Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session G16: Focus Session: Quantum Spins |
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Sponsoring Units: GMAG DMP Chair: Mark Meisel, University of Florida Room: 318 |
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G16.00001: Excitations in a perfect magnetized quantum spin ladder Andrey Zheludev, David Schmidiger, Sebastian Muehlbauer, Gvasaliya Severian, Pierre Bouillot, Corinna Kollath, Thierry Giamarchi, Tatiana Guidi, Robert Bewley, Georg Ehlers The strong-leg $S=1/2$ Heisenberg spin ladder system C$_7$(D$_{10}$N)$_2$CuBr$_4$ is investigated in applied magnetic fields using inelastic neutron scattering anf DMRG calculations. The spectrum in the high-field Tomonaga-Luttinger spin liquid phase is found to be qualitatively different from that in the low-field spin gap phase. In the former, numerous spectral featrures, including incommensurate excitations and multi-spinon continua are identified. In contrast, the latter is dominated by long-lived magnon excitations and two-magnon bound states [1]. An unprecedented quantitive agreement between experiment and numerical claculations is achieved.\\[4pt] [1] D. Schmidiger, P. Bouillot, S. Muhlbauer, S. Gvasaliya, C. Kollath, T. Giamarchi, A. Zheludev, Phys. Rev. Lett. {\bf 108}, 167201 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G16.00002: The Phase Diagram of the Quantum Magnet SrCu2(BO3)2 Sara Haravifard, Arnab Banerjee, Jonathan Lang, George Srajer, Daniel Silevitch, Stefan Klotz, Bruce Gaulin, Thomas Hansen, Hanna Dabkowska, Thomas Rosenbaum SrCu2(BO3)2(SCBO) is one of the few real-world materials that corresponds to the Shastry-Sutherland model, with corner-sharing Cu S=1/2 dimers lying on a square lattice. The application of pressure can be used to tune the ground state of the system. High-resolution x-ray synchrotron experiments on SCBO at pressures up to 6 GPa reveal new structural peaks as a result of lattice distortions at low temperatures that we associate with long-range antiferromagnetic order. Additionally we have conducted high-pressure neutron diffraction measurements at pressures up to 7 GPa investigating the magnetic structure of SCBO and its link to structural distortions as a function of temperature. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G16.00003: Asymmetric thermal linehape broadening in a dimerised antiferromagnet - evidence for strong correlations at finite temperature B. Lake, D.L. Quintero-Castro, A.T.M.N. Islam, E.M. Wheeler, C. Balz, M. Mansson, K.C. Rule, S. Gvasaliya, A. Zheludev In the conventional picture of thermal effects in magnetism, the excitations are long-lived at low temperatures and their lifetime decreases with temperature. The explanation is that thermally activated excitations collide with each other limiting their lifetimes - observed experimentally as a symmetric Lorentzian energy broadening of the lineshape. This is confirmed for gapless magnets with long-range magnetic order. Here the excitations interact only weakly and fluctuate among the large range of available states in an uncorrelated manner. The damping is due simply to loss of coherence associated with the reduced lifetime. The concept of thermal decoherence and symmetric Lorentzian linewidth broadening is assumed to apply to all magnetic systems. This presentation will discuss Sr$_{3}$Cr$_{2}$O$_{3}$ which is 3-dimensional network of antiferromagnetic dimers with gapped magnon excitations. High resolution inelastic neutron scattering reveals that its lineshape broadens $asymmetrically$ with increasing temperature. This indicates that far from becoming increasingly incoherent with temperature, the excitations behave collectively like a strongly correlated gas of hard-core Bosons. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G16.00004: Magnetism of Ba$_4$Ru$_3$O$_{10}$ revealed by density functional calculations: Structural trimers behaving as coupled magnetic dimers Andres Saul, Guillaume Radtke, Yannick Klein, Gwenaelle Rousse From a simple ionic picture, the only magnetically active ions in this compound are the three Ru$\sp{4+}$ atoms which form trimers of faced shared RuO$_6$ octahedral. The Ru atom in the middle of the trimer (named Ru(1)) is cristallographically inequivalent to the ones at the corners (named Ru(2)). A na\"ive analysis of the magnetic properties of this compound compatible with the expected low spin magnetic configuration of the Ru ions would predict a complicate magnetic order at low temperature involving the Ru(1) and Ru(2) ions and a high temperature susceptibility corresponding to three S=1 ions per unit cell. In spite of that, we demonstrate in this work, from density functional calculations, that under the influence of Ru-Ru covalent bonding, the structural trimers behave in an extended range of temperature from 0 to 600K, as strong ($S=1$) antiferromagnetic dimers. Our calculations of the effective exchange interactions show a strong intra-dimer interaction and a weaker inter-dimer one which explains the antiferromagnetic order observed below $T_N = 105K$ and the magnetic susceptibility in the intermediate and high temperature range (from $T_N$=105K up to 612~K). [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G16.00005: Wilson ratio of a Tomonaga-Luttinger liquid in a spin-1/2 Heisenberg ladder Tao Hong, K. Ninios, Y.H. Kim, T. Manabe, C. Hotta, G. Tremelling, S.N. Herringer, M.M. Turnbull, C. Landee, H.-J. Kang, K.P. Schmidt, G.S. Uhrig, H.B. Chan, C. Broholm, Y. Takano We report a comprehensive study of a strong-leg spin-1/2 ladder compound (C7H10N)2CuBr4 (DIMPY) by specific heat, magnetocaloric effect, magnetization and inelastic neutron scattering measurements. DIMPY is shown to be a perfect one-dimensional Heisenberg antiferromagnet with a spin gap$=$0.32 meV. Above a critical field Hc and at temperature below 1 K, the specific heat exhibits asymptotic linear-T behavior, characteristic of a Tomonaga-Luttinger liquid (TLL). In this field and temperature region, the specific heat in conjunction with the susceptibility yields the Wilson ratio $R_{W}$. The result supports the relation $R_{W}=$4$K$, where $K$ is the TLL parameter. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G16.00006: High magnetic field studies of a spin-half dimmer Kim Modick, Ross McDonald, John Singleton, Paul Goddard, Jamie Manson We present high magnetic field studies of an organic molecular magnet system comprising of spin half copper dimmers. DC and pulsed field magnetometry combined with EPR indicate a small ($\sim$ 2 K) singlet triplet gap, and can be used to infer the sign of the triplon dispersion. Furthermore the low magnetic-exchange energy scales combined with the relatively soft organic framework of exchange pathways, indicate that the magnetic order can be readily tuned by temperature, magnetic field and pressure. The anisotropy between the effective mass of the top and bottom of the triplon band are analyzed in terms of the relative upper and lower critical fields for the onset of triplon condensation and magnetic saturation respectively. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 1:03PM |
G16.00007: Magnetic engineering with molecular bricks Invited Speaker: Stephen Blundell Magnetic materials can be constructed using molecular components to build up novel and unusual architectures. This approach provides an exciting opportunity for exploring the physics of magnetism. Gaining control of the building blocks of magnetic materials and thereby achieving particular characteristics will make possible the design and growth of bespoke magnetic devices. While progress in the synthesis of molecular materials, and especially coordination polymers, represents a significant step towards this goal, the ability to tune the magnetic interactions within a particular framework remains in its infancy but promising advances are being made, including the production of single molecule magnets and a variety of extended structures. We have recently found a chemical method which achieves dimensionality selection via preferential inhibition of the magnetic exchange in an S = 1/2 antiferromagnet along one crystal direction, switching the system from being quasi-two- to quasi-one-dimensional while effectively maintaining the nearest-neighbour coupling strength [1]. We have also demonstrated that single molecule magnets can be used to store quantum information and have devised a strategy for extending the spin coherence time by chemical adjustment [2]. Very recently we have found that introduction of a molecular spacer layer can produce a greater than fourfold enhancement in the superconducting transition temperature of iron selenide [3]. The experimental techniques used in this work include ESR, muSR and high magnetic fields. \\[4pt] [1] P. A. Goddard, J. L. Manson, J. Singleton, I. Franke, T. Lancaster, A. J. Steele, S. J. Blundell, C. Baines, F. L. Pratt, R. D. McDonald, O. E. Ayala-Valenzuela, J. F. Corbey, H. I. Southerland, P. Sengupta, and J. A. Schlueter, Phys. Rev. Lett. 108, 077208 (2012);\\[0pt] [2] C. J. Wedge, G. A. Timco, E. T. Spielberg, R. E. George, F. Tuna, S. Rigby, E. J. L. McInnes, R. E. P. Winpenny, S. J. Blundell, and A. Ardavan, Phys. Rev. Lett. 108, 107204 (2012); \\[0pt] [3] M. Burrard-Lucas, D. G. Free, S. J. Sedlmaier, J. D. Wright, S. J. Cassidy, Y. Hara, A. J. Corkett, T. Lancaster, P. J. Baker, S. J. Blundell, S. J. Clarke, Nature Materials, 11, December 2012. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G16.00008: Low-temperature studies of a 2D Quantum Heisenberg Antiferromagnet Christopher Landee, Fan Xiao, Mark Turnbull, Juan Bartolom\'e A recent inelastic neutron scattering experiment of a 2D Quantum Heisenberg Antiferromagnet (2DQHAF) in an applied field [1] revealed novel features in the energy spectrum but the field was limited to \textless 0.3 H$_{\mathrm{SAT}}$ due to the exchange strength (J $=$ 17.5 K) of the material under study. (Quinolinium)$_{2}$CuBr$_{4}$$\cdot$2H$_{2}$O is known [2] to be a molecular-based version of a strongly 2D QHAF with a significantly smaller exchange strength of 6.2 K and a saturation field of 15 T. We report the low-temperature properties (T \textless\ 1.8 K) of (Quinolinium)$_{2}$CuBr$_{4}$$\cdot$2H$_{2}$O and discuss its applicability for further investigations.\\[4pt] [1] N. Tsyrulin, T. Pardina, R. R. P. Singh et al, \textit{Phys. Rev. Lett}. \textbf{102}, 197201: 1-4 (2009).\\[0pt] [2] R. T. Butcher, M. M. Turnbull, C. P. Landee et al, \textit{Inorg. Chem.} \textbf{49}, 427-434 (2010). [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G16.00009: Control of Crystal Structure and Magnetism in Copper(II) Fluoride Based Coordination Polymers John Schlueter, Saul Lapidus, Jamie Manson Whereas magnetic copper (II) halide (halide $=$ chloride or bromide) coordination polymers have been frequently studied, the copper(II) fluoride analogs have been much less investigated. This is due in part to synthetic challenges associated with solubility and reactivity. In analogy to cuprates, Cu-F-Cu linkages are expected to provide strong magnetic superexchange. The magnetic structure of such systems can be tuned by choice of ancillary ligand. Herein, we describe the use of various pyridines, diazines, and triazoles that have the ability to define the structural and magnetic dimensionality. Hydrogen bonding to the fluoride ligand provides an additional opportunity for designing molecule-based materials through a `crystal-engineering' approach. Competition for intermolecular interactions frequently enables stimuli responsive behavior, including pressure-induced phase transitions. This will be illustrated for the CuF$_{\mathrm{2}}$(H$_{\mathrm{2}}$O)$_{\mathrm{2}}$(pyrazine) coordination polymer and the five-coordinate CuF$_{\mathrm{2}}$(H$_{\mathrm{2}}$O)$_{\mathrm{2}}$(3-chloropyridine) molecular solid. The use of new synthetic methods, including the use of high pressure, will be described. Subtle changes in reaction conditions leads to significant changes in structural and magnetic properties. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G16.00010: Finite-temperature valence-bond-solid transition of quantum spins in two dimensions Songbo Jin, Anders Sandvik The $S=1/2$ Heisenberg model on the 2D square lattice with four- or six-neighbor spin interactions (JQ model) hosts a quantum phase transition between N\'eel and valence-bond-solid (VBS) ground states. The deconfined quantum critical (DQC) point, predicted by the theory of Senthil \emph{et al.}[1], may be realized in this model [2]. Here we study the finite-temperature phase transition between the VBS ($Z_4$ symmetry breaking) to the paramagnetic state. We find continuously changing exponents with the correlation-length exponent $\nu$ close to the Ising value far from the $T=0$ critical point, and diverging when the critical temperature $T_c \rightarrow 0^+$. This is in accord with the DQC theory, according to which the transition for $T_c \rightarrow 0^+ $ should approach a Kosterlitz-Thouless fixed point.\\[4pt] [1] T. Senthil, L. Balents, S. Sachdev, A. Vishwanath, and M. P. A. Fisher, Phys. Rev. B {\bf 70}, 144407 (2004).\\[0pt] [2] R. K. Kaul, R. G. Melko, A. W. Sandvik, arXiv:1204.5405. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G16.00011: Ordering in weakly coupled random singlet spin chains Matthias Thede, F. Xiao, Ch. Baines, C. Landee, E. Morenzoni, A. Zheludev We study the effect of bond randomness on long range magnetic ordering in quasi-one-dimensional antiferromagnets, where the introduction of arbitrary weak bond randomness gives rise to the so-called random singlet phase. We investigated weakly coupled spin chain systems by local (muon spin rotation/relaxation) and bulk measurements (susceptibility and specific heat). The material Cu(py)$_2$(Cl$_{1-x}$Br$_x$)$_x$ is an organic tunable spin chain which has an average intrachain coupling constant between J = 2.3 meV (x=0) and J = 4.5 meV (x = 1). The disorder free end materials order magnetically at T$_N$ = 1.15 K (x=0) and T$_N$ = 0.72 K (y=0), respectively. Bond disorder strongly affects the magnetically ordered phase. In apparent contradiction with chain mean field theory [1] bond randomness strongly suppresses both the ordered moment and the ordering temperature T$_N$ [2]. We will also report about similiar results in BaCu$_2$(Si$_1-x$Ge$_x$)$_2$O$_7$.\\[4pt] [1] A. Joshi et. al, Phys. Rev. B 67, 174403 (2003).\\[0pt] [2] M. Thede, et. al, arXiv:1208.6479 [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G16.00012: Pressure-Induced Ferromagnetic Interactions in the Molecule-based Magnet Mn(dca)$_2$ P.A. Quintero, M.K. Peprah, M.W. Meisel, D. Rajan, D.R. Talham Using SQUID magnetometry, we have studied the pressure dependence of the magnetization of the three-dimensional antiferromagnetic coordination polymer Mn(N(CN)$_2$)$_2$, referred to as Mn(dca)$_2$, up to 1.2 GPa and down to 5~K. The isostructural compounds M(dca)$_2$, where M = Fe, Co, and Ni, have been previously studied by others and are known to show variations in their transition temperatures of up to 26\% for pressures as large as \mbox{1.7 GPa.}\footnote{C. J. Nuttall \textit{et al.}, Mol. Cryst. Liq. Cryst. \textbf{343} (2000) 227.} Our results on Mn(dca)$_2$ indicate a linear dependence of the transition temperature on the applied pressure, where a change of $48\%$ is measured at 1.2 GPa. In addition, a marked difference in the behavior of the magnetization is observed above and below 0.8 GPa. Specifically, for P $<$ 0.8 GPa, the magnetization decreases with increasing pressure, and for P $>$ 0.8 GPa, the behavior is inverted. These results indicate that external pressure changes the angle along the \mbox{Mn-[N(1)-C-N(2)]-Mn} superexchange path, thereby favoring ferromagnetic interactions.\footnote{C. R. Kmety \textit{et al.}, Phys. Rev. B \textbf{62} (2000) 5576.} [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G16.00013: Quantum critical dynamics in the one-dimensional spin chain compound copper pyrazine dinitrate probed by NMR spectroscopy Hannes Kuehne, A.P. Reyes, P.L. Kuhns, A.A. Zvyagin, S. Grossjohann, W. Brenig, M. Guenther, H.-H. Klauss, C.P. Landee, M.M. Turnbull The metalorganic compound copper pyrazine dinitrate is known to be one of the best realizations of the antiferromagnetic $S = 1/2$ Heisenberg chain model with a comparatively small nearest neighbor exchange constant $J/k_B$ = 10.7 K. The zero temperature saturation field $B_c$ = 14.6 T corresponds to a quantum critical point (QCP), where the system is driven from a Luttinger liquid state to ferromagnetic polarization. With an emphasis on the vicinity of the QCP, a comprehensive comparison of our experimental findings from $^{13}$C NMR spectroscopy with both numerical (quantum Monte Carlo) and analytical (conformal field theory) approaches is presented. In particular, this comparison reveals a well-defined maximum of $1/T_1$ $(B,T)$ \textit{below} $B_c$ as the signature of essential one-dimensional spin-spin interactions in the Luttinger liquid regime. [Preview Abstract] |
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