K, indicating antiferromagnetic interactions among the Ce$^{3+}$ ions. The magnetic specific heat rises from a minimum at 15 K to a broad maximum at 2 K, before falling to a heavy fermion value of $\sim $0.8 J/CemolK$^{2}$ at the lowest temperature. The entropy reaches only 1/2Rln2 at the maximum in the specific heat, and the full doublet ground state Rln2 is only recovered at 15 K. Ce$_{2}$Pt$_{2}$Pb is an unusual material, in which a heavy fermion liquid with short range antiferromagnetic order emerges from a strongly frustrated and fluctuating paramagnetic state. [Preview Abstract] 
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