Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Z28: Focus Session: Kagome Antiferromagnets II |
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Sponsoring Units: GMAG DMP Chair: Collin Broholm, Johns Hopkins University Room: 205 |
Friday, March 6, 2015 11:15AM - 11:27AM |
Z28.00001: Simplex valence-bond crystal in the spin-1 kagome Heisenberg antiferromagnet Wei Li, Tao Liu, Andreas Weichselbaum, Jan von Delft, Gang Su We investigate the ground state properties of a spin-1 kagome antiferromagnetic Heisenberg model using tensor-network (TN) methods. We find a ground state with trimerization (simplex) valence-bond order, and obtain the energy per site $e_0=-1.4099$ ($D=16$) by accurate calculations directly in the thermodynamic limit. The symmetry between the two kinds of triangles is spontaneously broken, with a relative energy difference of $\delta \approx$ 20\%. The spin-spin, dimer-dimer, and chirality-chirality correlation functions are found to decay exponentially with a rather short correlation length, showing that the ground state is gapped. We thus identify the ground state be a simplex valence-bond crystal (SVBC). We also discuss the spin-1 bilinear-biquadratic Heisenberg model on a kagome lattice, and determine its ground state phase diagram, find a quantum phase transition between the SVBC and a ferro-quadrupolar nematic state. Moreover, we implement non-abelian symmetries, here spin SU(2), in the TN algorithm, which improves the efficiency greatly and provides insight into the tensor structures. [Preview Abstract] |
Friday, March 6, 2015 11:27AM - 11:39AM |
Z28.00002: Quantum Kagome Ice Juan Carrasquilla, Zhihao Hao, Roger Melko Two-dimensional quantum spin liquids (QSLs) are exotic phases of matter where magnetic moments remain disordered even at extremely low temperatures. Despite ongoing searches, QSLs remain elusive, due to a lack of concrete knowledge of the microscopic mechanisms that inhibit magnetic order in real materials. Here, we study a theoretical model for a broad class of frustrated magnetic rare-earth pyrochlore materials called ``quantum spin ices''. When subject to an external magnetic field along the [111] crystallographic direction, the resulting spin interactions contain a mix of geometric frustration and quantum fluctuations in decoupled two-dimensional kagome planes. Using large-scale quantum Monte Carlo simulations, we identify a simple set of interactions sufficient to promote a groundstate with no magnetic long-range order, and a gap to excitations, conjectured to be a $Z_2$ spin liquid phase. This suggests a systematic experimental procedure to search for two-dimensional QSLs within the broader class of three-dimensional pyrochlore quantum spin ice materials. [Preview Abstract] |
Friday, March 6, 2015 11:39AM - 11:51AM |
Z28.00003: The $J_1$-$J_2$ Kagome Heisenberg Model with Dzyaloshinskii-Moriya Interaction Tsezar Seman, Cheng-Chien Chen, Rajiv Singh, Michel van Veenendaal We study the static and dynamic properties of an $S=1/2$ kagome antiferromagnetic (AFM) Heisenberg model using large-scale exact diagonalization. We map out the phase diagram as functions of the next-nearest-neighbor exchange $J_2$ and the z-axis Dzyaloshinskii-Moriya interaction $D_z$. In particular, the phase boundary between a magnetically disordered state and a ${\bf Q}=0, 120^{\circ}$ AFM long-range order is identified through finite-size extrapolation of the transverse spin-spin correlation function. We also compute the dynamical structure form factors and the cross sections of two-magnon Raman scattering in different regimes of the phase diagram. The implications of our numerical results to the $S=1/2$ kagome compound herbertsmithite ZnCu$_3$(OH)$_6$Cl$_2$ will be discussed. [Preview Abstract] |
Friday, March 6, 2015 11:51AM - 12:03PM |
Z28.00004: Frustration and Dzyaloshinsky-Moriya anisotropy in the kagome francisites Cu$_3$Bi(SeO$_3)_2$O$_2$X) Alexander Tsirlin, Ioannis Rousochatzakis, Ronald Zinke, Johannes Richter Kagome spin lattice is an abundant source of magnetic frustration. We will present density-functional as well as analytical and numerical calculations that elucidate the microscopic magnetic model of spin-1/2 francisite materials Cu$_3$Bi(SeO$_3)_2$X$_2$ (X = Cl, Br). Their weakly distorted kagome lattice features ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor couplings that result in an infinitely degenerate classical ground state for the isotropic spin model restricted to Heisenberg exchanges. This degeneracy is lifted by quantum fluctuations, although the canted magnetic order observed experimentally is only marginally lower in energy than other competing states. We argue that in francisites this canted state is primarily stabilized by the Dzyaloshinsky-Moriya (DM) anisotropic exchange. We derive the hierarchy of the DM exchanges in francisites and explain qualitatively the anisotropic magnetic response of these frustrated quantum magnets. [Preview Abstract] |
Friday, March 6, 2015 12:03PM - 12:15PM |
Z28.00005: Using the Density-Matrix Renormalization Group to Explore a Proposed Hamiltonian for Volborthite Edward Parker Volborthite ($\mathrm{Cu_3 V_2 O_7 (OH)_2 \cdot 2H_2O}$) is a strongly geometrically frustrated sytem of spin-$1/2$ ions on a Kagom\'{e} lattice whose magnetic ordering temperature is more than two orders of magnitude below its Curie temperature. Measurements of its magnetization curve show an extremely broad magnetization plateau extending over a range of at least 100 Tesla. Density functional theory calculations suggest a nontrivial anisotropic spin coupling structure with both ferromagnetic and antiferromagnetic bonds. Prior studies of similar (but simpler) systems suggest the possibility of a spin nematic phase containing gapless bound states of two or more magnons, which can condense and spontaneously break the $U(1)$ spin symmetry about the applied field down to a discrete cyclic symmetry. We will report Density-Matrix Renormalization Group studies of this model to investigate plateau formation and possible spin nematic and spin density wave phases. Techniques include approximating the full 2-D lattice using interchain mean-field theory and spin ladders. [Preview Abstract] |
Friday, March 6, 2015 12:15PM - 12:27PM |
Z28.00006: Thermal transport study of S = 1/2 kagome frustrated system Volborthite Daiki Watanabe, Minoru Yamashita, Masaaki Shimozawa, Yoshitaka Suzuki, Hajime Ishikawa, Zenji Hiroi, Yuji Matsuda The nature of spin liquid states of 2D frustrated magnetic systems has been discussed over decades. Recently, some candidate materials of 2D quantum spin liquid have been reported from the absence of long range order in low temperatures. However, the elementary excitations which characterize the ground state have yet to be observed. It is suggested that thermal Hall measurement is a powerful probe to identify the elementary excitations of 2D quantum spin liquids[1]. Here we report the results of thermal-transport measurements of Volborthite (Cu$_3$V$_2$O$_7$(OH)$_2 \cdot $2H$_2$O) which possesses the 2D kagome planes. We observed double anomalies in the thermal conductivity around 1K. These anomalies correspond magnetic orderings reported by the NMR measurements and the specific heat measurements[2,3]. We will also talk about our thermal Hall measurements above the ordering temperature. [1] H. Katsura {\it et al.}, Phys. Rev. Lett. {\bf 104}, 066403 (2010). [2] H. Yoshida {\it et al.}, Nat. Commun. {\bf 3}, 860 (2012). [3] M. Yoshida {\it et al.}, J. Phys. Soc. Jpn. {\bf 81}, 024703 (2012). [Preview Abstract] |
Friday, March 6, 2015 12:27PM - 12:39PM |
Z28.00007: New Realisations of Frustrated Quantum Spin Systems from Vanadium Based Oxyfluorides Lucy Clark, Farida Aidoudi, Cameron Black, Russell Morris, Philip Lightfoot We recently presented the first example of a material containing a kagome network of antiferromagnetically interacting V$^{4+}$ $S={\frac{1}{2}}$ cations, DQVOF (Diammonium Quinuclidinium Vanadium OxyFluoride). The $S={\frac{1}{2}}$ kagome layers within DQVOF are separated by V$^{3+}$ $S=1$ cations. Our low temperature magnetic study of DQVOF suggested that the kagome layers remain decoupled from these inter-layer spins and that the system adopts a gapless QSL ground state [1]. Here, we will discuss how variations in the chemical methods used to prepare DQVOF can be employed to extend this family of frustrated V$^{4+}$ based oxyfluorides. In particular, we will focus on a new phase ImVOF (Imidazolium Vanadium OxyFluoride), which consists of V$^{4+}$ $S={\frac{1}{2}}$ kagome layers like DQVOF, but the connectivity between the kagome layers is remarkably different. Single crystal X-ray diffraction reveals that the inter-layer vanadium species in ImVOF also sit on a kagome network. Magnetic susceptibility data of ImVOF reveal an absence of long range magnetic magnetic order down to $2$ K despite significant antiferromagnetic exchange exchange (${\theta}{\sim} -50$ K), which suggests that interesting physics is at play.\\[4pt] [1] L. Clark et al., Phys. Rev. Lett. 110, 207208 ($2013$) [Preview Abstract] |
Friday, March 6, 2015 12:39PM - 12:51PM |
Z28.00008: ABSTRACT WITHDRAWN |
Friday, March 6, 2015 12:51PM - 1:03PM |
Z28.00009: projective variational study on low-temperature quantum magnetism in Na4Ir3O8 Ryuichi Shindou Na4Ir3O8 is one of candidate materials of three-dimensional quantum spin liquid Mott isnulator, where Ir J=1/2 spin forms a hyperkagome lattice, a corner-sharing triangle network lattice with spin frustration. Due to lack of spatial inversion symmetries and heavy atom nature of Iridium, the system possesses larger anisotropic exchange interactions. In fact, preceding theories based on ab-initio band calculation show that Dzyaloshinskii-Moriya (DM) interaction is on order of 10 percent of isotropic exchange interaction, which could play vital role of highly competing grand state energetics in Na4Ir3O8. From transport experiments, the system is also known to be in weak Mott insulating regime, where charge fluctuation cause larger multiple-spin interactions. Employing variational analyses based on projective(fermionic) construction of many-body spin wavefunctions, we will reconsider possible quantum spin ground states in the hyperkagome antiferromagnetic Heisenberg model with anisotropic exchange and multiple-spin interactions of Na4Ir3O8. Starting from these states, we will discuss possible magnetic excitations and compare them with reported experiments. [Preview Abstract] |
Friday, March 6, 2015 1:03PM - 1:15PM |
Z28.00010: Quantum Phase Transitions and De-Coupling of Magnetic Sublattices in the Quasi-Two Dimensional Ising Magnet Co3V2O8 in a Transverse Magnetic Field K. Fritsch, G. Ehlers, K. C. Rule, K. Habicht, M. Ramazanoglu, H. A. Dabkowska, B. D. Gaulin The application of a magnetic field transverse to the easy axis, Ising direction in the quasi-two dimensional Kagome staircase magnet, Co$_3$V$_2$O$_8$, induces three quantum phase transitions at low temperatures, ultimately producing a novel high field polarized state, with two distinct sublattices. New time-of-flight neutron scattering techniques, accompanied by large angular access, high magnetic field infrastructure allows the mapping of a sequence of ferromagnetic and incommensurate phases and their accompanying spin excitations. At least one of the transitions to incommensurate phases at $\mu_0H_{c1}\sim6.25$ T and $\mu_0H_{c2}\sim7$ T are discontinuous, while the final quantum critical point at $\mu_0H_{c3}\sim 13$ T is continuous. [Preview Abstract] |
Friday, March 6, 2015 1:15PM - 1:27PM |
Z28.00011: $\mathbf{^{17}O}$ Single Crystal NMR Evidence for a Gapped Spin-liquid Ground State in the S=1/2 Kagome Lattice $\mathbf{ZnCu_{3}(OH)_{6}Cl_{2}}$ Mingxuan Fu, Takashi Imai, Tianheng Han, Young. S. Lee The two-dimensional S=1/2 Kagome lattice in Herbersmithite $\rm{ZnCu_{3}(OH)_{6}Cl_{2}}$ is the best candidate for experimental realization of a quantum spin liquid ground state known to date. The recent discovery of a continuum of spinon excitations using inelastic neutron scattering\footnote{T. Han et al., Nature \textbf{492}(2012) 406} has drawn strong attention to its exotic magnetic properties. Understanding the nature of the paramagnetic ground state of $\rm{ZnCu_{3}(OH)_{6}Cl_{2}}$, however, remains a challenge, due to excess magnetic Cu defects occupying the interlayer Zn sites. We conducted single crystal NMR measurements of the $\rm{^{17}O}$ Knight shift, and succeeded in measuring the intrinsic spin susceptibility of the Kagome layer down to $\rm{T\sim0.01J(J\sim17 meV)}$ for the first time. We demonstrate that the intrinsic spin susceptibility decays exponentially at low temperatures, revealing the presence of a spin gap $\rm{\Delta \sim 0.1J}$. Moreover, we show that application of a high magnetic field suppresses the gap. These results provide direct evidence for a gapped spin-liquid ground state realized in $\rm{ZnCu_{3}(OH)_{6}Cl_{2}}$.\footnote{M. Fu et al., preprint.} [Preview Abstract] |
Friday, March 6, 2015 1:27PM - 1:39PM |
Z28.00012: Spin liquid state in the S=1 vanadium kagome YCa$_3$(VO)$_3$(BO$_3$)$_4$ Christopher Wiebe, Harlyn Silverstein, Jason Gardner, Haidong Zhou Over the last decade, the search for model kagome compounds has been fruitful for S=1/2 Cu$^{2+}$ spins in the minerals Herbertsmithite and Volborthite [1-2]. There are fewer comparable materials for S=1 analogues, but recent progress has been made with the discovery of YCa$_3$(VO)$_3$(BO$_3$)$_4$, which has a network of V$^{3+}$ kagome spins [3]. Previous reports were made of no magnetic ordering down to 1.5 K in this compound, despite strong antiferromagnetic exchange [3]. Here we report a new synthesis method for this material which reduces impurity levels, resulting in high quality polycrystalline samples of YCa$_3$(VO)$_3$(BO$_3$)$_4$. Neutron scattering experiments show no evidence for long-ranged magnetic ordering down to 50 mK, with gapped inelastic excitations developing similar to S=1/2 kagome compounds. Heat capacity and susceptibility measurements also show a lack of long-range magnetic order and a lack of spin glassiness, placing this compound as a new spin liquid candidate. \\[4pt] [1] J. S. Helton \textit{et al.} Phys. Rev. Lett. (2007) 98, 10724.\\[0pt] [2] Z. Hiroi \textit{et al.} J. Phys. Soc. Japan (2001) 70, pp 3377-3384.\\[0pt] [3] W. Miller \textit{et al.} Chem. Mater. (2011) 23 (5), pp 1315-1322. [Preview Abstract] |
Friday, March 6, 2015 1:39PM - 1:51PM |
Z28.00013: Infrared phonons as a probe of spin liquid in kagome antiferromagnet Herbertsmitite Andrei Sushkov, Gregory Jenkins, Dennis Drew, Tian-Heng Han, Young Lee Phonons are sensitive to magnetic interactions between ions through the spin-phonon coupling mechanism where the phonon resonance frequency is proportional to $< S_iS_j >$ spin-spin correlation function. In our earlier work [1], we observed strong magnetic phonon splitting at the magneto-structural transition in cubic spinel ZnCr2O4. In Herbertsmithite ZnCu3(OH)6Cl2, neither magnetic ordering nor structural transition was observed down to mK temperatures. Recent theoretical work predicts a resonating bond ground state in Herbertsmithite that breaks p6 chiral symmetry and lifts the degeneracy between two zone center optical phonon modes [2]. From fits to reflectivity spectra we have obtained the temperature dependence of all IR-active phonons polarized both in and perpendicular to the kagome plane. Observed signatures of magnetic interaction effects on the phonon parameters will be discussed. \\[4pt] [1] Sushkov, A. B., Tchernyshyov, O., Ratcliff, W., Cheong, S. W. \& Drew, H. D. Probing Spin Correlations with Phonons in the Strongly Frustrated Magnet ZnCr2O4. Phys. Rev. Lett. 94, 137202 (2005).\\[0pt] [2] Capponi, S., Chandra, V. R., Auerbach, A. \& Weinstein, M. p6 chiral resonating valence bonds in the kagome antiferromagnet. Phys. Rev. B 87, 161118 (2013). [Preview Abstract] |
Friday, March 6, 2015 1:51PM - 2:03PM |
Z28.00014: Barlowite: a new mother compound of spin liquids Tian-Heng Han, John Singleton, John Schlueter Experimental investigations of spin liquids start with finding suitable materials. Existing candidate compounds have limitations, even though the observation of spinons in herbertsmithite has marked a breakthrough. I will introduce a new mother compound of kagome spin liquids, barlowite, with its properties characterized using thermodynamic techniques. The advantages of barlowite will be discussed. [Preview Abstract] |
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