Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session H34: Focus Session: Frustrated and Low-D Magnetism -- Spin Chains and Ladders |
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Sponsoring Units: DMP GMAG Chair: Andrei Bernevig, Princeton University Room: E144 |
Tuesday, March 16, 2010 8:00AM - 8:12AM |
H34.00001: The magnetic structure of the zigzag chain family Na$_{x}$Ca$_{1-x}$V$_2$O$_4$ determined by muon-spin rotation Oren Ofer, Tatsuo Goko, Jess H. Brewer, Eduardo J. Ansaldo, Jun Sugiyama, Yutaka Ikedo, Kim Chow, Martin M\aa{}nsson, Hiroya Sakurai We present a muon spin rotation measurements on the complete
zig-zag chain Na$_x$Ca$_{1-x}$V$_2$O$_4$ family. In this family
we explore the magnetic properties from the metallic NaV$_2$O$_4$
to the insulating CaV$_2$O$_4$. We find a critical $x_c$ which
separates the low and high $x$ transition temperature and its
magnetic ground state. For $x |
Tuesday, March 16, 2010 8:12AM - 8:24AM |
H34.00002: Magnetic Ordering in the Frustrated Heisenberg Chain System CuCl$_2$ R. K. Kremer, M. G. Banks, C. Hoch, A. Simon, B. Ouladdiaf, J.-M. Broto, H. Rakoto, C. Lee, W.-H. Whangbo We report a detailed examination the magnetic structure of anhydrous cupric chloride CuCl$_2$ carried out by powder neutron diffraction, magnetic susceptibility and specific heat measurements as well as an evaluation of the spin exchange interactions by first principles density functional theory (DFT) calculations. Anhydrous CuCl$_2$ shows one dimensional antiferromagnetic behavior and long range antiferromagnetic ordering below a N$\acute{\rm e}$el temperature of 23.9 K. Neutron powder and single crystal diffraction reveal that, below 23.9 K, CuCl$_2$ undergoes a phase transition into an incommensurate magnetic structure (propagation vector (1,0.2257,0.5) with a spin-spiral propagating along $b$ and the moments confined in the $bc$ crystallographic plane. Our DFT calculations show that the spin-spiral results from competing ferromagnetic nearest neighbor and antiferromagnetic next-nearest neighbor spin-exchange interaction along the spin chains. Implications for possible multiferroic behavior of CuCl$_2$ are discussed. [Preview Abstract] |
Tuesday, March 16, 2010 8:24AM - 8:36AM |
H34.00003: Synthesis and structural characterization of 2Dioxane$\cdot$2H$_2$O$\cdot$CuCl$_2$: Metal-organic compound with Heisenberg antiferromagnetic S= 1 2 chains Tao Hong, R. Custelcean, B.C. Sales, B. Roesslie, D.K. Singh, A. Zheludev A novel organometallic compound 2Dioxane$\cdot$2H$_2$O$\cdot$CuCl$_2$ (CuDCl) has been synthesized and structurally characterized by X-ray crystallography. Magnetic susceptibility and zero-field inelastic neutron scattering have also been used to study its magnetic properties. It turns out that this material is a weakly coupled one-dimensional S=1/2 Heisenberg antiferromagnetic chain system with chain direction along the crystallographic \textbf{c} axis and the nearest-neighbor intra-chain exchange constant \emph{J}=0.85(4) meV. The next-nearest-neighbor inter-chain exchange constant $J'$ is also estimated to be 0.05 meV. Unlike other compounds, the Cl concentration in CuDCl can be arbitrarily tuned by Br substitution. As a result, the strength of superexchange coupling \emph{J} changes dramatically with different Br concentrations \emph{x}. [Preview Abstract] |
Tuesday, March 16, 2010 8:36AM - 9:12AM |
H34.00004: Spin excitations and magnon fractionalization in a quantum spin ladder Invited Speaker: Christian Ruegg The quantum spin ladder is arguably the most prototypical model system in theoretical quantum magnetism. Recent progress in the field was triggered by the development of efficient theoretical methods for simulations of relatively large finite-size systems (DMRG, ED); on the other hand, experimental work was for decades limited by the sparse number of model materials. The compound (C$_{5}$H$_{12}$N)$_{2}$CuBr$_{4}$ [1-3], representing the limit of exceptionally weak magnetic exchange realised in metal-organic materials, is a rare exception. It is well-suited for studies of the elementary excitations and the phase diagram of the quantum spin ladder. The phase diagram of quasi--1D arrays of quantum spin ladders in temperature and magnetic field is particularly rich: quantum disordered, quantum critical, spin Luttinger--liquid, BEC, and classically saturated phases can be studied and their characteristic excitations explored. We measured by inelastic neutron scattering the spin excitation spectra in (C$_{5}$H$_{12}$N)$_{2}$CuBr$_{4}$ as a function of applied magnetic field across these phases and the two intrinsic quantum critical points. Discrete magnon modes at low fields in the quantum disordered phase and at high fields in the saturated phase contrast sharply with a spinon continuum at intermediate fields characteristic of the spin Luttinger--liquid phase. By tuning the magnetic field, we drive the fractionalization of magnons into spinons and, in this deconfined regime, observe both commensurate and incommensurate continua. \\[4pt] [1] Ch. R\"uegg \textit{et al.}, Phys. Rev. Lett. {\bf 101}, 247202 (2008).\\[0pt] [2] B. Thielemann \textit{et al.}, Phys. Rev. B {\bf 79}, 020408(R) (2009).\\[0pt] [3] B. Thielemann \textit{et al.}, Phys. Rev. Lett. {\bf 102}, 107204 (2009). [Preview Abstract] |
Tuesday, March 16, 2010 9:12AM - 9:24AM |
H34.00005: CrSb$_{2}$ is a S=1 Quasi-one-Dimensional Antiferromagnet Matthew Stone, Mark Lumsden, Stephen Nagler, Brian Sales, David Mandrus, David Singh First principle calculations have predicted the existence of a quasi-one-dimensional antiferromagnet in the intermetallic compound CrSb$_{2}$ (S=1). This is unexpected given that most low-dimensional magnets rely on anisotropies in crystal structure or bonding to produce the anisotropic exchange constants which ultimately lead to their low-dimensionality. We present inelastic neutron scattering measurements of single-crystal samples of CrSb$_{2}$ which verify the existence of quasi-one-dimensional magnetism in this compound. Triple-axis and time-of-flight spectrometer measurements indicate a single spin-wave branch in all the three reciprocal space directions. The bandwidth of this excitation is only 25 meV along the H and K directions, but approximately 50 meV along the L reciprocal space direction. This corresponds to quasi-one-dimensional excitations propagating along the crystalline c-axis. These measurements provide the first conclusive evidence of a quasi-one-dimensional intermetallic antiferromagnet. [Preview Abstract] |
Tuesday, March 16, 2010 9:24AM - 9:36AM |
H34.00006: Ground state description of BiCu$_2$PO$_6$ and BiCu$_2$AsO$_6$ Deepa Kasinathan, Alexander Tsirlin, Oleg Janson, Helge Rosner Low dimensional spin systems have always been of interest to the physics community due to their inherent exotic magnetic properties. A further impetus for the study of low-dimensional spin systems was given by the discovery of spin-ladder materials, due to the fact that they are intermediate objects between 1D and 2D systems. Recent experiments by two groups [1,2] on a spin-ladder material BiCu$_2$PO$_6$ exhibited a gapped singlet ground state with a spin gap of about 34 K, though the strength of the spin-exchange interactions have remained controversial. No consensus has been reached on the correct spin-ladder model to describe this compound. We will report on our re-investigations of BiCu$_2$PO$_6$ and the related material BiCu$_2$AsO$_6$ using density functional theory based electronic structure calculations and as well as Transfer Matrix Renormalization Group (TMRG) calculations. Our results necessitate the inclusion of previously neglected exchange couplings to describe the spin-ladder model more adequately.\\[4pt] [1] B. Koteswararao, {\it et. al.}, Phys. Rev. B {\bf 76}, 052402 (2007).\\[0pt] [2] O. Mentr\'{e}, {\it et. al.}, Phys. Rev. B {\bf 80}, 180413 (2009). [Preview Abstract] |
Tuesday, March 16, 2010 9:36AM - 9:48AM |
H34.00007: Phase diagram of the frustrated spin ladder Oleg Starykh, Toshiya Hikihara We re-visit the phase diagram of the frustrated spin-$1/2$ ladder with two competing inter-chain antiferromagnetic exchanges, $J_\perp$ and $J_\times$. We suggest, based on the accurate renormalization group analysis of the low-energy Hamiltonian of the ladder, that marginal inter-chain current-current interaction plays central role in reducing the stability of the intermediate columnar dimer phase centered around the classical degeneracy line $J_\perp = 2J_\times$. Following this insight we then suggest that changing these competing inter-chain exchanges from the previously considered antiferromagnetic to the {\it ferromagnetic} ones eliminates the issue of the marginal interactions altogether and dramatically expands the region of stability of the columnar dimer phase. This analytical prediction is convincingly confirmed by the extensive density matrix renormalization group and exact diagonalization calculations. [Preview Abstract] |
Tuesday, March 16, 2010 9:48AM - 10:00AM |
H34.00008: Dynamical correlation functions in spin-$1/2$ ladders under a magnetic field Pierre Bouillot, Corinna Kollath, Lauchli Andreas, Mikhail Zvonarev, Thierry Giamarchi Our work is dedicated to magnetic properties of spin-$1/2$ ladders. These systems have recently generated a great interest due to the new experimental realization of $(Hpip)_2CuBr_4$. During the last few years, this compound has been the focus of numerous measurements like specific heat, magnetostriction, NMR and neutron scattering. We theoretically investigate these systems and determine the zero temperature dynamical correlations using time-dependent density matrix renormalization group. This numerical approach allows us to fully explore their spectrum for a broad range of magnetic fields. We are able to compute the high energy components that are not accessible by analytical methods. The calculated correlations are directly related to the neutron scattering cross section that we can predict with very good precision. [Preview Abstract] |
Tuesday, March 16, 2010 10:00AM - 10:12AM |
H34.00009: Low-temperature density matrix renormalization group study on spin-1/2 zigzag XY chains Shigetoshi Sota, Takanori Sugimoto, Takami Tohyama We have investigated the effect of the thermal fluctuation on spin-1/2 zigzag XY chains by using the Low-temperature dynamical density matrix renormalization group [1] that we have developed recently. At zero temperature, gapless and gapful excitations of the vector spin chirality are confirmed for systems with and without the chiral order, respectively, and change smoothly with temperature. On the other hand, at finite temperatures, the chirality excitation spectra show contrasting behaviors between the cases with and without the chiral order. In particular, we found a characteristic temperature in the gapful case, where the spectral weight changes dramatically. [1] S. Sota and T. Tohyama, Phys. Rev. B {\bf78}, 13101 (2008) [Preview Abstract] |
Tuesday, March 16, 2010 10:12AM - 10:24AM |
H34.00010: Entanglement spectrum in Spin Chains Ronny Thomale, Daniel P. Arovas, B. Andrei Bernevig We show that the entanglement spectrum can be used to define topological order in \emph{gapless} systems for which usual methods such as torus degeneracy fail. The gap \emph{fully} separates a series of generic, high `entanglement energy' levels, from a \emph{flat} band of levels with specific multiplicities that uniquely define the ground-state, and remains finite in the thermodynamic limit. We pick the appropriate set of quantum numbers, and then partition the system in this space. Despite the fact that the Laughlin state is bulk gapped while the antiferromagnetic spin chain state is bulk gapless, we show that the $S=1/2$ Heisenberg antiferromagnet in one dimension has an entanglement spectrum almost identical to that of the Laughlin Fractional Quantum Hall state in two dimensions, revealing the nature of their low-energy edge and bulk excitations respectively. [Preview Abstract] |
Tuesday, March 16, 2010 10:24AM - 10:36AM |
H34.00011: Characterization of multipolar liquid phases in frustrated spin chains from NMR relaxation rate Masahiro Sato, Toshiya Hikihara, Tsutomu Momoi, Akira Furusaki Recently, it has been shown that spin nematic (quadrupolar) or higher multipolar correlation functions exhibit a quasi long- range order in the broad region of the field-induced Tomonaga-Luttinger-liquid (TLL) phase in spin-1/2 zigzag chains. We point out [1] that the temperature dependence of the NMR relaxation rate $1/T_1$ in these multipolar TLLs is qualitatively different from that in more conventional TLLs of one-dimensional quantum magnets such as the spin-1/2 Heisenberg chain; $1/T_1$ decreases with lowering temperature in multipolar TLL. This behavior can be used as an indirect but definite signature for identifying the multipolar order. We also discuss some characteristic features of the field dependence of $1/T_1$ and spin dynamical structure factors. [1] M. Sato, T. Momoi, and A. Furusaki, PRB79, 060406(R) (2009). [Preview Abstract] |
Tuesday, March 16, 2010 10:36AM - 10:48AM |
H34.00012: Field-induced quantum phase transitions of the asymmetric three-leg spin tube Toru Sakai, Masahiro Sato, Kouichi Okunishi, Kiyomi Okamoto, Chigak Itoi The spin tube has attracted a lot of interest as a magnet with a nanotube structure. Various applicable functions of the spin tube are expected to be induced by magnetic field. The S=1/2 three-leg spin tube has a spin gap due to the strong frustration. Using the numerical dyagonalization and the density matrix renormalization group (DMRG) calculation, we revealed new quantum phase transitions between the spin-gap and gapless phases induced by an asymmetric interchain interactions [1]. Under high magnetic field this system also exhibits some interesting phenomena. The same numerical analysis on the magnetization process indicates that the 1/3 magnetization plateau appears for sufficently large rung interaction caused by two different mechanisms, depending on the asymmetry. Some magnetization cusps are also predicted by the DMRG calculation. In addition we found a field induced crossover between the effictive S=1/2 and S=3/2 magnon excitations and a field induced chiral order[2]. In the present study, we revealed a novel symmetry breaking at the 1/3 magnetization plateau. [1] T. Sakai, M. Sato, K. Okunishi, Y. Otsuka, K. Okamoto and C. Itoi, Phys. Rev. B 78 (2008) 184415. [2] M. Sato and T. Sakai, Phys. Rev. B 75 (2007) 014411. [Preview Abstract] |
Tuesday, March 16, 2010 10:48AM - 11:00AM |
H34.00013: Ground State phase diagram of the asymmetric spin tube up to 6 legs Ryo Igarashi, Masahiko Okumura, Susumu Yamada, Masahiko Machida We obtain the detailed ground state phase diagram of the asymmetric $S=1/2$ spin tube up to 6 legs using the two-dimensional density-matrix renormalization-group method. Spin gap is observed for the symmetric tube regardless of the leg size, but 3- and 5-leg system becomes gapless when one of the asymmetric interchain (rung) interaction is turned off. We determine the phase boundary between the spin gap phase and the gapless phase and discuss the leg-size dependence of the full gap region. [Preview Abstract] |
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