Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session D36: Focus Session: Bulk Properties of Complex Oxides -- Cobaltites and Vanadates |
Hide Abstracts |
Sponsoring Units: DMP GMAG Chair: John Mitchell, Argonne National Laboratory Room: E146 |
Monday, March 15, 2010 2:30PM - 3:06PM |
D36.00001: Impact of Na ordering on the electronic properties of the Co planes in Na$_{x}$CoO$_{2}$ Invited Speaker: In 2003 superconductivity at 4.5K has been discovered in Na$_{0.35}$CoO$_{2}$,1.3H$_{2}$O, a hydrated cobaltate. These layered cobaltates Na$_{x}$CoO$_{2}$ have some analogies with the cuprates,~as 2D conductivity occurs in the CoO$_{2}$ planes, where the Co are arranged on a triangular lattice, and doping can be modified by changing the Na content. But, contrary to the case of most cuprates for which dopant induced disorder is quite influential, the doping achieved in cobaltate samples is associated with the insertion of well ordered 2D Na structures and even 3D ordered stacks of the Na/Co planes. We found that the correlations between magnetic properties and Na structural order are uniquely probed locally through NMR/NQR measurements on both $^{23}$Na and $^{59}$Co nuclei. The Na ordering appears responsible for very distinct electronic ground state properties of the metallic CoO$_{2}$ planes, which do exhibit ferromagnetic in plane correlations for $x>$ 0.62 up to the highest Na content (except for x=1 which is a band insulator). These metallic ferromagnetic correlations do not yield any static magnetic order down to $T$=0$^{(1)}$ for $x<$0.75, beyond which an AF 3D order occurs (in plane ferromagnetic with AF 3D stacking$^{(2)})$. On the contrary for $x<$2/3, in plane AF correlations appear$^{(3)}$, though static 3D AF ordering at low $T$ is only achieved for $x$=1/2 $^{(4)}$. For $x>$0.5, the nature of the ground state and the anomalous magnetic properties are found to be linked with a charge segregation which localizes an ordered array of non magnetic Co$^{3+}$ sites in a metallic Co background responsible for the magnetic properties$^{(5)}$. This is contrary to many expectations for which Na order was expected to pin local magnetic moments in a metallic and less magnetic bath. NMR/ NQR experiments have allowed us recently to determine the atomic structure of the $x$=2/3 phase. We demonstrated that the associated Co charge order is somewhat peculiar as the nearly ferromagnetic metallic state results of hole delocalization on a simple kagome lattice of Co sites$^{(6)}$. Further efforts to determine the actual arrangement of the non magnetic cobalt sites for different Na concentrations is presently undertaken and should give hints to link the ground state properties with the Co charge arrangements. \\[4pt] [1] H. Alloul, I. Mukhamedshin, G. Collin , N. Blanchard, Europhysics Lett. \textbf{82}, 17002 (2008).\\[0pt] [2] S. P. Bayrakci S. P. et al., Phys. Rev. B, \textbf{69} (R) 100410 (2004).\\[0pt] [3] G. Lang, J. Bobroff, H. Alloul, G. Collin, N. Blanchard, Phys. Rev. B \textbf{78}, 155116 (2008).\\[0pt] [4] J. Bobroff, G. Lang, H. Alloul, N. Blanchard, G. Collin, Phys. Rev. Letters \textbf{96}, 107201 (2006).\\[0pt] [5] I.R. Mukhamedshin, H.Alloul, N. Blanchard, G. Collin, Phys. Rev. Letters \textbf{94}, 247602 (2005).\\[0pt] [6] H.Alloul, I.R. Mukhamedshin, T. A. Platova, A.V. Dooglav, Europhysics Lett. \textbf{85}, 47006 (2009) [Preview Abstract] |
Monday, March 15, 2010 3:06PM - 3:18PM |
D36.00002: Synthesis, Structure, and Thermoelectric Properties of Misfit-Layered Cobalt Oxides Gregory T. McCandless, Ho Nyung Lee, Rongying Jin Using two methods of crystal growth (flux and floating zone technique), high purity single crystals of misfit-layered cobalt oxides, Ca$_{3}$Co$_{4}$O$_{9}$ and Bi$_{2}$Sr$_{2}$Co$_{2}$O$_{8}$, have been grown. The latter compound has also a thin film form which was grown by pulsed laser deposition (PLD) method. In addition to structural information, physical property measurements (such as thermopower, resistivity and thermal conductivity) of these compounds will be presented. The correlation between structure and physical properties will be discussed. [Preview Abstract] |
Monday, March 15, 2010 3:18PM - 3:30PM |
D36.00003: Spin-state transition and phase separation in multi-orbital Hubbard model Sumio Ishihara, Ryo Suzuki, Tsutomu Watanabe Exotic phenomena in correlated electron systems are responsible for competition and cooperation between multi-electronic phases. In particular, in perovskite cobaltites, there is the spin-state degree of freedom, i.e., multiple spin states due to the different electron configurations in a single ion. The multiple spin states occur by changes in the carrier concentration, temperature and other parameters. In the lightly hole doped region between the low-spin band insulator (BI) and the high-spin (HS) ferromagnetic metallic (FM) states, several inhomogeneous features have been reported experimentally. We address the issues of the spin-state transition and the phase separation (PS) associated with this transition by analyzing the multi-orbital Hubbard model [1]. We examine the electronic structures in hole doped and undoped systems by the variational Monte-Carlo (VMC) method. We find that the electronic PS is realized between the nonmagnetic BI and the HS FM metal. We conclude that the different band widths play an essential role in the present electronic PS. [1] R. Suzuki, T. Watanabe, and S. Ishihara, Phys. Rev. B 80, 054410 (2009). [Preview Abstract] |
Monday, March 15, 2010 3:30PM - 3:42PM |
D36.00004: Orbital 120$^{\circ}$ model on pyrochlore lattice: MnV$_2$O$_4$ Gia-Wei Chern, Natalia Perkins Recent first-principles calculation on vanadium spinel MnV$_2$O$_4$ reveals a significant trigonal distortion at the vanadium site [1]. Its interplay with other known interactions in this compound, including spin-orbit coupling, Jahn-Teller effect, V-V and Mn-V exchanges, is yet to be understood. To make analytical calculations tractable, we present a theoretical model based on a large trigonal crystal field to describe orbital ordering in MnV$_2$O$_4$. At the single-ion level, the trigonal distortion leaves a doubly degenerate ground state and breaks the approximate rotational symmetry of $t_{2g}$ orbitals. We find that the effective interaction between these low-energy doublets is described by a quantum antiferromagnetic 120$^\circ$ model [2] on a pyrochlore lattice. We obtain the classical ground state and show its stability against quantum fluctuations. The corresponding orbital order consists of two inequivalent orbital chains with an additional modulation of electron density within the chain, consistent with experiments and {\em ab initio} calculations. Furthermore, in the presence of orbital ordering, single-ion spin anisotropy arising from relativistic spin-orbit interaction stabilizes the experimentally observed orthogonal magnetic structure. [1] S. Sarkar, T. Maitra, R. Valent\'i, and T. Saha-Dasgupta, Phys. Rev. Lett. {\bf 102}, 216405 (2009). [2] Z. Nussinov, M. Biskup, L. Chayes, and J. v. d. Brink, Europhys. Lett. {\bf 67}, 990 (2004). [Preview Abstract] |
Monday, March 15, 2010 3:42PM - 3:54PM |
D36.00005: Phonon dynamics of ferromagnetic Mott insulator Lu$_{2}$V$_{2}$O$_{7}$ S. J. Moon, A. Schafgans, D. N. Basov, W. S. Choi, T. W. Noh, J. Akimitsu We investigated phonon dynamics of ferromagnetic Mott insulator Lu$_{2}$V$_{2}$O$_{7}$ using infrared spectroscopy. Pyrochlore structure inherent to geometrical frustration could give rise to intriguing spin-phonon coupling effect. We observed that phonons showed anisotropic line shapes indicating their coupling to broad excitation. As temperature decreased, some phonons became strongly softened without an anomaly across the magnetic transition. In addition, the phonon spectra showed little change with the application of magnetic field. We will discuss possible effects of the coupling of phonon to spin and orbital. [Preview Abstract] |
Monday, March 15, 2010 3:54PM - 4:30PM |
D36.00006: Spin and Orbital Ordering in Vanadates Invited Speaker: Vanadate compounds provide ideal systems to study the interactions between spin, lattice and orbital degrees of freedom. This talk will discuss the physics of two vanadates, CaV$_{2}$O$_{4}$ and MgV$_{2}$O$_{4}$. In both materials the Vanadium ion is in the 3+ valence state resulting in two electrons in the 3d-shell and a spin of S=1. In CaV$_{2}$O$_{4}$ the V$^{3+}$ ions form quasi-one-dimensional zig-zag chains with frustrated first and second neighbor exchange interactions, while in MgV$_{2}$O$_{4}$ they lie on a frustrated pyroclore lattice. The electronic configuration of the V$^{3+}$ ions consists of two electrons in the three t$_{2g}$ levels giving rise to orbital degrees of freedom in these compounds. The orbital and magnetic degrees of freedom are strongly coupled because the magnetic interactions occur via direct overlap of the t$_{2g}$ orbitals and as a result orbital ordering has a strong impact on the exchange pathways and magnetic behaviour. Both compounds undergo structural phase transitions which either partially or fully lift both the orbital degeneracy and magnetic frustration, long-range antiferromagnetic order then occurs at a lower temperature. Heat capacity, DC susceptibility and neutron and x-ray scattering data will be presented. The results reveal that at high temperatures CaV$_{2}$O$_{4}$ behaves as a Haldane chain, but at low temperatures, it is a spin-1 ladder, while in MgV$_{2}$O$_{4}$ three-dimensional magnetism is replaced by one-dimensional behaviour at low temperatures. The results are discussed in term of orbital ordering. [Preview Abstract] |
Monday, March 15, 2010 4:30PM - 4:42PM |
D36.00007: Orbital ordering in CaV$_{2}$O$_{4}$: A neutron scattering study Oliver Pieper, B. Lake, A. Daoud-Aladine, M. Reehuis, T. Perring, M. Enderle, K. Rule, K. Prokes, B. Klemke, K. Kiefer, A. Niazi, J.Q. Yan, D.C. Johnston, A. Honecker CaV$_{2}$O$_{4}$ is a quasi-one dimensional spin-1 Heisenberg antiferromagnet. The magnetism arises from the partially filled t$_{2g}$-levels of the V$^{3+}$-ions, which in addition give an orbital degree of freedom to the system. In contrast to the isovalent vanadium spinel compounds, the low dimensionality in CaV$_{2}$O$_{4 }$already arises from the crystal structure. It contains weakly coupled double-chains of edge-sharing VO$_{6}$ octahedra, where the particular octahedral staggering creates a zigzag-like arrangement of the vanadium ions. This in return gives rise to strong magnetic direct exchange interactions between nearest and next nearest neighbor vanadium ions and to geometrical frustration. However, the strength of the exchange interactions is strongly influenced by the particular occupation of the t$_{2g}$-orbitals. Depending on the type and degree of octahedral distortion, the system can be interpreted as a frustrated Haldane chain or a spin-1 ladder. Here we use single crystal neutron diffraction and neutron spectroscopy to determine the spin structure as well as the complex excitation spectrum of CaV$_{2}$O$_{4}$. The results are analyzed theoretically and from this the leading exchange paths are deduced and discussed in terms of orbital ordering. [Preview Abstract] |
Monday, March 15, 2010 4:42PM - 4:54PM |
D36.00008: Synthesis, structure and electrical properties of a strontium manganese vanadate Qifan Yuan, R.L. Kallaher, V. Soghomonian Metal oxide frameworks offer materials opportunities in electrical energy storage and conversion applications. We present synthesis, structural and physical characterization, as well as electrical properties of a strontium manganese vanadate framework. Dark rectangular plates are isolated from hydrothermal reactions. The vanadate crystallizes in the monoclinic space group C2/m. The structure consists of octahedral manganese sites connected to each other through vanadate tetrahedral units, forming manganese vanadate layers. The layers are in turn connected to each other through octahedral strontium sites. Within the manganese vanadate layers, multiple oxidation states are present for the transition metal sites. Single crystals were contacted, and four point conductivity measurements obtained as a function of temperature. The electronic conductivity for manganese vanadate framework ranges from, dependent on crystal direction, 2 x 10$^{-6} \quad \Omega ^{-1}$ cm$^{-1}$ to 7 x 10$^{-7} \quad \Omega ^{-1}$ cm$^{-1}$. The conductivity within the manganese vanadate planes is greater than the conductivity between planes, as expected. Preliminary results intercalating ions into the framework show potential for charge storage in the material. We acknowledge partial support from NSF DMR-0943971. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700