Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session A29: Focus Session: Antiferromagnets on Triangular Lattices |
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Sponsoring Units: GMAG DMP Chair: Alexander Chernyshev, University of California, Irvine Room: 206A |
Monday, March 2, 2015 8:00AM - 8:12AM |
A29.00001: Magnetostriction to 100T in SrCu2(BO3)2: magnetic pantograph effect and tuning of J'/J ratio M. Jaime, A. Saul, G. Ratdke, M.B. Salamon, H.A. Dabkowska The magnetostriction of the frustrated spin dimer system SrCu$_2$(BO$_3$)$_2$ was measured in pulsed high magnetic fields with an optical fiber Bragg grating technique. Both longitudinal (c-axis) and transverse (ab-plane) magnetostriction were obtained to H = 60T for H // c-axis, observing a reduction of the unit cell volume as the sample is magnetized past the 1/3 saturation magnetization plateau. Modest changes in the lattice parameters, when combined with existing elastic nneutron scattering data, suggest significant changes in the Cu-O-Cu bond angles (superexchange) through a pantograph effect as the sample is driven into highly polarized magnetic states. Supporting computations reveal an increase of the ratio of intra- to inter-dimer exchange integrals (J'/J) with a decrease in the Cu-O-Cu angle, and the concomitant drop in unit cell volume. These results impact our reading of existing predictions for the (H,T) phase diagram, and predictions for the effect of hydrostatic pressures on the ground state. [Preview Abstract] |
Monday, March 2, 2015 8:12AM - 8:24AM |
A29.00002: Theory of lattice response to external magnetic field in SrCu$_2$(BO$_3$)$_2$: magnetostriction driven by panthograph effect Andres Saul, Guillaume Radtke, Marcelo Jaime, Myron Salamon, Hanna Dabkowska Recent magnetostriction experiments have shown that the macroscopic physical dimensions of the Shastry-Sutherland compound SrCu$_2$(BO$_3$)$_2$ change with the applied magnetic field mimicking the same complex behavior observed in the magnetization. Using Density Functional based methods we find that the driving force behind the magnetoelastic coupling is the Cu-O-Cu superexchange angle which, thanks to the orthogonal Cu$^{2+}$ dimers acting as pantographs, can shrink significantly (0.44\%) with minute (0.01\%) variations in the lattice parameters. The consequence is a reduction of the order of $\sim$10\% in the antiferromagnetic intra-dimer exchange integral $J$, sufficient to compensate the elastic energy loss in the deformation. [Preview Abstract] |
Monday, March 2, 2015 8:24AM - 8:36AM |
A29.00003: NMR in Pulsed Magnetic Fields on the Orthogonal Shastry-Sutherland spin system SrCu2(BO3)2 Raivo Stern, Jonas Kohlrautz, Hannes K\"uhne, Liz Greene, Jochen Wosnitza, J\"ugen Haase SrCu$_2$(BO$_3$)$_2$ is a quasi-two-dimensional spin system consisting of Cu$^{2+}$ ions which form orthogonal spin singlet dimers, also known as the Shastry-Sutherland lattice, in the ground state. Though this system has been studied extensively using a variety of techniques to probe the spin triplet excitations, including recent magnetization measurements over 100 T, microscopic techniques, such as nuclear magnetic resonance (NMR), could provide further insight into the spin excitations and spin-coupling mechanisms. We demonstrate the feasibility of performing NMR on real physics system in pulsed magnets. We present $^{11}$B NMR spectra measured in pulsed magnetic fields up to 53 T, and compare those with prior results obtained in static magnetic fields. Herewith we prove the efficacy of this technique and then extend to higher fields to fully explore the spin structure of the 1/3 plateau. [Preview Abstract] |
Monday, March 2, 2015 8:36AM - 8:48AM |
A29.00004: Identifying topological order in the Shastry-Sutherland model via entanglement entropy David Ronquillo, Michael Peterson It is known that for a topologically ordered state the area law for the entanglement entropy shows a negative universal additive constant contribution, $-\gamma$, called the topological entanglement entropy. We theoretically study the entanglement entropy of the two-dimensional Shastry-Sutherland quantum antiferromagnet using exact diagonalization on clusters of 16 and 24 spins. By utilizing the Kitaev-Preskill construction [A. Kitaev and J. Preskill, Phys. Rev. Lett. 96, 110404 (2006)] we extract a finite topological term, $-\gamma$ , in the region of bond-strength parameter space corresponding to high geometrical frustration. Thus, we provide strong evidence for the existence of an exotic topologically ordered state and shed light on the nature of this model's strongly frustrated, and long controversial, intermediate phase. [Preview Abstract] |
Monday, March 2, 2015 8:48AM - 9:00AM |
A29.00005: Magnetic vortex crystals in frustrated 3D Mott insulators Zhentao Wang, Yoshitomo Kamiya, Andriy Nevidomskyy, Cristian Batista Topological spin textures, such as skyrmions, are of great interest to the field of spintronics and usually arise due to the interplay of Dzyaloshinskii-Moriya and exchange couplings. By contrast, using the BCC and FCC lattices as examples, here we demonstrate that frustrated spin exchange interactions alone can produce topological vortex crystals near the magnetic field-induced saturation transition of 3D bulk Mott insulators. Because of the magnetic frustration, the magnon spectrum of the high-field fully polarized state has multiple degenerate minima at different Q-vectors. This quantum paramagnet becomes gapless and goes through a Bose-Einstein condensation at the saturation field (quantum critical point). In this limit, we apply the dilute bosonic gas approximation to study the rich topological structures produced due to multi-Q condensation. We find that the vortex crystal phases span sizable regions in the phase diagrams of frustrated 3D Mott insulators with isotropic Heisenberg interactions, and are further stabilized by exchange anisotropies. Vortex strings emerge in the direction of the magnetic field and, depending on the distributions of the condensed modes, can form different exotic patterns. [Preview Abstract] |
Monday, March 2, 2015 9:00AM - 9:12AM |
A29.00006: Magnetic phase diagram of the S$=$1/2 triangular-lattice antiferromagnet Ba$_{3}$CoSb$_{2}$O$_{9}$ Yoshitomo Kamiya, Cristian Batista To explain the recently reported magnetic phase diagram of the spin-1/2 triangular-lattice compound Ba$_{3}$CoSb$_{2}$O$_{9}$ [1-3], we present a semiclassical mean-field theory for the easy-plane XXZ model on the stacked triangular-lattice with a small inter-layer coupling. Quantum effects are incorporated by deriving effective interactions from the linear spin-wave analysis of the two-dimensional model. This analysis reproduces the main experimental observations, such as the 1/3-magnetization plateau (B \textbar \textbar $a)$, a cusp near 1/3 of the saturated moment (B \textbar \textbar $c)$, and a small step anomaly in the high field regime. The predicted spin configurations are compared against the NMR measurements on this compound. This work was done in collaboration with G. Koutroulakis (Los Alamos), T. Zhou (UCLA), J. D. Thompson (Los Alamos), H. D. Zhou (Univ. of Tennessee), and S. E. Brown (UCLA).\\[4pt] [1] Y. Shirata \textit{et al.}, Phys. Rev. Lett. \textbf{108}, 057205 (2012)\\[0pt] [2] H. D. Zhou\textit{ et al}., Phys. Rev. Lett. \textbf{109}, 267206 (2012)\\[0pt] [3] T. Susuki \textit{et al}., Phys. Rev. Lett. \textbf{110}, 267201 (2013)\\[0pt] [4] T. Zhou, G. Koutroulakis, YK \textit{et al}., arXiv: 1308.6331 [Preview Abstract] |
Monday, March 2, 2015 9:12AM - 9:48AM |
A29.00007: Exotic magnetism on the quasi-FCC lattices of the d$^{3}$ double perovskites La$_{2}$NaB'O$_{6}$ (B' $=$ Ru, Os) Invited Speaker: Adam Aczel B-site ordered double perovskites with quantum spins S $=$ 1/2 (d$^{1})$ and S $=$ 1 (d$^{2})$ on the B' site have attracted a great deal of recent interest, due to the possibility of studying 4d and 5d magnetism combined with magnetic frustration on the face-centered-cubic (FCC) lattice. There has been less focus on d$^{3}$ systems, as they are generally expected to behave more classically and yield simple, commensurate magnetic ground states. In contrast, we find evidence for long-range and short-range ($\xi =$ 70 {\AA} at 4 K) incommensurate magnetic order on the quasi-FCC lattices of the monoclinic double perovskites La$_{2}$NaRuO$_{6}$ and La$_{2}$NaOsO$_{6}$ respectively. Incommensurate magnetic order on the FCC lattice has not been predicted by mean field theory, but may arise via a delicate balance of inequivalent nearest neighbor and next nearest neighbor exchange interactions. Furthermore, in the Ru system with long-range order, inelastic neutron scattering reveals a spin gap $\Delta $ $=$ 2.75 meV. Magnetic anisotropy is generally minimized in the more familiar octahedrally-coordinated 3d$^{3}$ systems, so the large gap observed for La$_{2}$NaRuO$_{6}$ may result from the significantly enhanced value of spin-orbit coupling in this 4d$^{3}$ material. [Preview Abstract] |
Monday, March 2, 2015 9:48AM - 10:00AM |
A29.00008: The incommensurate-commensurate phase transition of anisotropic XXZ model on the triangular lattice Xuefeng Zhang, Shi-Jie Hu, Axel Pelster, Sebastian Eggert We investigate the XXZ model on the triangular lattice with anisotropic ferromagnetic xy coupling and antiferromagnetic z coupling. In previous studies, an incommensurate supersolid phase was found when introducing anisotropy of interactions in different directions. However, the mechanism of the incommensurate phase is still unknown. In the strong coupling region where interactions in the z direction are larger than in the xy plane, we found that quantum fluctuation of domain wall excitations can reduce the energy and cause the incommensurate order. By using quantum Monte Carlo simulations, we confirmed our theoretical analysis and found that the number of domain walls and the incommensurate k-vector monotonically increase with the anisotropy. The physics is dimensionally reduced to 1d. [Preview Abstract] |
Monday, March 2, 2015 10:00AM - 10:12AM |
A29.00009: Spin phonon induced magnetic soft mode in triangular antiferromagnet h-RMnO$_3$ Joosung Oh, Manh Duc Le, Hasung Sim, Je-Geun Park, T.G. Perring The relief of geometrical magnetic frustration by spin-lattice coupling is an extensively studied subject: For example, theory shows that in triangular lattice antiferromagnets, the spin-phonon coupling can stabilize a collinear magnet order over the non-collinear 120$^{\circ}$ order. We report inelastic neutron scattering measurements on the triangular lattice antiferromagnets (Y/Lu)MnO$_3$, showing evidence of magnon-phonon hybridization at the Brillouin zone boundary. Furthermore, a magnetic soft mode is observed at lower energy at the same momentum transfer. The exchange striction model within the linear approximation qualitatively explains the observed features while the 1/$S$ expansion calculation shows that a third of the observed softening is caused by magnon-magnon interactions. Our results demonstrate how the spin-phonon coupling $and$ quantum effect cooperatively develops the roton-like minimum in triangular lattice antiferromagnets with a 120$^{\circ}$ structure. [Preview Abstract] |
Monday, March 2, 2015 10:12AM - 10:24AM |
A29.00010: Magnetic phase diagram and multiferroicity of Ba$_{3}$MnNb$_{2}$O$_{9}$: A spin- $\frac{5}{2}$ triangular lattice antiferromagnet with weak easy-axis anisotropy M. Lee, E.S. Choi, X. Huang, J. Ma, C.R. Dela Cruz, M. Matsuda, W. Tian, Z.L. Dun, S. Dong, H.D. Zhou We have performed magnetic, electric, thermal, and neutron powder diffraction (NPD) experiments as well as density functional theory (DFT) calculations on Ba$_{3}$MnNb$_{2}$O$_{9}$. All results suggest that Ba$_{3}$MnNb$_{2}$O$_{9}$ is a spin-$\frac{5}{2}$ triangular lattice antiferromagnet (TLAF) with weak easy-axis anisotropy. At zero field, we observed a narrow two-step transition at $T_{\mathrm{N1}}$ = 3.4 K and $T_{\mathrm{N2}}$ = 3.0 K. The neutron diffraction measurement and the DFT calculation indicate a 120$^{\circ}$ spin structure in the $ab$ plane with out-of-plane canting at low temperatures. With increasing magnetic field, the 120$^{\circ}$ spin structure evolves into up-up-down ($uud$) and oblique phase showing successive magnetic phase transitions, which fits well to the theoretical prediction for the 2D Heisenberg TLAF with classical spins. Multiferroicity is observed when the spins are not collinear but suppressed in the $uud$ and the oblique phase. We discuss the results in comparison with our previous works on its sister compounds with small spins, Ba$_{3}$NiNb$_{2}$O$_{9}$ (S = 1) (J. Hwang $et$ $al$., Phys. Rev. Lett. {\bf 109}, 257205 (2012) and Ba$_{3}$CoNb$_{2}$O$_{9}$ (S = $\frac{1}{2}$) (M. Lee $et$ $al$., Phys. Rev. B {\bf 89}, 104420 (2014)). [Preview Abstract] |
Monday, March 2, 2015 10:24AM - 10:36AM |
A29.00011: Indirect RIXS study of bimagnon excitations in triangular-lattice quantum Heisenberg antiferromagnet Trinanjan Datta, Cheng Luo, Zengye Huang, Dao-Xin Yao Bimagnon correlations in triangular-lattice quantum Heisenberg antiferromagnet can be probed by the resonant inelastic X-ray scattering (RIXS) technique. Utilizing an interacting spin wave theory within the Bethe-Salpeter approximation scheme, we compute the K-edge indirect RIXS spectra for the nearest neighbor Heisenberg model with a general S for the entire magnetic brillouin zone. The non-collinear spin arrangement in the triangular lattice geometry supports the intrinsic spontaneous single-magnon decay or recombination. Based on our calculation, we find that the RIXS spectra display a peak at the antiferromagnetic wave vector $(4\pi/3, 0)$ corresponding to the triangular lattice, which is in contrast to the square lattice case. The major contribution to the RIXS spectra originates from the decay vertices arising from the three-magnon interaction terms, with the quartic interaction contributions subdued. Our results indicate that the spontaneous decay and recombination of magnons inherent to the triangular lattice model can be oberved in the RIXS spectra without a disintegration. [Preview Abstract] |
Monday, March 2, 2015 10:36AM - 10:48AM |
A29.00012: Stripes and antiphase boundaries in CaFe$_{2}$O$_{4}$ Chris Stock, Efrain Rodriguez, Mark Green, Nara Lee, S.-W. Cheong We report on the magnetic structure and spin dynamics in CaFe$_{2}$O$_{4}$ based upon an orthorhombic structure [1]. The magnetic structure consists of two competing magnetic phases based upon stripes of S$=$5/2 Fe$^{3+}$ ions. The magnetic dynamics illustrate that the coupling is primarily two dimensional. On application of a magnetic field, antiphase magnetic boundaries can be introduced into the lattice and frozen in at low temperatures. We investigate the structure and dynamics of these domains using polarized and unpolarized neutron scattering and discuss how the triangular geometry allow these localized defects to be energetically favorable. \\[4pt] [1] L. M. Corliss et al. Phys. Rev. \textbf{160}, 408 (1967). [Preview Abstract] |
Monday, March 2, 2015 10:48AM - 11:00AM |
A29.00013: Anomalous weak ferromagnetism in $R_{1-x}$Y$_{x}$B$_{4}$ ($R = $ Sm, Gd, Tb, Dy, Ho, Er) B.Y. Kang, Myungsuk Song, B.K. Cho, J.Y. Kim Since the report of magnetic properties of rare earth tetraborides, $R$B$_{4}$ ($R=$rare-earth elements), $R$B$_{4}$ compounds have received a great attention over last decades because it shows various interesting magnetic ground states depending on rare-earth elements. $R$B$_{4}$ exhibits antiferromagnetic ordering at low temperature and is classified as the Shastry-Sutherland lattice, which is a geometrically frustrated system. In this system, the disturbance of a delicate balance can lead to new electronic and magnetic states. In this study, single crystals of $R_{1-x}$Y$_{x}$B$_{4}$ ($R=$Sm, Gd, Tb, Dy, Ho, Er), (x$=$0, 0.1, 0.2, 0.3, 0.35, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1) were synthesized using the high-temperature Al solution growth method. Interestingly, weak ferromagnetism was found to emerge at the N\'eel temperature for the Y-doped single crystals of $R_{1-x}$Y$_{x}$B$_{4}$. The magnitude of spontaneous magnetic moment was found to be correlated with the Y substitution ratio, which have maximum value at about 30{\%} of Y-concentration. The weak ferromagnetism reveals also a strong magnetic anisotropy depending on rare-earth elements. The observed data indicate that the weak ferromagnetism is not due to an individual atomic effect but a systematic bulk effect. The exotic antiferromagnetic properties will be discussed in detail in terms of yttrium substitution and magnetic and geometrical structures. [Preview Abstract] |
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