Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session M15: Focus Session: Spin/orbital Frustration and Short-range Order |
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Sponsoring Units: GMAG DMP Chair: Collin Broholm, Johns Hopkins University Room: 317 |
Wednesday, March 20, 2013 8:00AM - 8:36AM |
M15.00001: Spin-orbital quantum liquid on the honeycomb lattice Invited Speaker: Philippe Corboz The symmetric Kugel-Khomskii can be seen as a minimal model describing the interactions between spin and orbital degrees of freedom in transition-metal oxides with orbital degeneracy, and it is equivalent to the SU(4) Heisenberg model of four-color fermionic atoms. We present simulation results for this model on various two-dimensional lattices obtained with infinite projected-entangled pair states (iPEPS), an efficient variational tensor-network ansatz for two dimensional wave functions in the thermodynamic limit. This approach can be seen as a two-dimensional generalization of matrix product states - the underlying ansatz of the density matrix renormalization group method. We find a rich variety of exotic phases: while on the square and checkerboard lattices the ground state exhibits dimer-N\'eel order and plaquette order, respectively, quantum fluctuations on the honeycomb lattice destroy any order, giving rise to a spin-orbital liquid. Our results are supported from flavor-wave theory and exact diagonalization. Furthermore, the properties of the spin-orbital liquid state on the honeycomb lattice are accurately accounted for by a projected variational wave-function based on the pi-flux state of fermions on the honeycomb lattice at 1/4-filling. In that state, correlations are algebraic because of the presence of a Dirac point at the Fermi level, suggesting that the ground state is an algebraic spin-orbital liquid. This model provides a good starting point to understand the recently discovered spin-orbital liquid behavior of Ba$_3$CuSb$_2$O$_9$. The present results also suggest to choose optical lattices with honeycomb geometry in the search for quantum liquids in ultra-cold four-color fermionic atoms. [Preview Abstract] |
Wednesday, March 20, 2013 8:36AM - 9:12AM |
M15.00002: Spin-orbital short-range order in the honeycomb-based quantum magnet Ba$_{3}$CuSb$_{2}$O$_{9}$ Invited Speaker: Satoru Nakatsuji The realization of quantum correlated matter beyond one dimension has been vigorously pursued in geometrically frustrated spin systems for decades. In frustrated magnetic materials, however, symmetry breaking of orbital and chemical origin is usually found to induce semi-classical spin freezing. In this talk, I present a contrast case where spins and possibly orbitals remain in a liquid state down to low temperature even in a highly disordered structure of 6H-perovskite Ba$_{3}$CuSb$_{2}$O$_{9}$. Our comprehensive experimental analysis indicates that the geometrical frustration of Wannier's Ising antiferromagnet on a triangular lattice can be exploited to build a nano-structured bipartite honeycomb lattice from electric dipolar spin-1/2 molecules. Despite a strong local Jahn-Teller distortion about the Cu$^{2+}$ ion, the resulting spin-orbital random bond lattice not only retains hexagonal symmetry averaged over time and space, but it supports a gapless excitation spectrum without spin freezing down to ultralow temperatures. This is the work based on the collaboration with K. Kuga, K. Kimura, R. Satake, N. Katayama, E. Nishibori, H. Sawa, R. Ishii, M. Hagiwara, F. Bridges, T. U. Ito, W. Higemoto, Y. Karaki, M. Halim, A. A. Nugroho, J. A. Rodriguez-Rivera, M. A. Green, C. Broholm. [Preview Abstract] |
Wednesday, March 20, 2013 9:12AM - 9:24AM |
M15.00003: Spin-orbital entanglement due to dynamical Jahn-Teller effect Joji Nasu, Sumio Ishihara Quantum spin liquid (QSL) state is one of the fascinating themes in correlated electron systems. Recently, a new candidate of the QSL state is experimentally reported in a layered copper oxide Ba$_{3}$CuSb$_{2}$O$_{9}$. In this material, a Cu$^{2+}$ has the $e_{g}$ orbital degree of freedom and the dynamical Jahn-Teller effect (DJTE) is suggested to play a key role for the emergence of the QSL state. Motivated from the recent experiments in Ba$_{3}$CuSb$_{2}$O$_{9}$, we study the DJTE in the spin-orbital coupled system and examine a possibility of the QSL state in a spin-orbital system with lattice vibrations. In particular, we focus on the competitive or cooperative phenomena between the superexchange (SE) interaction and the DJTE. A SE interaction Hamiltonian is derived from the $d$-$p$ type Hamiltonian and the DJTE Hamiltonian for the low-lying vibronic states is represented by the orbital pseudo-spin and the lattice vibration. We analyze the model, where these two interactions are taken into account on a honeycomb lattice, by using the cluster mean-field approximation with the exact diagonalization (ED) method and the combined method of the quantum Monte-Carlo method and ED method. We find that magnetic orders are unstable in a wide parameter region and a spin-singlet dimer state associated with an orbital order is realized. With increasing the DJTE, the orbital order is strongly suppressed and a resonance state of the spin-orbital dimers appears. We confirm that the spin and orbital degrees of freedom are strongly entangled with each other in this resonance state. [Preview Abstract] |
Wednesday, March 20, 2013 9:24AM - 9:36AM |
M15.00004: Raman phonon study of Jahn-Teller distortion in Ba$_3$CuSb$_2$O$_9$ Natalia Drichko, Collin Broholm, Kenta Kimura, Rieko Ishii, Satoru Nakatsuju The frustrated magnet Ba$_3$CuSb$_2$O$_9$ does not exhibit either structural or magnetic ordering down to the lowest measured temperatures and is of great current interest as a spin-liquid candidate. It has been proposed recently that the lack of ordering is due to a static or dynamic Jahn-Teller distortion that leads to orbital disorder [1]. We use phonon Raman scattering at temperatures between 20 and 380 K to investigate Jahn-Teller distortion in crystals with different Sb:Cu stoichiometry. We focus on phonons in the range of 500-800 cm$^{-1}$ attributable to oxygen vibrations. In addition to signatures of the strong disorder due to Cu-Sb site mixing present in these materials, we observe mode-splitting due to a static Jahn-Teller distortion below 200 K in samples that undergo a transition to an orthorhombic phase. In contrast, samples that remain hexagonal to the lowest temperatures do not show such mode splitting. References: [1] S. Nakatsuji et al. Science 336, 559 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 9:36AM - 9:48AM |
M15.00005: Orbital short range correlation in Ba$_3$CuSb$_2$O$_9$ Yusuke Wakabayashi, Yuki Ishiguro, Kenta Kimura, Satoru Nakatsuji, Satoshi Tsutsui, Alfred Q.R. Baron, Tsuyoshi Kimura Ba$_3$CuSb$_2$O$_9$ is consist of short range honeycomb lattice of $S=1/2$ Cu$^{2+}$ with the Weiss temperature $-55$~K[1]. Because of the similar energy scale of the spin and orbital degrees of freedom, the interaction between them is important in this system. We have studied the behavior of the orbital degree of freedom, which can fluctuate under an effect of frustrated spin system, by means of x-ray diffuse scattering method. Measurements were performed with a four-circle diffractometer at BL-3A of the Photon Factory, KEK, Japan. Clear Huang scattering that reflects lattice strain induced by the Jahn-Teller distortion was observed. The orbital correlation provides additional scattering intensity around the $\Gamma$ point in low temperatures. The lifetime of the strain field was examined by inelastic x-ray experiments performed at BL-35XU of the SPring-8, Japan. Quasielastic intensity corresponding to the Huang scattering had slightly broader energy width than the instrumental resolution, and the lifetime was estimated as 3 picoseconds.\\[4pt] [1] S. Nakatsuji et al., Science, {\bf 336}, 559 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 9:48AM - 10:00AM |
M15.00006: Local Probe Studies of the Quantum Honeycomb Antiferromagnet Ba$_3$CuSb$_2$O$_9$ Jeffrey Quilliam, Fabrice Bert, Edwin Kermarrec, Christophe Payen, Cath\'erine Guillot-Deudon, Pierre Bonville, Philippe Mendels The 6H-perovskites, Ba$_3M$Sb$_2$O$_9$, have generated an enormous amount of interest in the last two years following the possible discovery of quantum spin liquid physics in two such materials. We present local probe studies (muon spin rotation and nuclear magnetic resonance) on the spin-1/2 honeycomb antiferromagnet Ba$_3$CuSb$_2$O$_9$. We show that the system presents no spin freezing down to temperatures as low as 20 mK. NMR measurements show evidence of a spin gap and suggest that the material has a random singlet ground state rather than the alternative spin-orbital liquid state. [Preview Abstract] |
Wednesday, March 20, 2013 10:00AM - 10:12AM |
M15.00007: Electronic structure and the suppression of the Jahn-Teller distortion in the quantum antiferromagnet Ba$_3$CuSb$_2$O$_9$ K.V. Shanavas, Z. Popovic, S. Satpathy In recent years, the field of geometrically frustrated materials have regained interest by the dicsovery of several candidates for quantum spin liquids. The antiferromagnet Ba$_3$CuSb$_2$O$_9$ is one such material where the $S=\frac{1}{2}$ on a triangular (more recently hexagonal) lattice leads to frustration. Using density functional methods, we study the electronic structure of the material, both in the triangular lattice as well as the honeycomb structure. For both structures, a simple tight-binding description involving the Cu ($e_g$) orbitals describes the band structure rather well, confirming the central role of these orbitals in the physics of the problem. It has been suggested that the Jahn-Teller effect could play an important role in the properties of the system. We find that in spite of the presence of the Cu ($d^9$) ion, the Jahn-Teller coupling is surprisingly weak in the material, which suppresses any Jahn-Teller distortion of the CuO$_9$ octahedra in the compound. [Preview Abstract] |
Wednesday, March 20, 2013 10:12AM - 10:24AM |
M15.00008: Ba NMR studies of the triangular lattice antiferromagnets Ba$_3$MSb$_2$O$_9$ (M=Co, Ni) T. Zhou, G. Koutroulakis, S.E. Brown, H.D. Zhou, J.G. Cheng, J.S. Brooks Ba$_3$MSb$_2$O$_9$, with M=Co, Ni are triangular lattice magnetic systems with near-neighbor antiferromagnetic exchange. For M=Co (S=1/2), the ground state is ordered and there are field-induced changes to the symmetry, whereas for the Ni (S=1) system there is no evidence for a phase transition to a lower-symmetry phase. Here we report on Ba nuclear magnetic resonance (NMR) spectroscopy and spin-lattice relaxation measurements for both systems. For example, the temperature dependence of the relaxation rate is independent of temperature for the Ni-based compound, and is similar to what is observed for the high-symmetry phase of the Co compound. The spin structures for the ordered phases of the Co material are also explored. [Preview Abstract] |
Wednesday, March 20, 2013 10:24AM - 10:36AM |
M15.00009: Physical Properties of new A$_{2}T$O$_{3}$ ($A = $ Na, Li, $T = $ Ru, Rh, Ir) materials Yogesh Singh, Soham Manni, Philipp Gegenwart The layered iridates $A_{2}$IrO$_{3}$ ($A \quad =$ Na, Li) have recently been suggested to be spin-orbit driven Mott insulators with their magnetism being consistent with an extended Kitaev-Heisenberg model [1-6]. While Na$_{2}$IrO$_{3}$ was found to lie deep in a magnetically ordered region, Li$_{2}$IrO$_{3}$ was suggested to lie close to the spin-liquid state expected in the strong Kitaev limit [6]. To explore the effect of chemical pressure and the effect of varying the spin-orbit coupling we have synthesized the new materials Li$_{2}$RhO$_{3}$, Na$_{2}$RuO$_{3}$, and Na$_{2}$Ir$_{\mathrm{1-x}}$Ru$_{\mathrm{x}}$O$_{3}$. We will present magnetic, electrical transport, and heat capacity measurements on these materials.\\[4pt] [1] Y. Singh and P. Gegenwart, Phys. Rev. B \textbf{82}, 064412 (2010).\\[0pt] [2] Y. Singh et al., Phys. Rev. Lett. \textbf{108}, 127203 (2012)\\[0pt] [3] S. K. Choi et al., Phys. Rev. Lett. \textbf{108}, 127204 (2012).\\[0pt] [4] F. Ye et al., Phys. Rev. B \textbf{85}, 180403 (2012)\\[0pt] [5] R. Commin, et al., Phys. Rev. Lett. (in press) 2012.\\[0pt] [6] J. Chaloupka, G. Jackeli, and G. Khaliullin, Phys. Rev.Lett. \textbf{105}, 027204 (2010). [Preview Abstract] |
Wednesday, March 20, 2013 10:36AM - 10:48AM |
M15.00010: Neutron Scattering Study on the Spin-Orbital Coupling in Mn$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$V$_{2}$O$_{4}$(x$=$0.2, 0.4, and 0.6) Jie Ma, Masaaki Matsuda, Huibo Cao, Haidong Zhou Two consecutive magnetic transitions have been reported in MnV$_{2}$O$_{4}$ compounds: the first transition is collinear and is from paramagnetic to ferrimagnetic state; The second transition, which is noncollinear, is accompanied by a tetragonal distortion, which produces an excitation gap in the magnetic spectrum [1]. However, the V-V distance is interfered with Co doping, and there is no structural phase transition observed in CoV$_{2}$O$_{4}$ down to 10 K [2]. In order to study the Co-doping effects on the structural and magnetic properties of Mn$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$V$_{2}$O$_{4}$, elastic and inelastic neutron scattering is applied in our experiments and the interaction between magnetism and orbital will be discussed.\\[4pt] [1] V. O. Garlea, et al.,Phys. Rev. Lett. \textbf{100}, 066404 (2008);\\[0pt] [2] A. Kismarahardja, et al., Phys. Rev. Lett. \textbf{106}, 056602 (2011) [Preview Abstract] |
Wednesday, March 20, 2013 10:48AM - 11:00AM |
M15.00011: Vibronic excitations in the orbitally active A-site spinels FeSc$_2$S$_4$, FeCr$_2$O$_4$, and FeCr$_2$S$_4$ J. Deisenhofer, M. Schmidt, Z. Wang, Yu. Goncharov, D.V. Quach, J.R. Groza, A. Loidl, V. Tsurkan We investigated the low-lying excitations of the spinels FeSc$_2$S$_4$, FeCr$_2$O$_4$, and FeCr$_2$S$_4$ by THz spectroscopy. FeSc$_2$S$_4$ reportedly is in a spin-orbital singlet ground state [1,2], while the other two compounds exhibit complex magnetically ordered ground states and orbital ordering transitions [3]. In all compounds we observed excitations which we assign to transitions between vibronic levels of the Fe2+ ions in tetrahedral environment. We will discuss the evolution of these excitations in the case of orbital ordering transition and the competition of spin-orbit coupling and electron-phonon interaction as a source for (spin-)orbital frustration in these systems. \\[4pt] [1] A. Krimmel et al. Phys Rev Lett. 94, 237402 (2005).\\[0pt] [2] G. Chen et al. Phys Rev Lett. 102, 096406 (2009)\\[0pt] [3] V. Tsurkan, et al., Phys. Rev. B 81, 184426 (2010). [Preview Abstract] |
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