Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session J15: Focus Session: Frustrated 1D Magnetism |
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Sponsoring Units: GMAG Chair: Michel Kenzelmann, ETH Zurich, Switzerland Room: Colorado Convention Center Korbel 4E |
Tuesday, March 6, 2007 11:15AM - 11:51AM |
J15.00001: Quantum helimagnetism of the frustrated spin 1/2 chain LiCuVO4 Invited Speaker: Henrik M. Ronnow Inelastic neutron scattering, susceptibilit y, and high-field magnetization identify LiCuVO4 as a nearest-neighbour ferromagnetic, next-nearest-neigh bour rustrated, quasi-onedimensional helimagnet,whic h is largely influenced by quantum fluctuations. Complementary band structure calculations provide a microscopic model with the correct sign and magnitude of the major exchange integrals. \newline \newline In collaboration with M. Enderle, C. Mukherjee, B. Fak, R. K. Kremer, J.-M. Broto, H. Rosner, S.-L. Drechsler, J. Richter, J. Malek, A. Prokofiev, W. Assmus, H. Rakoto, M. Rheinstadter, \'Ecole Polytechnique F\'ed\'erale de Lausanne. [Preview Abstract] |
Tuesday, March 6, 2007 11:51AM - 12:03PM |
J15.00002: Properties of frustrated one dimensional S=1/2 Heisenberg antiferromagnets in zero and applied fields Michael Banks, Reinhard Kremer The Phase diagram for a S = 1/2 $J_{1}$-$J_{2}$ exchange model on a one dimensional chain offers phases with no classical analogue. We will present a summary of experimental findings on quasi-one dimensional S = 1/2 (Cu$^{2+}$) chain systems with competing interactions along the chain, which lie in the frustrated helix part of this phase diagram. Compounds in this region of the phase diagram have similar magnetic properties, including a common magnetic structure with an incommensurate helix which is polarized along the chain. The application of a magnetic field applied in the helix plane offers a rich phase diagram with yet unknown phases. This will be discussed primarily for the quasi one dimensional compound LiCuVO$_{4}$. [Preview Abstract] |
Tuesday, March 6, 2007 12:03PM - 12:15PM |
J15.00003: Specific-heat and magnetocaloric-effect study of the S=1/2 frustrated-chain antiferromagnet Cu$_{3}$(CO$_{3})_{2}$(OH)$_{2}$ Y. Yoshida, B. Andraka, Y. Takano, H. Tsujii, Y. Inagaki, H. Kikuchi Azurite, Cu$_{3}$(CO$_{3})_{2}$(OH)$_{2}$, is a natural mineral in which the S=1/2 spins of Cu$^{2+}$ form frustrated chains. The magnetization of this material exhibits a plateau at 1/3 of the saturation value, at magnetic fields between 16 T and 26 T applied in the chain direction, and the specific heat at zero field shows a broad peak at 4 K followed by a sharp peak at 1.8 K [1]. We have measured specific heat up to 18 T and found that the broad peak gets smaller with increasing field and disappears above 10 T. The sharp peak begins to separate into a peak and a shoulder at 5 T, and the temperature difference between these anomalies becomes larger up to 13 T. These observations are well supported by magnetocaloric-effect data taken at the same time. In the plateau region, an exponential temperature dependence is observed in the specific heat at low temperatures, indicating an energy gap for low-lying excited states. Detailed specific-heat and magnetocaloric-effect data and the magnetic phase diagram up to 18 T will be presented. [1] H. Kikuchi \textit{et al,} Phys. Rev. Lett. \textbf{94}, 227201 (2005) [Preview Abstract] |
Tuesday, March 6, 2007 12:15PM - 12:27PM |
J15.00004: Commensurate Fluctuations in the Pseudogap and Incommensurate Spin-Peierls Phases of TiOCl J.P. Clancy, B.D. Gaulin, K.C. Rule, J.P. Castellan, F.C. Chou We have performed x-ray scattering measurements on the unconventional spin-Peierls system TiOCl and the closely related doped compound Ti$_{(1-x)}$Sc$_{x}$OCl (x = 0.01, 0.03). In pure TiOCl these measurements reveal the presence of commensurate dimerization peaks within both the incommensurate spin-Peierls phase (T$_{c1}<$ T $<$ T$_{c2})$ and the so-called pseudogap phase (T$_{c2}<$ T $<$ T*). This commensurate scattering is slightly shifted in Q-space relative to the commensurate long-range ordered state below T$_{c1}$, and has a fairly narrow width in Q, suggesting correlation lengths greater than 100 angstroms. Below T* $\sim $ 130K, the integrated intensity of the scattering over the commensurate and incommensurate positions grows continuously as a function of decreasing temperature. In addition, measurements performed on the doped compound show that the substitution of non-magnetic Sc$^{3+}$ ions (S = 0) onto Ti$^{3+}$ (S = 1/2) sites appears to suppress commensurate fluctuations and prevent the development of a commensurate long-range ordered state. [Preview Abstract] |
Tuesday, March 6, 2007 12:27PM - 12:39PM |
J15.00005: High-pressure structural investigation of the spin-Peierls compound TiOCl. A. Prodi, J. Hu, E. Abel, F.C. Chou, Y.S. Lee TiOCl is a S=1/2 layered Mott insulator which displays a spin-Peierls state at low-temperatures involving the dimerization of the Ti ions. Although carrier localization is expected to be weak, it has proven difficult to dope charges in the system through partial chemical substitution. To explore if TiOCl could instead host a bandwidth-driven insulator-to-metal transition under external pressure, we investigated the crystal structure at pressures up to 17 GPa by means of powder and single crystal synchrotron x-ray diffraction in diamond anvil cells. [Preview Abstract] |
Tuesday, March 6, 2007 12:39PM - 12:51PM |
J15.00006: NMR and $\mu $SR studies of Sc (S=0) doping effects in a spin-Peierls system TiOCl T. Imai, E. Mehes, F.L. Ning, S. Golin, S.R. Saha, A. Aczel, J. Rodriguez, F.C. Chou, G.M. Luke TiOCl is a model one-dimensional S=1/2 chain system with an incommensurate phase below T$_{c}\sim $95K and a spin-Peierls ground state below T$_{c}$'$\sim $65K. We report a detailed NMR and $\mu $SR investigation of the local lattice and spin environment in undoped and Sc (S=0) doped Ti$_{1-x}$Sc$_{x}$OCl (x = 0, 0.01, and 0.03). Based on $^{35}$Cl NMR lineshape measurements, we show that all Ti$_{1-x}$Sc$_{x}$OCl samples exhibit signatures of short-range Peierls distortion starting from as high as T*$\sim $130K. Moreover, the NMR line splits into two peaks below T$_{c}\sim $95K, corresponding to two types of local lattice geometries. Only undoped TiOCl develops sharp double peaks below T$_{c}$'=65K. $\mu $SR measurements reveal no evidence for a magnetic long range order down to 2K, even for x=0.03. We will compare our results with x-ray scattering measurements (J.P. Clancy et al., this meeting) and doped CuGeO$_{3}$. [Preview Abstract] |
Tuesday, March 6, 2007 12:51PM - 1:03PM |
J15.00007: Hole-doping of Ca$_{1-x}$Na$_x$V$_2$O$_4$ ($x = 0 - 0.5$) With Zig-Zag Vanadium Chains A. Niazi, D.C. Johnston CaV$_2$O$_4$ crystallizes in an orthorhombic $Pnam$ structure with $S=1$ zig-zag V chains along the $c$-axis. In this low-dimensional, insulating system the triangular arrangement of V atoms with $J_1\approx J_2$ leads to competing frustrating antiferromagnetic (AF) interactions. Our recent studies on powders and single crystals of CaV$_2$O$_4$ show long-range AF ordering at a N$\acute{\rm e}$el temperature $T_{\rm N} \sim 75 - 78$~K (with a monoclinic distortion at $\sim 145-150$~K) and signatures of partial spin-freezing below 20~K. We have tried doping CaV$_2$O$_4$ into the metallic state by substitution at the Ca site to drive V into fractional valence states. We have succeeded in replacing Ca upto 50\% by Na at 1200~$^{\circ}$C. Powder XRD patterns of our Na-substituted samples are nearly single-phase CaV$_2$O$_4$-type, while the $c$-axis lattice parameter decreases sharply - Thus Na indeed substitutes for Ca instead of occupying interstitial positions. The room temperature resistance of Na-doped sintered pellets decreases significantly. High field ($H$ = 1 T) dc magnetization measurements show a steep fall in $T_{\rm N}$ while low field ($H$ = 100 Oe) data suggest onset of spin-glass like behavior as the Na content increases. We shall present our results and discuss the evolution from a partially disordered AF insulator to a spin-glass. [Preview Abstract] |
Tuesday, March 6, 2007 1:03PM - 1:15PM |
J15.00008: $^{51}$V NMR Study of the Magnetic Structure of the Frustrated Zig-Zag Spin-1 Chain Compound CaV$_2$O$_4$ X. Zong, B.J. Suh, A. Niazi, D.C. Johnston $^{51}$V NMR measurements have been performed on a single crystal of orthorhombic (at room temperature) CaV$_2$O$_4$ in zero applied magnetic field and with a small perturbing field up to $H = 2$~T, at temperatures well below the N\'eel temperature $T_N = 78$~K\@. The $c$-axis is parallel to the chains. At $H = 0$, a broad $^{51}$V NMR spectrum with a peak at 237~MHz was observed. The effective local hyperfine field $H_{{\rm eff}} = 21.2$~T corresponding to the peak frequency 237~MHz is in good agreement with expectation for the V$^{3+}$ $S = 1$ spin state. In $\vec{H}\perp c$, the spectrum splits into two parts that are equally separated from the peak position at zero field. The separation of the parts depends strongly both on the magnitude and direction of $\vec{H}$ with respect to the crystal axes. Our NMR results are consistent with a collinear antiferromagnetic spin structure with the spin direction along the \textit{b}-axis, which together with the magnetization data suggest that the antiferromagnetic long-range order arises from an order-from-disorder mechanism. We also present the temperature and orientation dependence of the spin-lattice relaxation rate $1/T_1$. [Preview Abstract] |
Tuesday, March 6, 2007 1:15PM - 1:27PM |
J15.00009: Bilinear-Biquadratic Spin 1 Heisenberg Zig-Zag Chain Philippe Corboz, Andreas Laeuchli, Hirokazu Tsunetsugu Recent theoretical studies raised the possibility of a realization of spin nematic states in the S=1 triangular lattice compound NiGa2S4. We study the bilinear-biquadratic spin 1 Heisenberg chain in a zig-zag geometry by means of Density Matrix Renormalization Group (DMRG) and Exact Diagonalization (ED). We present the phase diagram focusing on antiferromagnetic interactions. Adjacent to the known Haldane / double Haldane and the extended critical phase with dominant spin nematic correlations we find a trimerized phase with a non-vanishing energy gap. We discuss results for different order parameters, energy gaps, correlation functions and the central charge, and make connection to field theoretical predictions for the phase diagram. [Preview Abstract] |
Tuesday, March 6, 2007 1:27PM - 1:39PM |
J15.00010: Static Holes in Geometrically Frustrated Bow Tie Ladder George Martins, Wolfram Brenig Doping of the geometrically frustrated bow-tie spin ladder \footnote{C. Waldtmann, A. Kreutzmann, U. Schollwock, K. Maisinger, and H.-U. Everts, Phys. Rev. B {\bf 62}, 9472 (2000).} with static holes is investigated by a complementary approach using exact diagonalization and hard-core quantum dimers. Results for the thermodynamics in the undoped case, the singlet density of states, the hole-binding energy, and the spin correlations will be presented. We find that the static holes polarize their vicinity by a localization of singlets in order to reduce the frustration. As a consequence the singlet polarization cloud induces short range repulsive forces between the holes with oscillatory longer range behavior. For those systems we have studied, most results for the quantum dimer approach are found to be qualitatively if not quantitatively in agreement with exact diagonalization. The ground state of the undoped system is non-degenerate with translationally invariant nearest-neighbor spin correlations up to a few unit cells, which is consistent with a spin liquid state or a valence bond crystal with very large unit cell. [Preview Abstract] |
Tuesday, March 6, 2007 1:39PM - 1:51PM |
J15.00011: Magnetic and transport properties of a one dimensional frustrated t-J model for vanadate nanotubes Jose Riera, Sebastian Costamagna We propose a one-dimensional model consisting of a chain with a t-J Hamiltonian coupled to a Heisenberg chain in a frustrated geometry to describe the appearance of the ferromagnetic phase which has been experimentally observed in vanadate nanotubes. This model contains a mechanism of frustration suppressed by doping suggested by L. Krusin-Elbaum, {\it et al.} [Nature {\bf 431}, 672 (2004)]. We study, using numerical techniques in small clusters, the relation between magnetic order and transport properties in the proposed model, and we perform a detailed comparison of the properties of this model with those of the ferromagnetic Kondo lattice model (FKLM). For this comparison, a number of results for the latter model, obtained using the same numerical techniques, will be provided to complement those results already available in the literature. We conclude that it that does not appear to be a true ferromagnetic order in the proposed model, but rather an incommensurate ferrimagnetic one, and contrary to what happens in the FKLM, electronic transport is somewhat suppressed by this ferrimagnetic order. [Preview Abstract] |
Tuesday, March 6, 2007 1:51PM - 2:03PM |
J15.00012: Quantized Berry phase for itinerating singlets in one-dimensional t-J mod Isao Maruyama, Yasuhiro Hatsugai The quantized Berry phase as a local order parameter of gapped quantum liquids is proposed for characterization of a topological or quantum order in various models including strongly correlated electron systems[1]. We apply the scheme to calculate the quantized Berry phase in the $t-J$ model, where the Berry phase is quantized as trivial or non-trivial value, i.e., 0 or $\pi$, due to some anti-unitary symmetry. One-dimensional $t-J$ model with a few electrons gives a realization of itinerating singlets when the exchange energy $J$ is large. Although the charge excitation is gapless, the spin gap is finite. Then we can calculate the Berry phase by treating low energy states as a degenerated multiplet. To use a local spin singlet as a order parameter, we define the Berry phase by a local spin twist. It is found that the Berry phase is quantized actually and becomes uniform and nontrivial when the number of electrons $N=4n+2$, with $n$ being an integer [2]. \newline [1] Y. Hatsugai, cond-mat/0603230 to appear in J. Phys. Soc. Jpn.\newline [2] I. M. and Y.Hatsugai. unpublished [Preview Abstract] |
Tuesday, March 6, 2007 2:03PM - 2:15PM |
J15.00013: Field-Induced New Phases in the S=1/2 Three-Leg Spin Tube Toru Sakai, Masahiro Sato, Yuichi Otsuka, Kouichi Okunishi, Kiyomi Okamoto The S=1/2 three-leg frustrated antiferromagnetic spin tube is investigated using the numerical diagonalization, density matrix renormalization group, and effective field theory techniques. Varing one of the three rung couplings, a quantum phase transition between the spin gap and gapless phases was predicted in our previous work[1]. The present phenomenological renormalization approach gives a phase diagram of the spin gap. In addition we study the magnetization process of the regular triangle spin tube. Our present effective field theory between the lower critical field where the spin gap vanishes, and the upper critical field where the magnetization is saturated, predicts two field-induced new phases appear; the one has the vector chiral order and the other has an inhomogeneous distribution of the magnetization. These two phases coexist with the Tomonaga-Luttinger liquid phase. The prediction is confirmed by the numerical diagonalization and finite-size scaling analyses.\newline [1] T. Sakai, M. Matsumoto, K. Okunishi, K. Okamoto and M. Sato: Physica E 29 (2005) 633. [Preview Abstract] |
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