Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session B23: Quantum Spin Chains I |
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Sponsoring Units: GMAG Chair: Christopher Landee, Clark University Room: Baltimore Convention Center 320 |
Monday, March 13, 2006 11:15AM - 11:27AM |
B23.00001: Calculated And Experimental Exchange Interactions In A Strong-rail Spin Ladder; Christopher Landee, A. Shapiro, M. M. Turnbull, J. Novoa, J. Ribas Low-dimensional magnetic lattices with moderate exchange interactions can be conveniently formed by combining organic cations with tetrahalocuprates. In these compounds, the ultimate structure is ultimately determined by size, shape and hydrogen-bonding capacity of the cation with the magnetic interactions occurring through halide-halide contacts. A variety of these compounds have been studied [1] and techniques have been developed for calculating the magnetic exchange interactions from first principles [2,3]. We report on the synthesis, structure and magnetic properties of (2,3-dimethylpyridinium)$_{2}$CuBr$_{4}$, which is the second spin ladder to be found with a stronger rail interactions (J$_{rung}$/k$_{B}$ = 9 K, J$_{rail}$/k$_{B}$ = 17 K). Comparison will be made between the exchange constants calculated from first principles and those obtained from experimental data. 1. C. P. Landee \textit{et al}, \textit{Phys. Rev. B} \textbf {63} 100402 (2001). 2. M. Deumal \textit{et al}, \textit{Polyhedron }\textbf{22 } 2235-2239 ( 2003). 3. M. Deumal \textit{et al}, \textit{Euro. J. Inorg. Chem.} \textbf{2005}, 4697-4706. [Preview Abstract] |
Monday, March 13, 2006 11:27AM - 11:39AM |
B23.00002: Novel Raman resonance in ladder spin systems Alexander Donkov, Andrey Chubukov We consider Raman intensity in spin $S$ two-leg- spin-ladder, with the goal to understand recent experiments[1,2]. We argue that the Raman intensity has a pseudo-resonance peak whose width is very small at large $S$. The pseudo-resonance originates from the existence of a local minimum in the magnon excitation spectrum, and is located slightly below twice the magnon energy at the minimum. The physics behind the peak is surprisingly similar to that in the excitonic scenario for the neutron and Raman resonances in a $d-$wave superconductor. We also consider mid-infrared X-ray scattering in 2D systems and compare the results with recent measurements [3]. \newline \newline [1] A. Gozar et al, Phys. Rev. Lett. {\bf 87}, 197202 (2001). \newline [2] S. Sugai and M. Suzuki, Phys stat sol (b) {\bf 215}, 653 (1999). \newline [3] J. P. Hill, G Blumberg et al, [unpublished] [Preview Abstract] |
Monday, March 13, 2006 11:39AM - 11:51AM |
B23.00003: Theory of spin gaps in the dimerized quarter-filled two-leg rectangular ladder Yongguo Yan, Sumit Mazumdar Despite the extensive work on even-leg spin-ladders and 1/2- filled band electron-ladders \footnote{E. Dagotto and T.M. Rice, Science {\bf 271}, 618 (1996).} , little is known currently about commensurate non-1/2-filled band electron ladders. We report density-matrix renormalization group calculations of spin gaps in the 1/4-filled band correlated two-leg rectangular ladder with dimerization along the legs of the ladder. In contrast to the uniform ladder, spin gaps in the dimerized ladder can be significant. Very large spin gaps, as are found experimentally in quarter-filled band organic charge-transfer solids with coupled pairs of quasi-one-dimensional stacks \footnote{E. Ribera et al., Chem. Eur. J. 5, 2025 (1999).}, however, occur within the model only for large leg dimerization and electron hopping along the rungs that are nearly equal to the hopping along the legs. [Preview Abstract] |
Monday, March 13, 2006 11:51AM - 12:03PM |
B23.00004: Magnetic and electrical properties of the double chain-type compounds NaRh$_2$O$_4$ and NaMn$_2$O$_4$ K. Yamaura, Q. Huang, M. Moldovan, D.P. Young, E. Takayama-Muromachi Both compounds share the same structure basis in which the 6- fold coordinated transition metal, RhO$_6$/MnO$_6$, forms a double chain-type unit by sharing the octahedra edges (CaFe$_2 $O$_4$-type structure, not spinel-like). We were interested in the compounds because those magnetic and electrical properties might reflect nature of the correlated $4d/3d$ electrons on the anisotropic basis. The compounds were recently synthesized for the first time. The structure, magnetic and electrical properties were investigated by neutron diffraction, magnetic susceptibility, and electrical transport measurements of those high-quality polycrystalline samples. The data indicate NaRh$_2$O$_4$ is metallic down to 1.8 K and the magnetic susceptibility shows a broad cusp around $\sim100$ K. NaMn$_2$O$_4$ is highly electrically insulating with and without presence of a magnetic field. The manganese oxide goes into an anti-ferromagnetically ordered state at low temperatures through two-steps process at $\sim$40 K and $\sim$10 K. We will give a detailed report about the properties of the two new compounds. [Preview Abstract] |
Monday, March 13, 2006 12:03PM - 12:15PM |
B23.00005: Spinless Fermionic ladders in a magnetic field Sam Carr, Boris Narozhny, Alexander Nersesyan We study a system of interacting spinless fermions hopping on a two-leg ladder subject to an external magnetic field perpendicular to the ladder. At $1/2$-filling, we find a series of quantum phase transitions as a function of the magnetic field: these are either $U(1)$ Gaussian phase transitions between two phases with distinct types of long-range order or Berezinskii-Kosterlitz-Thouless transitions between ordered and gapless phases. At $1/4$-filling, we also find long-range order: a bond density wave or a staggered flux phase, each supporting excitations with fractional charge. [Preview Abstract] |
Monday, March 13, 2006 12:15PM - 12:27PM |
B23.00006: Magnetic Susceptibility and Heat Capacity of Single Crystal CaV$_2$O$_4$ Containing $S=1$ $J_1$-$J_2$ Antiferromagnetic Spin Chains A. Niazi, D. C. Johnston, S. Bud'ko, D. L. Schlagel, T. A. Lograsso, A. Honecker The compound CaV$_2$O$_4$ has an orthorhombic $Pnam$ structure and contains spin $S=1$ zig-zag ($J_1$-$J_2$) chains running along the crystallographic $b$-axis. Structural considerations suggest that $J_1\approx J_2$, which would result in competing frustrating antiferromagnetic (AF) interactions in this low-dimensional system. Previous studies on powders have suggested collinear spin models, a gapless chiral phase, or spin freezing below 20~K\@. We have grown single crystals of CaV$_2$O$_4$ for the first time and carried out magnetization and heat capacity measurements down to 1.8~K\@. We observe long-range AF ordering at $T_{\rm N} = 54$~K with the easy axis being the $c$-axis, as reflected by anisotropic susceptibility $\chi$ below $T_{\rm N}$. The magnetic specific heat $C_{\rm p}^{\rm mag}(T)$ up to 100 K, obtained after subtracting the lattice contribution of nonmagnetic isostructural ${\rm CaSc_2O_4}$, shows a clear signature of long-range magnetic order at $T_{\rm N}$. However, the molar entropy attains only $\approx 1/4$ of its maximum value 2Rln(3) by 100~K, indicating strong short range order above $T_{\rm N}$ and large values $J_1,\ J_2 > 100$ K\@. This is consistent with our estimates $J_1 \approx J_2 \sim$ 150--300 K obtained by comparison of our $\chi(T)$ data with exact diagonalization calculations of $\chi(T)$. [Preview Abstract] |
Monday, March 13, 2006 12:27PM - 12:39PM |
B23.00007: Profiles of magnons and spinons in a spin ladder David Clarke, Oleg Tchernyshyov, Jean-Baptiste Fouet, Fr\'ed\'eric Mila We have studied quantum phase transitions between magnetization plateaux and gapless incommensurate states in a frustrated spin ladder placed in a magnetic field \cite{Mila98}. The transitions are triggered by a condensation of magnons (spinons) at the edge of an integer (half-integer) magnetization plateau. In this talk we will describe magnetization patterns formed in a finite ladder just off a magnetization plateau, i.e. at low concentrations of magnons or spinons. In these regimes the magnons and spinons behave as noninteracting fermions carrying spins $S^z = \pm 1$ and $\pm 1/2$, respectively. The spinons also act as domain walls, thus causing a dramatically different magnetization pattern from that of the magnons. The analytical results obtained agree well with numerical data. \begin{thebibliography}{1} \bibitem{Mila98} F. Mila, Eur. Phys. J. B {\bf 6}, 201 (1998). \end{thebibliography} [Preview Abstract] |
Monday, March 13, 2006 12:39PM - 12:51PM |
B23.00008: Ferromagnetic-antiferromagnetic alternating exchange spin-dynamics in the low-dimensional quantum magnet DMACuCl$_{3}$ Matthew Stone, Wei Tian, Mark Lumsden, Garrett Granroth, David Mandrus, Jae-Ho Chung, Stephen Nagler The spin ½ magnet dimethylammonium trichlorocuprate, DMACuCl$_{3}$, has been considered both an alternating ferromagnetic-antiferromagnetic chain along the crystalline a-axis as well as a collection of isolated ferromagnetic and antiferromagnetic spin pairs. However, no spectroscopic probes have been applied until now to determine the underlying nature of the spin dynamics. We present results of single crystal inelastic neutron scattering measurements which clearly indicate that the magnetic excitations in DMACuCl$_{3 }$propagate along the crystalline b-axis, not the proposed a-axis. A single magnetic excitation is observed with a gap of 0.98 meV and a bandwidth of approximately 0.67 meV. The quasi-one-dimensional excitation spectrum is dominated by a strongly coupled ferromagnetic spin-pair or dimer with weaker antiferromagnetic interdimer interactions. In light of this classification, we make comparisons to previously measured specific heat and magnetization measurements. We also place a limit on inter-dimer interactions perpendicular to the chain axis and propose a potential multi-chain model for DMACuCl$_{3}$. [Preview Abstract] |
Monday, March 13, 2006 12:51PM - 1:03PM |
B23.00009: Copper Phenazine Dichloride, a S=1/2 Heisenberg Antiferromagnetic Chain R. T. Schneider, C. P. Landee, M. M. Turnbull, F. F. Awwadi Low-dimensional antiferromagnets provide good testing grounds for predictions of cooperative quantum behavior. S=1/2 copper ions bridged by organic ligands such as pyrazine form antiferromagnetic chains with exchange strengths weak enough to be overcome by available magnetic fields. [1] Phenazine (Phz) (C$_{12}$H$_{8}$N$_{2})$ is similar to pyrazine in its ability to form magnetic chains but has a different electronic structure resulting in different exchange strengths. We report on the synthesis, crystal structure, magnetic susceptibility, and high field magnetization of Cu(Phz)Cl$_{2}$. Its behavior can be described as that of a Heisenberg antiferromagnetic chain with an exchange constant $\vert $J$\vert $/k$_{B}$ = 33 K. The exchange strengths of chains with pyrazine, quinoxaline, and phenazine will be compared. [1] M. B. Stone et al, \textit{Physical Review Letters} \textbf {91}, 037205 (2003). [Preview Abstract] |
Monday, March 13, 2006 1:03PM - 1:15PM |
B23.00010: Real-Space Treatment of Spin-Peierls Transitions: Gap Equation of the S = 1/2 Heisenberg Antiferromagnetic Chain Zoltan Soos, Sharon Bewick The spin-Peierls transition at T$_{SP}$ of s = 1/2 Heisenberg AF chains with linear spin-phonon (sp-ph) coupling is modeled by exact solution up to 18 spins. Spin correlations alter the gap relation between T$_{SP}$ and the singlet-triplet gap, E$_{ST}$, from the free-fermion or mean-field (mf) result by 30{\%}. Direct solution accounts for the spin-Peierls transition, magnetic susceptibility and magnetization of the prototypical TTF$^{+}$ (tetrathiafulvalene) spin chain in TTFCu(BDT). Strong sp-ph coupling is required for size convergence. Exact analysis of finite chains conserves total spin, while mf or bosonization approximations for spinless fermions in infinite chains only conserve the z component and have axial rather than spherical symmetry in spin space. [Preview Abstract] |
Monday, March 13, 2006 1:15PM - 1:27PM |
B23.00011: Phonon spectra in Spin-Peierls transitions Franz Michel, Hans Gerd Evertz We present phonon and spin spectra from precise Quantum Monte Carlo simulations of Spin-Peierls systems, in one and two dimensions with acoustical and with optical phonons. We introduce a new technique which allows the direct integration of phonons for a given spin configuration. Together with cluster simulations for spins and a mapping from SSE to continuous time this allows efficient simulations of large systems at finite temperature and close to T=0. [Preview Abstract] |
Monday, March 13, 2006 1:27PM - 1:39PM |
B23.00012: Spin excitations in $S=1/2$ AFM chains with alternating $g$-tensor and the Dzyaloshinskii-Moriya interaction. S.A. Zvyagin, J. Wosnitza, A.K. Kolezhuk, J. Krzystek, R. Feyerherm The magnetic excitation spectrum of copper pyrimidine dinitrate (Cu-PM), an $S=1/2$ antiferromagnetic chain system with alternating $g$-tensor and the Dzyaloshinskii-Moriya interaction, is probed using ESR spectroscopy in magnetic fields up to 25 T. The field-induced gap is measured $directly$. Signatures of a soliton and three breather branches are identified, and their frequency-field dependences are described in frame of the quantum field sine-Gordon model. Furthermore, the ESR linewidth and shift behavior in Cu-PM in the $perturbative~spinon$ regime is studied as function of temperature and field. Excellent quantitative agreement between theoretical predictions (Phys. Rev. Lett. 82, 5136 (1999)) and experiment is obtained. The results are published in: S.A. Zvyagin $et~al.$, Phys. Rev. Lett. 93, 027201 (2004); $ibid.$ 95, 012707 (2005). [Preview Abstract] |
Monday, March 13, 2006 1:39PM - 1:51PM |
B23.00013: Dzyaloshinskii-Moriya interaction in LiCu$_2$O$_2$ Laszlo Mihaly, Helmuth Berger, Laszlo Forro The far-infrared optical spectrum of the spin 1/2 helimagnet LiCu$_2$O$_2$ was investigated in magnetic fields up to 14Tesla. In the paramagnetic state a spin resonance line was observed corresponding to a $g$-factor of about 2. The resonance broadens and practically disappears around the phase transition temperature of T=25K, but it is recovered at lower temperatures at a new position. The 2.5K field dependence of the spin susceptibility and resonance frequency is characteristic of a magnetically ordered system, where the Dzyaloshinskii-Moriya interaction causes a finite gap of 1.5meV in the spin wave spectrum at q=0. [Preview Abstract] |
Monday, March 13, 2006 1:51PM - 2:03PM |
B23.00014: Meron and Haldane-Shastry spin chain states in quantum dots Milica Milovanovic, Aleksandra Petkovic We study solutions of the Coulomb interaction problem, in the lowest Landau level approximation, of a quantum dot with N = 4 and N = 6 electrons, without Zeeman term, and above the maximum density or filling factor one state. By making maps of spin correlations of the solutions we are able to connect excitations of the dot with states of the (one-dimensional) Haldane-Shastry spin chain. We also argue that the solutions of the problem without Zeeman term can be described as meron quasiparticle excitations analogous to vortex excitations in the completely polarized state. As a result we will describe how these meron excitations and states of the dot can be connected with spinon states of the Haldane-Shastry chain. This connection will enable us to associate orbital spin currents to quantum dot states. [Preview Abstract] |
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