Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session L27: Focus Session: Low-dimensional Spin Systems |
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Sponsoring Units: GMAG Chair: Christopher Wiebe, Florida State University Room: Morial Convention Center 219 |
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L27.00001: Specific heat and magnetocaloric effect of the S=1/2 spin-ladder compound (CH$_{3})_{2}$CHNH$_{3}$CuCl$_{3}$ Younghak Kim, Yasuo Yoshida, Yasumasa Takano, Hiroyuky Tsujii, Keishi Kanada, Takehiro Saito, Akira Oosawa, Takayuki Goto (CH$_{3})_{2}$CHNH$_{3}$CuCl$_{3 }$is the best laboratory model for the S=1/2 spin ladder comprising ferromagnetic rungs and antiferromagnetic legs [1]. We have determined the magnetic phase diagram of this compound in fields up to 18 T by means of specific-heat and magnetocaloric-effect measurements for two crystal orientations, with either the so-called $B$ or $C$ plane perpendicular to the field direction. For both orientations, we find power-law dependences of the critical field of the long-range antiferromagnetic order on temperature, behavior indicative of the Bose-Einstein condensation of spin triplets due to the cancellation of the energy gap by the magnetic field. [1] T. Masuda \textit{et al}., Phy. Rev. Lett. \textbf{96, }047210 (2006). [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L27.00002: Neutron scattering study of strong- and weakly-coupled, spin-1/2 spin-ladders Bella Lake, S. Notbohm, D.A. Tennant, T.G. Perring, C.D. Frost, R.I. Bewley, P. Manuel, K.P. Schmidt, G.S. Uhrig, P. Ribeiro, C. Sekar, R. Klingeler, C. Hess, G. Krabbes This presentation will discuss two-leg spin-ladders where the magnetic ions have spin-1/2 moments and antiferromagnetic exchange interactions. In the limit of strong rung coupling, the magnetic spectrum is dominated by the gapped magnon mode of an antiferromagnetic dimer, introduction of leg coupling modulates this mode. In the limit of weak rung coupling the excitations are similar to the multi-spinon continuum of the one-dimensional, spin-1/2, Heisenberg antiferromagnet, although the gap remains in the presence of infinitesimal rung coupling. Cyclic exchange interactions are often found in spin-ladders and reduce the gap size. Inelastic neutron scattering measurements will be presented for two ladders. La$_{4}$Sr$_{10}$Cu$_{24}$O$_{41}$ has strong rung coupling and excitations consisting of a gapped one-magnon mode and a two-magnon continuum. In contrast CaCu$_{2}$O$_{3}$ has a weak rung interaction and a substantial cyclic exchange which drives the system gapless and quantum critical. The excitations are similar to a multi-spinon continuum of a spin-1/2 chain, however the presence of rung coupling is revealed by a modulation parallel the rung at low energies. [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L27.00003: Incommensurate correlations in a S=1/2 4-leg quantum spin tube Andrey Zheludev, Ovidiu Garlea, Louis-Pierre Regnault, Klaus Habicht Inelastic neutron scattering is used to investigate magnetic excitations in the quasi-one- dimensional quantum spin-liquid system Cu2Cl4-D8C4SO2. Contrary to previously conjectured models, the appropriate Heisenberg Hamiltonian is equivalent to that of a S=1/2 4-leg spin-tube with almost perfect one dimensionality and no bond alternation [1]. A partial geometric frustration of rung interactions induces a small incommensurability of short-range spin correlations. In high magnetic fields, a Bose-Einstein condensation of magnons induces a quantum phase transition to a incommensurate helimagnetic ordered state. Research at ORNL was funded by the United States Department of Energy, Office of Basic Energy Sciences- Materials Science, under Contract No. DE-AC05-00OR22725 with UT-Battelle, LLC. [1] V. O. Garlea , A. Zheludev, L.-P. Regnault, J.-H. Chung, Y. Qiu, M. Boehm, K. Habicht and M. Meissner, Phys. Rev. Lett., in press (2007); arXiv:0710.0891. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L27.00004: Magnetic and transport properties of the mixed valent vanadium oxides LuV$_4$O$_8$ and YV$_4$O$_8$ S. Das, A. Niazi, D.C. Johnston Mixed valent vanadium oxides have been subjects of wide interest owing to their exotic properties such as heavy fermion behavior below 10 K in spinel LiV$_2$O$_4$ and isosymmetrical phase transition and charge ordering in YbV$_4$O$_8$. \textit{L}V$_4 $O$_8$ (\textit{L} = Y, Lu) crystallize in a structure similiar to that of orthorhombic CaFe$_2$O$_4$, containing four inequivalent V sites arranged in zig-zag chains. The formal oxidation state of V in these compounds is 3.25. In this study, the magnetic and transport properties of these \textit{L}V$_4 $O$_8$ compounds are reported. The magnetic susceptibility indicates two possible structural phase transitions in YV$_4$O$_8 $ in the temperature ($T$) range 70--90 K\@. Anomalies in the heat capacity are also seen in this temperature range. The zero- field-cooled/field-cooled magnetic susceptibilities show possible canted antiferromagnetic ordering in both YV$_4$O$_8$ and LuV$_4$O$_8$ for $T < 50$~K\@. For LuV$_4$O$_8$, the dc electrical resistivity of a sintered pellet measured using a standard four-probe technique shows a strong increase at $T < 100 $~K\@. [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L27.00005: Single Crystal Growth and Characterization of Quasi 1-D, Magnon BEC Candidate Pb$_2$V$_3$O$_9$ Benjamin Conner, Haidong Zhou, Christopher Wiebe, Luis Balicas, Youn-jung Jo Recent success in the floating zone growth of single crystal Pb$_2$V$_3$O$_9$ will be highlighted. Pb$_2$V$_3$O$_9$ has been studied as part of the class of antiferromagnetic 1D spin 1/2 dimer compounds that are believed to be driven through a phase transition to a BEC state with applied magnetic field. Magnetic susceptibility and specific heat measurements will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L27.00006: Magnetic structure of frustrated Haldane chain compound CaV$_{2}$O$_{4}$ B. Lake, O. Pieper, A. Daoud-Aladine, M. Reehuis, K. Prokes, M. Enderle, A. Niazi, J.Q. Yan, D.C. Johnston While the Haldane chain, (Heisenberg spin-1 chain with nearest neighbor antiferromagnet interactions) has been much studied and shown to have gapped magnon excitations. The effect of frustrated and single ion-anisotropy has not been investigated experimentally. Theory suggests that frustration can enhance the multi-particle spectrum and the combination of frustration and anisotropy can drive the system into a gapless chiral phase. CaV$_{2}$O$_{4}$ is a candidate for such a system. This compound consists of two inequivalent one-dimensional chains consisting of spin-1 V$^{3+}$ ions which have antiferromagnetic first and second neighour interactions. Interchain coupling gives rise to long-range order below $T_{N}$= 71K. Neutron diffraction measurements will be described. They reveal collinear spin order within the chains and canting between chains, with the average spin direction along the \textbf{b} axis. The ordered spin moment was also measured and found to be 1 $\mu _{B}$ per Vanadium; this reduction of 50{\%} from full spin ordering suggests loss of moment due to quantum fluctuations. Finally, preliminary inelastic neutron scattering reveals a gap due to single-ion anisotropy. In addition a steep dispersion along the \textbf{c} (chain) direction and much weaker dispersions along \textbf{a} and \textbf{b} confirm the one-dimensional nature of CaV$_{2}$O$_{4}$. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L27.00007: The local density of states of a finite quantum wire: New insights from DMRG and bosonization Imke Schneider, Michael Bortz, Alexander Struck, Sebastian Eggert We consider interacting Fermions on a finite one-dimensional lattice. By using an adapted DMRG algorithm we are able to calculate the energy and spatially resolved local density of states (LDOS) for the lattice model directly without using time-dependent correlations. We compare to analytic expressions for individual energy levels in systems with open boundary conditions from Luttinger Liquid theory. In this way, a detailed understanding of the LDOS for each individual energy level can be obtained in both fermionic and bosonic pictures. Certain degeneracies of the Luttinger Liquid spectrum are lifted in the lattice model by band curvature and interaction effects, leading to a large number of states and energy levels in the LDOS. The standing waves in the LDOS reveal the collective bosonic excitations explicitly. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L27.00008: Real-time dynamics of spinons and holons in one-dimensional correlated electron systems H. Onishi, I. Gonzalez, R. Melko, E. Dagotto To clarify the properties of spin-charge separation in correlated electron systems, we study the real time evolution of wavepackets carrying spinons and holons in a one-dimensional Hubbard model, using the time-dependent density-matrix renormalization group method. In the bulk, while spinons and holons move with the same velocity in the non-interacting chain, it is observed that in the interacting Mott insulator they move in opposite directions each with its own individual velocity. Thus, spin-charge separation is clearly observed. On the other hand, when the Mott insulator is connected to a non-interacting chain, the wavepackets created in the Mott insulator moving toward the non-interacting region exhibit interesting reflection and transmission properties at the interface between the two regions, that will be discussed in this presentation. The differences between Mott and band insulators are also discussed. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L27.00009: Excitation Spectrum Gap and Spin-Wave Stiffness of XXZ Heisenberg Chains: Global Renormalization-Group Calculation Ozan S. Sariyer, A. Nihat Berker, Michael Hinczewski The anisotropic XXZ spin-$\frac{1}{2}$ Heisenberg chain is studied using renormalization-group theory, thoughout the entire temperature and anisotropy ranges in both ferromagnetic and antiferromagnetic regions.[1] We obtain, for all anisotropies, the antiferromagnetic spin-liquid spin-wave stiffness and the Isinglike ferromagnetic excitation spectrum gap, exhibiting the spin-wave to spinon crossover. In the latter case, we also obtain a crossover in the leading algebraic behavior of the specific heat. A number of other purely quantum characteristics are found: The in-plane interaction $s_{i}^{x}s_{j}^{x}+s_{i}^ {y}s_{j}^{y}$ induces an antiferromagnetic correlation in the out-of-plane $s_{i}^{z}$ component. Conversely, an antiferromagnetic $s_{i}^{z}s_{j}^{z}$ interaction induces a correlation in the $s_{i}^{xy}$ component. As another purely quantum effect, (i) in the antiferromagnet, the value of the specific heat peak is insensitive to anisotropy and the temperature of the specific heat peak decreases from the isotropic (Heisenberg) with introduction of either type (Ising or XY) anisotropy; (ii) in complete contrast, in the ferromagnet, the value and temperature of the specific heat peak increase with either type of anisotropy. \newline \noindent [1] O.S. Sariyer, A.N. Berker, and M. Hinczewski, arXiv:0704.1064v1 [cond-mat.stat-mech]. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L27.00010: Dispersive excitations in the $S=1$ antiferromagnet Ba$_3$Mn$_2$O$_8$ Matthew Stone, Mark Lumsden, Eric Samulon, Yiming Qiu, Cristian Batista, Ian Fisher We present powder inelastic neutron scattering measurements on the $S=1$ dimerized antiferromagnet Ba$_3$Mn$_2$O$_8$. The measured $T=1.4$~K magnetic spectrum exhibits a spin-gap of $\Delta = 0.99$~meV and a dispersive spectrum with a bandwidth of approximately $1.67$~meV. Comparison to a coupled dimer model of the dispersion and scattering intensity yields an accurate description of the exchange constants in Ba$_3$Mn$_2 $O$_8$. Interdimer exchange between the stacked triangular lattice dimer bilayers is found to be two orders of magnitude weaker than the intradimer exchange resulting in a quasi-two- dimensional frustrated bilayer triangular lattice. The wave- vector dependent scattering intensity also confirms the proposed $S=1$ dimer bond. Temperature dependent measurements of the magnetic excitations indicate the presence of both singlet- triplet and thermally activated triplet-quintet excitations, with temperature dependent damping and spin-gap consistent with current models for weakly coupled quantum spin-liquids. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L27.00011: Field dependence study on the ordering temperature of [Cu(pz)$_{2}$(NO$_{3})$](PF$_{6})$ Fan Xiao, Christopher Landee, Mark Turnbull [Cu(pz)$_{2}$(NO$_{3})$](PF$_{6})$ is a tetragonal 2D quantum Heisenberg antiferromagnet with an exchange strength of 10.9 K and a zero-field ordering temperature of 3.05 K. Its magnetization has been studied along the two principal directions at various fields and temperatures. The ordering temperature increases with field by as much as 19{\%} in 5 Tesla. This behavior can be explained as the effects of both weak 3D interactions (J'/J $\approx $ 10$^{-3})$ and field-induced Heisenberg-XY crossover. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L27.00012: Raman scattering for the Heisenberg S=1/2 antiferromagnet Natalia Perkins, Wolfram Brenig We study the full Raman intensity for the Heisenberg $S=1/2$ antiferromagnet on the triangular lattice by simultaneously considering the effects of the renormalization of the spectrum by $1/S$ corrections, and the final state magnon-magnon interactions. The analysis of the Raman intensity without final state interactions shows, that it has two peaks, corresponding to two maxima of the bare magnon spectrum $E_k$. Then we calculate Raman intensity with the renormalized spectrum. We obtain that at the energy at which the renormalized dispersion has a plateau, and, therefore, density of states is large, the Raman intensity is strongly enhanced. We also derive explicit expressions for the vertex functions to order $1/S$, and calculate Raman intensity including $1/S$ self-energy and the vertex corrections on equal footing. The vertex corrections is calculated by a summation of ladder diagrams with magnon-magnon interactions. Once interactions are included, the peak smears out and shifts to lower energies. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L27.00013: The central peak problem in the 2d anisotropic heisenberg model Bismarck Costa, Anderson Lima The origin of the central peak in the neutron scattering function, $S(q,\omega)$, for the classical 2d anisotropic Heisenberg model has been a puzzle for several years. Wiesler et al (Z. Phys. B 93, 277-297, 1994) studied the compound $CoCl_2-GIC$ by using neutron scattering. More recently some numerical simulations of the model (H.G. Evertz and D. P. Landau, PRB 54, 12302-12317, 1996) came out. Their results are inconsistent with any analytical study so far. The analytical works suppose that the central peak is due to vortex motion. The theory mimics a successful approach for the one-dimensional version of the model where the central peak is due to kink motion. In this work we show that the central peak in the 2d anisotropic heisenberg model can be explained by a vortex-anti-vortex number fluctuation due to local diffusion and a creation-annihilation process. The phenomenology we propose gives the correct behavior for $S(q,\omega)$ when compared with experimental as well spin dynamics results. [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L27.00014: Magnetic susceptibilities of rectangular Heisenberg S=1/2 antiferromagnets Tom Valleau, Rob Butcher, Brian Keith, Christopher Landee, Mark Turnbull, Anders Sandvik Rectangular antiferromagnets are two-dimensional systems with inequivalent exchange strengths (J', J) along the two principle axes with J' $\equiv $ $\alpha $J, $\alpha \quad <$1. They have an intermediate dimensionality that can vary continuously from 1D ($\alpha $ = 0 ) to square 2D ($\alpha $ = 1). There exist a number of physical realizations of rectangular antiferromagnets (CuPzBr$_{2}$, CuPzCl$_{2}$, CuPz(N$_{3})_{2}$ where Pz = pyrazine) but there has been no previous mechanism for interpreting their susceptibilities in terms of two exchange parameters. We have simulated the susceptibility of the rectangular S=1/2 Heisenberg antiferromagnet using the stochastic series expansion quantum Monte Carlo method [1] and used the results to interpret our experimental data. For example, copper pyrazine diazide, CuPz(N$_{3})_{2}$, has a primary exchange of 15.5 K and an anisotropy parameter $\alpha $ = 0.4. The stronger exchange is due to the superexchange pathway through the pyrazine molecule and the weaker corresponds to the azide bridges. [1] A. Sandvik, PRB 59, R14157 (1999). [Preview Abstract] |
Tuesday, March 11, 2008 5:18PM - 5:30PM |
L27.00015: Magnetic and Thermal Properties of the Spin $S$~=~1/2 Zig-Zag Spin Chain Compound In$_2$VO$_5$ $^*$ Yogesh Singh, David Johnston The structure of In$_2$VO$_5$ consists of zig-zag V$^{4+}$ (spin $S$~=~1/2) chains along the $b$-axis. Prior to our work, there were two theoretical reports on this material.$^{1,2}$ One report suggested that the nearest-neighbor and next-nearest-neighbor interactions between the V$^{4+}$ moments would be anti-ferromagnetic and frustrated,$^1$ while the second report suggested that both these interactions should be ferromagnetic.$^2$ An experimental study of the physical properties of this material had not been reported. We measured magnetic susceptibility $\chi$, ac susceptibility $\chi_{\rm ac}$ and specific heat $C$ versus temperature $T$ on In$_2$VO$_5$ and $\chi$ and $C$ versus $T$ on the isostructural, nonmagnetic compound In$_2$TiO$_5$. The $\chi(T)$ data for In$_2$VO$_5$ showed that the dominant magnetic exchange between the V$^{4+}$ moments was ferromagnetic above 150~K\@. However, the $\chi(T)$ and the frequency dependence of the $\chi_{\rm ac}(T)$ data indicate that below 3~K the system is in a spin-glass state indicating the presence of disorder and frustrated interactions at these temperatures. Our $C$ and entropy $S$ data suggest that there may be a structural change below 140~K in In$_2$VO$_5$ which could possibly change the interactions between the V$^{4+}$ moments.\\ 1. I. M. Volkova, J. Phys.: Condens. Matter {\bf 19}, 176208 (2007).\\ 2. U. Schwingenschlogl, Phys. Rev. B {\bf 75}, 212408 (2007).\\ $^*$Supported by the USDOE under Contract No.\ DE-AC02-07CH11358. [Preview Abstract] |
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