Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Y4: Focus Session: Frustrated and Low-Dimensional Magnets |
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Sponsoring Units: GMAG Chair: Matthew Stone, Oak Ridge National Laboratory Room: 112/110 |
Friday, March 7, 2014 8:00AM - 8:12AM |
Y4.00001: Resonating Valence Bond states for low dimensional S=1 antiferromagnets Zheng-Xin Liu, Yi Zhou, Tai-Kai Ng We study $S=1$ spin liquid states in low dimensions. We show that the resonating-valence-bond (RVB) picture of $S=1/2$ spin liquid state can be generalized to $S=1$ case. For $S=1$ system, a many-body singlet (with even site number) can be decomposed into superposition of products of two-body singlets. In other words, the product states of two-body singlets, called the singlet pair states (SPSs), are over complete to span the Hilbert space of many-body singlets. Furthermore, we generalized fermionic representation and the corresponding mean field theory and Gutzwiller projected stats to $S=1$ models. We applied our theory to study 1D anti-ferromagnetic bilinear-biquadratic model and show that both the ground states (including the phase transition point) and the excited states can be understood excellently well within the framework. Our method can be applied to 2D $S=1$ antiferromagnets. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y4.00002: A new S $=$ 1/2 frustrated ferromagnetic chain with random exchange Christopher Landee, Susan Herringer, Mark Turnbull, Jordi Ribas, Juan Novoa We report on a new frustrated ferromagnetic Heisenberg chain, CuCl$_{2}$(2-Cl-3-Mepy) in which the Cu(II) ions are linked into well-isolated chains via bichloride bridges. The pyridine ligand is two-site disordered, leading to three unique magnetic superexchange pathways occurring at random throughout the chains. Magnetic susceptibility measurements (1.8 -- 300 K) show dominant ferromagnetic interactions; a maximum in the $\chi $T product is observed near 30 K, but no maximum in $\chi $ occurs. \textit{DFT} calculations of the exchange strengths predict ferromagnet exchange strengths of 81(5), 55(5), and 23(3) K occurring at random, but with nearly constant next-nearest-neighbor interactions of -13(2) K. We note that within uncertainties the ratios of the nnn interaction to the smaller two ferromagnetic interactions exceed or equals the quantum critical ratio of 0.25 at which the ground state of the system changes. Comparison of the experimental $\chi $T data to simulations of random configurations will be presented. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y4.00003: Excitations of quasi-one-dimensional field-induced spin density wave and nematic phases Oleg Starykh, Leon Balents We study the excitation spectrum and dynamical response functions for several quasi-one-dimensional spin systems in magnetic fields without dipolar spin order transverse to the field. This includes both nematic phases, which harbor ``hidden'' breaking of spin-rotation symmetry about the field direction and have been argued to occur in high fields in certain frustrated chain systems with competing ferromagnetic and antiferromagnetic interactions, and spin density wave states, in which spin-rotation symmetry is truly unbroken. Using bosonization, field theory, and exact results on the integrable sine-Gordon model, we establish the collective mode structure of these states, and show how they can be distinguished experimentally. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y4.00004: Dzyaloshinskii-Moriya induced inter-chain frustration in a new spin-chain compound Wolfram Lorenz, Manuel Haelg, Kirill Yu. Povarov, Andrey Zheludev We present a first study of the Dzyaloshinski-Moriya spin-chain material K$_2$CuSO$_4$Br$_2$, highlighting the peculiar frustration of inter-chain couplings between nearest neighbor chains in this compound. Bulk magnetization and specific heat data are found well consistent with spin-chain models. Inelastic neutron scattering data support the pronounced one-dimensionality of the compound. In detail, intra-chain exchange amounts to $20.4$ K, yet magnetic long-range-order sets in only at $100$ mK. An extraordinary magnetic phase diagram is observed, which may be attributed to inter-chain frustration of helical intra-chain correlations. The relevant Dzyaloshinskii-Moriya exchange is evidenced by ESR-data. Funding by SNSF division 2 is acknowledged. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y4.00005: Spontaneously magnetized Tomonaga-Luttinger liquid in frustrated quantum antiferromagnets Shunsuke Furuya, Thierry Giamarchi Spontaneous symmetry breaking and Nambu-Goldstone bosons (NGB) going with it are fundamental for a wide range of physical systems. NGB which governs low-energy physics is affected by dimensionality. In 1D quantum antiferromagnet, although the true long-range antiferromagnetic order is absent, we can consider a counterpart of the NGB of the antiferromagnetic order, that is, the Tomonaga-Luttinger liquid (TLL). Here TLL is related to the short-range antiferromagnetic order and described by a critical relativistic field theory. This nature of TLL is consistent with a general statement of NGB in higher dimensions that the NGB originating from the antiferromagnetic (ferromagnetic) order has a relativistic (non-relativistic) dispersion relation. In this context, it would be very surprising to find a TLL structure compatible with ferromagnetic order. In this talk, I will present a theory of spontaneously magnetized TLL which is realized in quasi-1D geometrically frustrated quantum antiferromagnets. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y4.00006: Anisotropic magneto-thermal transport in the frustrated spin ladder BiCu$_{2}$PO$_{6}$ B.-G. Jeon, B. Koteswararao, G.J. Shu, F.C. Chou, K.H. Kim We report the thermal conductivity ($\kappa )$ of $S=$1/2 frustrated two-leg ladder BiCu$_{2}$PO$_{6}$\textbf{. }In this material, an enhanced heat transport along the leg direction (\textbf{b}-axis) is observed above 10 K. This is in good accordance with the previous observation of dispersive, anisotropic magnetic excitation along the reciprocal \textbf{b}-axis [1], which suggests the presence of anisotropic magnetic heat transport along the leg direction. The suppression of the $\kappa (T)$ and its magnetic field dependence are also observed around 15 K which probably leads to the clear double-peak feature in the $\kappa (T)$ at about 6 K and 60 K. Based on the Debye model, this can be interpreted by the resonant scattering between phonon and magnetic energy levels which affects the phonon heat transport.\\[4pt] [1] K. W. Plumb \textit{et al}., Phys. Rev. B \textbf{88}, 024402 (2013). [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y4.00007: Successive magnetic-phase transitions in a frustrated two-leg spin ladder Takanori Sugimoto, Michiyasu Mori, Takami Tohyama, Sadamichi Maekawa Recently, successive phase transitions induced by magnetic field have been observed in a frustrated two-leg spin ladder BiCu$_2$PO$_6$, in which the frustration is introduced by the next-nearest-neighbor antiferromagnetic exchange interaction in the leg direction. First, we calculate the magnetic-field dependence of magnetization by using density-matrix renormalization-group method. In this calculation, we find a phase transition induced by strong frustration, which does not appear in a non-frustrated two-leg spin ladder. The phase transition emerges as a jump of susceptibility with unsaturated finite magnetization. We also investigate the origin of the phase transition by using the bond-operator mean-field approximation. We conclude that the phase transition is the same type as the Lifshitz transition. Our study is useful to analyze experimental data of BiCu$_2$PO$_6$. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y4.00008: Magnetic Ordering In SrHo$_{2}$O$_{4}$ Jiajia Wen, Seyed Koohpayeh, Tyrel McQueen, Wei Tian, Haifeng Li, Jiaqiang Yan, Ovidiu Garlea, Collin Broholm We report the experimental observation of unusual magnetic ordering in a frustrated magnet SrHo$_{2}$O$_{4}$. The Ho$^{3+}$ ions in this material form a lattice consisting of edge-sharing triangles making chains extending along the c direction. The chains in turn form a honeycomb-like structure in the a-b plane. Despite a Curie Weiss temperature of -16.9 K, magnetic long range ordering (LRO) occurs at the much lower Ne\'{e}l temperature (T$_{\mathrm{N}})$ of 0.7 K. Above T$_{\mathrm{N}}$, single crystal neutron scattering experiments carried out on MACS(NCNR) shows diffusive patterns of elastic scattering in momentum space, revealing anisotropic short-range correlation with extended correlations along the chain direction. Single crystal diffraction measurement down to 0.3 K at HB1A(ORNL) shows coexistance of a long range ordering component and a short range ordering one below T$_{\mathrm{N}}$ . A proper partition of the magnetic lattice allows excellent modeling of the experimental data, and show that an interplay of frustration, crystal field effects, and low dimensionality lay behind the deeply suppressed and unusual partial order in this system. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y4.00009: Evidence for SrHo$_2$O$_4$ and SrDy$_2$O$_4$ as model 1D J$_1$-J$_2$ zig-zag chain materials Amy Poole, Vladimir Pomjakushin, Anne-Christine Uldry, Bobby Prevost, Alexandre Desilets-Benoit, Andrea Bianchi, Britt Hansen, Robert Cava, Michel Kenzelmann This presentation will focus on the Ho and Dy members of the recently discovered Sr$R_2$O$_4$ family of frustrated rare-earth ($R$) materials. Despite the $R$ sites forming a honeycomb in the $ab$ plane, we demonstrate that the physics is dominated by 1D correlations in the $c$-direction and that these can be mapped onto the Ising $J_1$-$J_2$ spin chain model. Three-dimensional magnetic order is suppressed from $\theta_w$ = -16.9 K to T$_N$ = 0.66 K in SrHo$_2$O$_4$ and is not observed to 50 mK in SrDy$_2$O$_4$. Our neutron powder diffraction measurements indicate that prior to the 3D order in the SrHo$_2$O$_4$ compound a 1D magnetically correlated state exists and that a similar state is found at the lowest measured temperatures in SrDy$_2$O$_4$. Fits to the diffuse data allow us to identify the 1D nature of the material and the key structural motifs are found from the data collected in the long-range ordered phase of SrHo$_2$O$_4$. Point charge calculations are fitted to the inelastic neutron scattering data and identify a large Ising anisotropy. The combination of neutron scattering and novel modelling techniques has allowed us to unambiguously determine the magnetic structure, identify the key interactions in the system and understand the 1D nature of the system. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y4.00010: Global phase diagram of the stacked frustrated triangular Ising system in a transverse field: a quantum Monte Carlo study Han Ma, Jie Lou, Alexei Tsvelik, Naoki Kawashima, Yan Chen The global phase diagram of the stacked frustrated triangular Ising magnet in a transverse field is obtained by using continuous time quantum Monte Carlo method. As the inter-plane interaction is strengthened, an first-order transition from a ferrimagnetic phase with two equivalent sublattices (FR2) to a partially disordered antiferrimagnetic phase (AF) occurs at small transverse field. In the quasi-one dimensional case, i.e. antiferromagnetically coupled transverse Ising chains, which corresponds to the realistic material CoNb2O6, our simulation reveals the existence of the low-field FR2 phase. In contrast, in the quasi-two dimensional limit, i.e. weakly coupled triangular Ising magnet, upon increasing the transverse magnetic field, the FR2 and AF phases successively appear in the order. In the vicinity of the ordered-disordered phase transition, the nature of phases can hardly be identified within our computational ability. At large transverse field, the paramagnetic phase trivially appears. Future experiments on CoNb2O6 at low temperature are expected to evidence the different magnetic patterns of this frustrated magnet based on our results in the quasi-one dimensional limit. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y4.00011: Low temperature magnetic ordering in the frustrated zigzag ladder system BaNd$_{2}$O$_{4}$ A.A. Aczel, L. Li, J.-Q. Yan, F. Weickert, V.S. Zapf, M. Jaime, L. Civale, R. Movshovich, V. Keppens, D. Mandrus The AR$_{2}$O$_{4}$ family (R $=$ rare earth) have recently been attracting interest as a new series of frustrated magnets, with the magnetic R atoms forming zigzag chains running along the c-axis. We have investigated polycrystalline BaNd$_{2}$O$_{4}$ with a combination of low temperature magnetization, heat capacity, and neutron diffraction measurements. This material has a Curie-Weiss temperature of -24 K, while our zero field heat capacity measurements indicate a magnetic transition of only 1.7 K, indicative of a high magnetic frustration index. Combined magnetization and neutron diffraction data show evidence for a complex, canted antiferromagnetic ground state with a propagation vector of (0 0.5 0.5) and the spins lying in the ac-plane. Furthermore, low temperature magnetization and heat capacity measurements as a function of applied field reveal that the order can be completely suppressed in an applied field of only 3.5 T. Direct comparison of these results to previous work on SrR$_{2}$O$_{4}$ shows that there is a rich diversity of magnetic behavior in this family of frustrated magnets, likely due to a competition between single ion anisotropy, dipole-dipole interactions, and exchange interactions. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y4.00012: Variational study of the $J_1-J_2$ Antiferromagnetic Heisenberg model on square lattice using correlated valence bond state Ling Wang, Olexei Motrunich We propose a variational ansatz in the context of resonating valence bond state for the $J_1-J_2$ Antiferromagnetic model on square lattice in the strongly frustrated regime. The ansatz modifies the nearest neighbor valence bond state by considering short range bond-bond correlations and their local resonances. The correlated bond-bond pairs have their local sign factor, which can produce the correct sign structure (nodal structure) as can be checked on $4\times 4$ and $6\times 6$ torus. The variational energy (up to size $10\times 10$) is highly competitive with the recent DMRG study on torus and the slave fermion variational Monte Carlo study with two step lanczos projection. We will discuss phases in the intermediate coupling region. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y4.00013: Reentrant Berezinskii-Kosterlitz-Thouless Transitions in a Triangular Ising Thin Film Yoshitomo Kamiya, Shi-Zeng Lin, Gia-Wei Chern, Cristian Batista We study the triangular lattice Ising model with a finite number of vertically stacked layers and demonstrate a low temperature reentrance of one or two Berezinskii-Kosterlitz-Thouless transitions, which results in an extended disordered regime down to T $=$ 0. This regime exhibits a novel and peculiar low-temperature thermodynamics such as the enhanced short-range magnetic correlation as temperature is increased. Numerical results are complemented with the derivation of an effective dimer theory that quantitatively describes the low temperature physics. Qualitative features of the global phase diagram are obtained by mapping the classical spin model into the single-layer quantum Ising model. (Reference: Shi-Zeng Lin, Yoshitomo Kamiya, Gia-Wei Chern, and Cristian Batista, \underline {arXiv:1310.3468}) [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y4.00014: Simulation of the magnetism of the 2D XY spin system in finite magnetic field Chuntai Shi, Sungho Han, Clare Yu Experiments implicate surface spins as the source of flux and inductance noise in SQUIDs. There is experimental evidence that interactions between these surface spins cannot be ignored. We investigate the effect of applying an external magnetic field on an interacting surface spin system. As a candidate model of the surface spins, we present Monte Carlo simulations of the classical 2D XY spin model on a square lattice. We monitor the time evolution of the susceptibility of the system after an external magnetic field is applied. We also look at the local magnetic field at a site produced by the neighboring spins. We monitor how the distribution of local fields changes after the external field is applied. [Preview Abstract] |
Friday, March 7, 2014 10:48AM - 11:00AM |
Y4.00015: High magnetic field phases of the $J_1$-$J_2$-$J_3$ triangular antiferromagnet Giacomo Marmorini, Tsutomu Momoi We present a thorough study of the $J_1$-$J_2$-$J_3$ triangular lattice antiferromagnet close to the saturation field, where the magnetic structure is determined by the condensation of magnons. We focus on the case of ferromagnetic $J_1$, that is particularly rich because frustration effects can allow for magnons of different (commensurate or incommensurate) wave-vectors to condense simultaneously. Our calculation includes an interlayer coupling $J_0$, that can be taken as small as $10^{-4}$ (in units of $J_1$), in which case the system is nearly two-dimensional. Besides the well-known spiral and fan phases, we find a new double-$q$ phase (superposition of two modes), dubbed ``01'' phase, whose features (including a new type of multiferroic behavior) can be seen as intermediate between the two. Furthermore, in some regions of the parameter space, we show that a dilute gas of magnon can not be stable and phase separation (corresponding to a magnetization jump) is expected. Related to this, we discuss the presence of quantum tricritical points. In the $J_1$-$J_2$ model ($J_3=0$) bound states of two and three magnons may also appear, but it is an open issue whether or not they form a stable condensate and then give rise to nematic or octupolar order. [Preview Abstract] |
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