Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F7: Focus Session: Low-D Quantum Spins I |
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Sponsoring Units: GMAG Chair: Chris Landee, Clark University Room: 106 |
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F7.00001: Quantum magnetism in low dimensions and large magnetic fields Invited Speaker: Thierry Giamarchi The ability to control the properties of magnetic insulators by magnetic fields large enough to fully polarize the system has opened a host of possibilities. In addition to the intrinsic interest of such questions for magnetic systems, is has been shown that such systems could be efficiently used as quantum simulators to emulate problems pertaining to itinerant fermionic or bosonic systems. The magnetic field can then be viewed as similar to a gate voltage controlling the number of ``particles'' allowing an unprecedented level of control. In parallel with the experimental developments, progress on the theoretical front both on the numerical and the analytical side, have allowed a remarkable level of accuracy in obtaining the physical properties and in particular the correlation functions of these systems. A comparison between theoretical predictions without adjustable parameters or fudging with results from NMR, Neutrons or other probes such as ESR is thus now possible. This has allowed for example to test {\it quantitatively} the physics of Tomonaga-Luttinger liquids and also to tackle the effects of the interactions between spinons by comparing the physics of weak rung ladders with the one of strong rung ones. Comparison between the neutron results and theoretical calculations of the correlation functions has also been demonstrated as a way to reconstruct efficiently the Hamiltonian from the experimental data. I will review the recent results obtained in this domain with the different experimental compounds and will discuss the open questions and challenges. This concerns in particular the issues of finite temperatures, higher dimensional systems and effects of disorder. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F7.00002: The structure-magnetism relationship in a molecule-based magnetic system: magnetic order and quantum disorder in one and zero dimensions Tom Lancaster, Stephen Blundell, Johannes M\"{o}ller, Saman Ghannadzadeh, Paul Goddard, Peter Baker, Francis Pratt, Jamie Manson We have synthesized two distinct molecule-based magnets from the same starting components. These show different structural motifs which promote contrasting exchange pathways and consequently lead to markedly different magnetic ground states. Through examination of their structural and magnetic properties we show that [Cu(pyz)(H$_{2}$O)(gly)$_{2}$](ClO$_{4}$)$_{2}$ may be considered a quasi one-dimensional quantum Heisenberg antiferromagnet while the related compound [Cu(pyz)(gly)](ClO$_{4}$), which is formed from strongly antiferromagnetically interacting Cu$^{2+}$ dimers, remains disordered down to at least 0.03~K in zero field, but shows a field-temperature phase diagram reminiscent of that seen in materials showing a Bose-Einstein condensation of magnons. We emphasise the use of muon-spin relaxation as a probe of these materials, which has allowed us to determine magnetic ordering that is invisible to many conventional measurement techniques. This is especially useful in low-dimensional magnetism where strong thermal and quantum fluctuations often make transitions to states of long-range magnetic order difficult to observe. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F7.00003: Fractional spinon excitations in the quantum Heisenberg antiferromagnetic chain material CuSO$_4$.5D$_2$O Martin Mourigal, Mechthild Enderle, Axel Kl\"opperpieper, Jean-S\'ebastien Caux, Anne Stunault, Henrik R{\O}nnow One of the simplest quantum many-body systems is the spin-1/2 Heisenberg antiferromagnetic chain, a linear array of interacting magnetic moments. Its exact ground state is a macroscopic singlet entangling all spins in the chain. Its elementary excitations, called spinons, are fractional spin-1/2 quasiparticles created and detected in pairs by neutron scattering. Theoretical predictions show that two-spinon states exhaust only 71\% of the spectral weight and higher-order spinon states, yet to be experimentally located, are predicted to participate in the remaining. By accurate absolute normalization of our inelastic neutron scattering data on the spin-1/2 Heisenberg antiferromagnetic chain compound CuSO$_4$.5D$_2$O, we account for the full spectral weight to within 99(8)\% [1]. Our data thus establish and quantify the existence of higher-order spinon states. The observation that, within error bars, the experimental line shape resembles a rescaled two-spinon one with similar boundaries allows us to develop a simple picture for understanding multi-spinon excitations. \\[4pt] [1] Nature Physics {\bf 9}, 435--441 (2013) [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F7.00004: Characterization of the Spin-1/2 Linear-Spin-Chain Ferromagnet CuAs$_{2}$O$_{4}$ Kevin Caslin, Reinhard Kremer, Fereidoon Razavi, Armin Schulz, Alfonso Munoz, Franz Pertlik, Jia Liu, Mike Whangbo, Joseph Law We are investigating Cu$^{2+}$ ($S=$1/2) linear-spin-chains systems exhibiting low-dimensional magnetism. Linear-spin-chains are formed when CuX$_{6}$ (X$=$O,Cl,Br,...) Jahn-Teller distorted octahedra link together via their trans-edges. Most often, these spin-chains support ferromagnetic (FM) nearest-neighbor (NN) and antiferromagnetic (AFM) next-nearest-neighbor (NNN) spin-exchange interactions, sometimes leading to an incommensurate spin-spiral structures with multiferroic behavior. There exists a magnetic phase diagram which can predict the intra-chain behavior using a ratio of spin-exchange constants, $\alpha =$Jnn/Jnnn. A quantum critical point exists on a boundary at $\alpha =$- 4, small spin exchange perturbations on a system with an $\alpha $ ratio in the vicinity of this point may induce a pronounced response of the system. In this study, we report on CuAs$_{2}$O$_{4}$ mineral name trippkeite, featuring CuO$_{2}$ ribbon chains. Trippkeite is an exceptional spin-chain system because it shows long-range FM ordering and has an $\alpha$ ratio close to -4. Measurements of magnetic susceptibility, heat capacity, Raman spectroscopy, and electron paramagnetic resonance were performed. DFT calculations and TMRG simulations were also carried out. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F7.00005: Dimerizations in spin-$S$ antiferromagnetic chains with three-spin interaction Zheng-Yuan Wang, Shunsuke C. Furuya, Masaaki Nakamura, Ryo Komakura We discuss spin-$S$ antiferromagnetic Heisenberg chains with three-spin interactions, next-nearest interactions, and bond alternation. First, we prove rigorouslly that there exist parameter regions of the exact dimerized ground state in this system. This is a generalization of the Majumdar-Ghosh model to arbitral $S$. Next, we discuss the ground state phase diagram of the models by introducing several effective field theories and universality classes of the transitions are described by the level-$2S$ $\mathrm{SU}(2)$ Wess-Zumino-Witten model and the Gaussian model. Finally, we determine the phase diagrams of $S=1$ and $S=3/2$ systems by using exact diagonalization and level spectroscopy method. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F7.00006: Spin-orbital entanglement or separation? Understanding elementary excitations in a spin-orbital chain Krzysztof Wohlfeld, Cheng-Chien Chen, Michel van Veenendaal, Thomas P. Devereaux Recent theories have suggested {\it separation} of elementary spin and orbital excitations in anisotropic spin-orbital chains with evidence coming from a number of experiments on various copper oxides [1]. However, it is well-known that elementary excitations in an idealized spin-orbital chain with isotropic SU(4) symmetric interactions contain {\it entangled} spin and orbital quantum numbers [2]. Using a combined analytical and numerical approach, we show that a common description of the excitations in these two limits is possible: the spin and orbital spectra can be described in terms of fractionalized `RVB-like' spinons and antispinons where each excitation carries both spin and orbital quantum numbers, thus showing spin-orbital entanglement. Spin-orbital separation occurs solely in the highly anisotropic limit, and such a description is allowed only due to a particular choice of the spin and orbital basis.\\[4pt] [1] Nature 485, 82 (2012); PRL 107, 147201 (2011); arxiv:1307.6180; arxiv:1310.8346.\\[0pt] [2] PRL 81, 3527 (1998). [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F7.00007: Thermal transport and spin-phonon coupling in the one-dimensional antiferromagnetic spin chain compound CuSb$_2$O$_6$ Narayan Prasai, Joshua Cohn, Alwyn Rebello, Michael Smith, John J. Neumeier We report thermal conductivity ($\kappa$) measurements on single crystals of the $S=1/2$ antiferromagnetic spin-chain compound CuSb$_2$O$_6$ over the temperature range $5{\rm K}\leq {\rm T}\leq 300 {\rm K}$. Similar measurements on the non-magnetic analog compound, ZnSb$_2$O$_6$, allow for a comparison of the lattice thermal conductivities. The role of spin-phonon coupling and twinning on the anisotropic thermal transport of CuSb$_2$O$_6$ will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F7.00008: Magnetic model of BaCuSi$_2$O$_6$ revisited: Bose-Einstein condensation of magnons on a non-frustrated spin lattice Alexander A. Tsirlin, Vladimir V. Mazurenko, Maria V. Valentyuk, Raivo Stern Bose-Einstein condensation (BEC) of magnons remains one of the most intricate collective phenomena observed in quantum magnets. In Han Purple the BEC physics is heavily influenced by structural peculiarities related to the low-temperature structural distortion taking place around 100 K. The crystal structure comprises structural and magnetic dimers forming bilayers, and the BEC transition is formally 2D. Frustrated couplings between the bilayers are believed to be responsible for this effect, because at low enough temperatures the bilayers become decoupled. We challenge this scenario using extensive density-functional (DFT) calculations. We will show that DFT can well reproduce the couplings of $J_A\simeq 50$~K and $J_A\simeq 60$~K in two nonequivalent bilayers. Our calculations also yield a new scenario of the interdimer exchange that takes place between the top site of one dimer and the bottom site of the neighboring dimer rather than top-to-top and bottom-to-bottom. This scenario is verified by INS data and by magnetostructural correlations for the superexchange. The new regime of the interdimer couplings implies that BaCuSi$_2$O$_6$ lacks any appreciable magnetic frustration, individual bilayers are not decoupled, and other explanations for the 2D BEC physics should be sought [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F7.00009: High-resolution thermal expansion measurements of BaCuSi$_4$O$_{10}$ and BaCuSi$_2$O$_6$ Sueli Masunaga, Alwyn Rebello, J.J. Neumeier BaCuSi$_4$O$_{10}$ and BaCuSi$_2$O$_6$ were used in many ancient Chinese artifacts as synthetic pigments, and recently named as Han Blue and Han Purple, respectively.\footnote{E. W. FitzHugh \emph{et al}., Stud. Conserv. \textbf{37}, 145 (1992).} Besides being important synthetic pigments of ancient and modern times, these compounds have attracted scientific and technological interest due to their luminescent properties.\footnote{S. M. Borisov \emph{et al}., Anal. Chim. Acta \textbf{787}, 219 (2003); G. Pozza \emph{et al}., J. Cult. Herit. \textbf{1}, 393 (2000); S. M. Borisov \emph{et al}., Anal. Chem. \textbf{85}, 9371 (2013).} Moreover, Han Purple is a spin-dimer compound with an interesting phase diagram and a potential solid state device for exploring quantum effects in magnetic field induced Bose-Einstein condensation.\footnote{M. Jaime \emph{et al}., Phys. Rev. Lett. \textbf{93}, 087203 (2004).} In this work, we study BaCuSi$_2$O$_6$ and BaCuSi$_4$O$_{10}$ single crystals grown by floating zone method and flux growth technique, respectively. The results of thermal expansion, specific heat, and magnetization measurements of these compounds will be presented in detail. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F7.00010: Thermal phase transitions to valence-bond-solid states in the two dimensional SU(N) Heisenberg models Takafumi Suzuki, Kenji Harada, Haruhiko Matsuo, Synge Todo, Naoki Kawashima The two-dimensional (2D) SU(N) Heisenberg model with n-fold singlet projectors, namely the $JQ_n$ model [1], is believed to be a good example to study the deconfined critical (DC) scenario[2], because this hosts a quantum phase transition between valence-bond-solid (VBS) and magnetic ordered states at $T=0$. The DC scenario tells us that the universality should be same and independent on the broken lattice-rotation symmetry: the same criticality is observed in both $JQ_{3}$ model on the honeycomb lattice ($Z_{3}$) and $JQ_{2}$ model on the square lattice ($Z_{4}$) [3]. However, the thermal phase transition to the VBS phase may be drastically affected by the breaking symmetry. In this study, we consider the SU(N) $JQ_{n}$ models on square and honeycomb lattices and study the thermal phase transition to the VBS phases. From the QMC calculations, we discuss the critical properties for (1) the lattice dependence, (2) N dependence, and (3) coupling ratio $Q_{n}/J$ dependence. [1] T. Senthil, et al., Science 303, 1490 (2004); M. Levin and T. Senthil, Phys. Rev. B 70, 220403(R) (2004). [2] A. W. Sandvik, Phys. Rev. Lett. 98, 227202 (2007). [3] K. Harada, et al., arXiv:1307.0501. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F7.00011: ABSTRACT MOVED TO Q4.00015 |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F7.00012: Giant magnetic anisotropy and quantum tunneling of the magnetization in Li$_2$(Li$_{1-x}$Fe$_x$)N Anton Jesche, R. William McCallum, Srinivasa Thimmaiah, Jenee L Jacobs, Valentin Taufour, Andreas Kreyssig, Robert S. Houk, Sergey L. Bud'ko, Paul C. Canfield The magnetic anisotropy of 3$d$ transition metals is usually considered to be weak, mainly due to the widely known paradigm of orbital quenching. However, a rare interplay of crystal electric field effects and spin-orbit coupling causes a large orbital contribution to the magnetic moment of iron in Li$_2$(Li$_{1-x}$Fe$_x$)N. This leads, not only to large magnetic moments of $\sim$\,5\,$\mu_{\rm B}$ per iron atom but, also, to an enormous magnetic anisotropy field that extrapolates to more than 200 Tesla. Magnetic hysteresis emerges for $T \leq 50$\,K and the coercivity fields of more than 11 Tesla exceed even the hardest 4$f$ electron based ferromagnets. Li$_2$(Li$_{1-x}$Fe$_x$)N not only has a clear and remarkable anisotropy, generally not associated with iron moments, but also shows time-dependence more consistent with molecular magnets. In particular for low iron concentrations $x \ll 1$ the spin-inversion is dominated by a macroscopic tunneling process rather than by thermal excitations. It is shown that the huge magnetic anisotropy makes Li$_2$(Li$_{1-x}$Fe$_x$)N (i) an ideal model system to study macroscopic quantum effects at elevated temperatures and (ii) a basis for novel magnetic functional materials. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F7.00013: Molecular $j_{\rm eff}$ states in ternery transition metal chalcogenides $AM_4X_8$ Heung-Sik Kim, Jino Im, Myung Joon Han, Hosub Jin Spin-orbit-coupling(SOC)-induced $j_{\rm eff}$ states, reported in several iridium oxide compounds, is the key ingredient in understanding the interesting cooperation between SOC and the electron correlations. From our density functional theory calculations we suggest that, a series of ternery transition metal chalcogenides $AM_4X_8$ ($A$ = Ga, $M$ = 4$d$ and 5$d$ transition metal atoms, $X$ = chalcogen atoms) host the $j_{\rm eff}$ states in a molecular form. Wide range of the bandwidth covered with the external or chemical pressure enable one to access a broad range of electron correlation strength in a single compound. Implications of our results in both the weak and strong coupling regime are discussed. Our finding provides an ideal playground in exploring the $j_{\rm eff}$ physics and the resulting emergent phenomena. [Preview Abstract] |
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