Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session S7: Focus Session: Low-D Quantum Spins II |
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Sponsoring Units: GMAG Chair: Tom Lancaster, Durham University Room: 106 |
Thursday, March 6, 2014 8:00AM - 8:12AM |
S7.00001: Haldane gap evolution in quasi-one-dimensional spin-1 Heisenberg antiferromagnets Keola Wierschem, Pinaki Sengupta We study a spatially anisotropic spin-1 antiferromagnetic Heisenberg model on simple cubic lattices that is equivalent to a system of coupled chains with interchain coupling $J$ and intrachain coupling set to unity. In the limit of uncoupled chains ($J=0$), the ground state is known to be gapped as per the Haldane conjecture. As the coupling is turned on, this gapped phase persists up to a critical value $J_c$ beyond which there is a quantum phase transition to the gapless N\'{e}el state with long range magnetic order. Using the stochastic series expansion quantum Monte Carlo method, we accurately determine $J_c$ and calculate the string order parameter in the gapped phase for $0 |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S7.00002: S=2 quasi-one-dimensional spin waves in CrCl2 Matthew Stone, Georg Ehlers, Garrett Granroth We examine the magnetic excitation spectrum in the $S=2$ Heisenberg antiferromagnet CrCl$_2$. Inelastic neutron scattering measurements on powder samples are able to determine the significant exchange interactions in this system. A large anisotropy gap is observed in the spectrum below the N\'{e}el temperature and the ratio of the two largest exchange constants is $J_c / J_b = 9.1 \pm 2.2$. However, no sign of a gapped quantum spin liquid excitation was found in the paramagnetic phase. The research was performed at Oak Ridge National Laboratory's Spallation Neutron Source and was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S7.00003: Antiferromagnetic Ordering of Mn(III)F(salen) M.W. Meisel, Tong Wang, S.E. Brown, M. Botko, E. \v{C}i\v{z}m\'ar, O.N. Risset, D.R. Talham Due to a report suggesting Mn(III)F(salen), salen = H$_{14}$C$_{16}$N$_2$O$_2$, is an $S = 2$ Haldane system with $J/k_B = 50$~K and no long-range order down to 2~K based on standard magnetometry studies,\footnote{T.~Birk \emph{et al.}, Inorg.~Chem.~{\bf50} (2011) 5312.} specific heat and NMR measurements were performed. Using small single crystals, specific heat studies revealed the presence of an anomaly near 23~K, and this response was robust in fields up to 9~T. The $^1$H NMR results performed on a single crystal in 1~T revealed a sharp transition characteristic of antiferromagnetic ordering at 22.5~K. Measuring the magnetic response of the same single crystal in a commercial magnetometer reveals the presence of a subtle feature, near 23~K, that is not resolved with as-grown, randomlly oriented microcrystalline samples. These findings provide insight into the results obtained in torque magnetometry, EPR, and neutron scattering data.\footnote{J.-H.~Park \emph{et al.}, Acta Phys.~Pol.~A, in press.} [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 9:12AM |
S7.00004: Molecular-based 2D S $=$ 1/2 Heisenberg Antiferromagnetic Layers and Ladders Invited Speaker: Christopher Landee Low dimensional Quantum Heisenberg Antiferromagnets (QHAF) have long provided materials [1] with which to examine the influence of dimensionality and exchange anisotropy on critical behavior of cooperative systems. Molecular-based QHAF have provided materials with low exchange strengths ($\approx $ 10 K), facilitating examination of the compounds up to the saturation fields using current facilities. This presentation will provide an overview of recent developments of two classes of molecular magnets: 2D QHAF and spin ladders. Recent specific heat studies of Cu(pz)$_{2}$(ClO$_{4})_{2}$ in fields up to 45 tesla have determined the (H,T) phase diagram for this quasi-2D QHAF; the results will be compared to the results of QMC simulations of the diagram as a function of the intralayer exchange $J$, the interlayer exchange $J'$, and the XY-exchange anisotropy parameter. Developments in the study of spin ladders include the discovery of Luttinger liquid behavior for two molecular-based spin layers: the strong-rung ladder BPCB, (piperidinium)$_{2}$CuBr$_{4}$ [2] and the strong-rail ladder DIMPY, (2,3-dimethypyridinium)$_{2}$CuBr$_{4}$ [3]. The properties of a new, isotropic spin ladder will be reported. \\[4pt] [1] L. J. de Jongh and A. R. Miedema, Adv. Phys. \textbf{50}, 947 (2001).\\[0pt] [2] M. Klanj\v{s}ek \textit{et al}, Phys. Rev. Lett. \textbf{101}, 137207 (2008); Ch. R\"{u}egg \textit{et al}, Phys. Rev. Lett. \textbf{101}, 247207 (2008).\\[0pt] [3] K. Ninios \textit{et al}, Phys. Rev. Lett. \textbf{108}, 097201 (2012); D. Schmidiger \textit{et al}, Phys. Rev. Lett. \textbf{108}, 167201 (2012). [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S7.00005: $\mu^+$SR study of spin dynamics in the one dimensional Heisenberg antiferromagnet Cu(pyz)(NO$_3$)$_2$ Fan Xiao, Tom Lancaster, Rob Williams, Johannes Moeller, Stephen Blundell, Francis Pratt, Peter Baker, Jamie Manson We present the results of longitudinal-field muon spin relaxation ($\mu^+$SR) measurements on the one-dimensional quantum Heisenberg antiferromagnet (1DQHAF) Cu(pyz)(NO$_3$)$_2$ (pyz=pyrazine). The intrachain coupling strength $J/k_{\mathrm{B}}$ in this compound is 10.6 K and the ordering temperature $T_{\mathrm{N}}$ is 0.11 K. Spin dynamics were studied with $\mu^+$SR using applied longitudinal fields at two temperatures ($T$=0.33 K and 1.4 K) between $T_{\mathrm{N}}$ and $J/k_{\mathrm{B}}$. For $B>5$ mT, the nuclear contribution to the relaxation rate is quenched and the data can be fitted to an exponential decay along with a background correction. The relaxation rate $\lambda$ was found to follow a $\lambda\propto B^{-1/2}$ power law between 10 mT and 100 mT at both temperatures, suggesting diffusive spin transport of excitations in this 1DQHAF. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S7.00006: Spin Pseudogap in Ni-Doped SrCuO$_2$ Gediminas Simutis, Martin Mansson, Severian Gvasaliya, Alexander Chernyshev, Ashwin Mohan, Surjeet Singh, Christian Hess, Bernd B\"{u}chner, Andrei Savici, Alexander Kolesnikov, Andrea Piovano, Toby Perring, Igor Zaliznyak, Andrey Zheludev Effect of spin-1 impurities on the spectrum of an archetypical Heisenberg antiferromagnetic spin-$\frac{1}{2}$ chain SrCuO$_2$ is studied by inelastic neutron scattering [1]. We find that a spin pseudogap appears in the spectrum upon introduction of the impurities. We show that the pseudogap is a generic feature of quantum spin chains with dilute defects. This allows us to express the dynamic structure factor in a universal scaling form even for the system with fragmented chains. A simple model based on chain fragmentation shows good quantitative agreement with the experimental data for a broad temperature range.\\[4pt][1] Simutis et al, PRL 111, 067204 (2013) [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S7.00007: Interplay of covalency and correlations in the edge shared spin 1/2 $A_{3}T_{2}$O$_{4}$ chain compounds ($A$ = Na, K; $T$ = Cu, Ni) Deepa Kasinathan, Klaus Koepernik, Helge Rosner Na$_{3}$Cu$_{2}$O$_{4}$, K$_{3}$Cu$_{2}$O$_{4}$ and K$_{3}$Ni$_{2}$O$_{4}$ belong to a new class of quasi-1D insulating cuprates which feature strongly buckled, one-dimensional $^{1}_{\infty}$CuO$_{2}$ ribbon-like chains consisting of edge-sharing CuO$_{4}$ plaquettes. Structural analysis of the metal-oxygen bond lengths and thermodynamic measurements[1,2,3] imply that these systems are intrinsically charge ordered ($\ldots$ (Ni/Cu)$^{2+}$-(Ni/Cu)$^{3+}$-(Ni/Cu)$^{2+}$-(Ni/Cu)$^{3+}\ldots$) and show dominant antiferromagnetic interactions. No electronic structure analysis of these systems exist to date. Using density functional theory based calculations (LDA, Wannier functions, LDA+$U$), we analyze the microscopic origin of the magentic interactions in these systems. The main interaction along the chains are the second neighbor superexchanges. Nonetheless, a careful analysis of the first neighbor interaction between the magnetic (Cu$^{2+}$/Ni$^{3+}$) cation and the non-magnetic cation (Cu$^{3+}$/Ni$^{2+}$) is necessary. We report on the interplay of covalency, crystal field splitting and correlations in these systems. [1] Z. Anorg. Allg.Chem. vol. 462, 92 (1980). [2] J. Solid State Chem. vol. 178, 3708 (2005). [3] Z. Anorg. Allg. Chem. vol. 637, 1101 (2011). [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S7.00008: Localization of Spinons in Random Majumdar-Ghosh Chains Guillaume Roux, Arthur Lavarelo We study the effect of disorder on frustrated dimerized spin-1/2 chains at the Majumdar-Ghosh point. Using variational methods and density-matrix renormalization group approaches, we identify two localization mechanisms for spinons which are the deconfined fractional elementary excitations of these chains. The first one belongs to the Anderson localization class and dominates at the random Majumdar-Ghosh point. There, spinons remain gapped and localize in Lifshitz states whose localization length is analytically obtained. The other mechanism is a random confinement mechanism which induces an effective interaction between spinons and brings the initially gapped antiferromagnetic chain into a gapless and partially polarized phase for arbitrarily small disorder. This Imry-Ma mechanism induces domains which statistics is analyzed. Last, the connection to the real-space renormalization group method suited for the strong disorder limit is discussed. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S7.00009: Generation of chiral solitons in antiferromagnetic chains by a quantum quench Barbara Bravo, Ariel Dobry, Diego Mastrogiuseppe, Claudio Gazza In classical nonlinear physics, solitons are peculiar solutions which can be characterized by constant velocity and shape. In a recent paper [1], pursuing the understanding of the extension of the soliton concept to the quantum regime, the easy-axis ferromagnetic $XXZ$ model was chosen to analyze the evolution of a localized wave packet. It was shown that, besides the delocalization due to the uncertainty principle, they are in qualitative agreement with their classical counterparts. Following the objective of deciphering the quantum soliton term, we analyze the time evolution of a magnetic excitation in a spin-$\frac12$ antiferromagnetic Heisenberg chain after a quantum quench. By a modulation of the magnetic exchange, we prepare a static soliton of total spin $\frac{1}{2}$ as an initial state. Using bosonization and a time dependent density matrix renormalization group algorithm, we show that the excitation evolves to a state composed of two counter-propagating chiral states, which interfere to yield $<\!\!S^z\!\!> = \frac14$ for each mode. These dynamically generated states remain considerably stable in time. We propose spin-Peierls materials and ultracold-atom systems as experimental scenarios to conduct and observe this mechanism. [1] Phys. Rev. B 85, 184433 (2012). [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S7.00010: Theoretical study of the electronic and magnetic properties of $\beta$-TeVO$_4$ Andres Saul, Guillaume Radtke The $\beta$ phase of this compound can be described by zigzag chains formed by VO$_5$ distorted square pyramids sharing corners. This oxide, with V$^{4+}$ ions as magnetic centers, can be thus seen as a realization of a quasi-one-dimensional Heisenberg S=1/2 Hamiltonian. The corner-sharing of the VO$_5$ pyramids could lead to the prediction of AFM nearest neighbor interactions mediated by a weak super-exchange mechanism opening the possibility of complex magnetic properties due to competing next nearest-neighbors or inter-chain interactions. In this work we have studied its electronic and magnetic properties using density functional calculations. In particular, we evaluated the magnetic couplings on the basis of broken-symmetry formalism. We have performed extensive calculations comparing the results of the standard GGA (PBE) functional to the hybrid PBE0 functional and two different GGA+U implementations (SIC and AMF). The overall picture that arises from our calculations is of a frustrated AFM system with small FM nearest neigbors interactions but larger AFM nearest neighbors couplings. We discuss our results in the framework of the Kugel-Khomskii model using a projection of the electronic structure in localized Wannier functions. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S7.00011: Incommensurate dynamic correlations and continuum scattering in BiCu$_2$PO$_6$ Kemp Plumb, G.J. Shu, G.E. Granroth, A.T. Savici, Zahra Yamani, M. Matsuda, F.C. Chou, Young-June Kim We report comprehensive inelastic neutron scattering measurements on single crystals of the frustrated two-leg ladder BiCu$_2$PO$_6$, whose ground state is a spin liquid phase with no static magnetic correlations down to 5 K. A combination of triple-axis and time-of-flight experiments were performed to explore magnetic excitations over a broad range of phase space. Operation of the instruments in a high resolution configuration enabled a detailed measurement of the dynamical structure factor over many Brillouin zones; revealing an extremely rich and highly unusual magnetic excitation spectrum. Two branches of steeply dispersing long-lived spin excitations are observed with gaps of 1.90(9) meV and 3.95(8) meV. Significant frustrating next-nearest-neighbor interactions along the ladder-leg drive the minimum of each excitation branch to incommensurate wavevectors 0.574$\pi$ and 0.553$\pi$ for the lower and upper energy branches respectively. Intriguingly, the spin excitations merge into a broad continuum near the top of each excitation band which persists to an upper boundary of 40 meV. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S7.00012: Full magnetic dispersion relation in the frustrated quasi-1D ferromagnet Ca$_2$Y$_2$Cu$_5$O$_{10}$ M. Matsuda, J. Ma, V. O. Garlea, S. Nishimoto, S.-L. Drechsler, R. O. Kuzian, T. Ito, H. Yamaguchi, K. Oka Ca$_2$Y$_2$Cu$_5$O$_{10}$ consists of edge-sharing CuO$_2$ chains, in which Cu$^{2+}$ ions carry spin 1/2. The nearest-neighbor ($J_1$) and the next-nearest-neighbor interaction ($J_2$) are ferromagnetic and antiferromagnetic, respectively. For the $J_1$-$J_2$ model the theory predicts that when the ratio $\alpha$(=$|J_2/J_1|$) becomes larger than 0.25, the ground state becomes a spiral state. For the aforementioned compound, Kuzian et al. determined $\alpha$ to be 0.19, which is close to the critical value [1]. However, the parameters were fitted using the observed data up to $\sim$10 meV, above which the magnetic excitations were found to be broadened [2]. In order to determine the overall dispersion relation, we performed inelastic neutron scattering experiments using the HYSPEC neutron spectrometer at the SNS. We succeeded in observing the full magnetic dispersion that extends up to $\sim$55 meV. As previously observed, the magnetic excitations appeared to almost vanish at $\sim$11.5 meV. We also found another noticeable gap-like behavior at $\sim$28 meV. We re-evaluate the essential exchange coupling parameters and discuss the origin of gap-like regions in the spin-wave dispersion. [1] R. O. Kuzian et al., PRL109, 117207 (2012). [2] M. Matsuda et al., PRB63, 180403 (2001). [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S7.00013: A hierarchy of ``meson" bound state excitations in the 1D ferromagnetic Ising chain CoNb$_2$O$_6$ Christopher Morris, Seyed Koopayeh, Anirban Ghosh, Oleg Tchernyshyov, Tyrel M. McQueen, N. Peter Armitage, Rolando Vald\'es Aguilar, Jason Krizan, Robert J. Cava The quantum magnet CoNb$_2$O$_6$ was recently demonstrated to be an excellent realization of the one-dimensional ferromagnetic Ising spin chain. Low energy spin-flip excitations in the chains were recently observed via inelastic neutron scattering.\footnote{R. Coldea, \textit{et al}, Science \textbf{327}, 177 (2010)} The energy spectrum of these excitations was shown to have a interesting energy scaling governed by symmetries of the E8 exceptional Lie group. Here, time-domain terahertz spectroscopy (TDTS) is used to investigate these optically active spin flip excitations in CoNb$_2$O$_6$. A series of nine spin flip bound states is observed, whose energies can be modeled exceedingly well by the Airy function solutions to a 1D Schr{\"o}dinger equation. Additionally, a novel bound state of excitations on neighboring chains is observed just below the onset of a two particle continuum. [Preview Abstract] |
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