Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session W31: Focus Session: New Materials and Properties of Complex Oxides |
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Sponsoring Units: DMP GMAG Chair: Amos Sharoni, University of California, San Diego Room: Morial Convention Center 223 |
Thursday, March 13, 2008 2:30PM - 2:42PM |
W31.00001: Magnetic and Structural Properties of Sr-Doped Ba$_{2-x}$Sr$_{x}$CoO$_{4}$ Hao Sha, Jiandi Zhang, Q. Huang, V.O. Garlea, B.C. Sales, D. Mandrus, R. Jin We have studied the structural and magnetic properties of a newly synthesized compound Ba$_{2-x}$Sr$_{x}$CoO$_{4}$ with different doping ($x)$ levels. Monoclinic Ba$_{2}$CoO$_{4}$ is an antiferromagnetic (AFM) insulator with N\'eel temperature $T_{N}$ = 25 K and a two-dimensional character with spins aligned in the \textit{ac} plane. The isovalent Sr doping causes changes in both crystal structure and magnetic properties. With increasing $x$, $T_{N}$ initially increases then decreases after reaching the maximum at $x$=0.5. Correspondingly, its crystal structure changes from monoclinic ($x <$ 0.5) to orthorhombic ($x\ge $0.5) at room temperature. The correlation between crystal structure and physical properties will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W31.00002: Electronic structure changes in novel $J_{eff}$=1/2 system: Ruddlesden-Popper series Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$ (n=1, 2, and $\infty )$ S.J. Moon, J.S. Lee, W.S. Choi, T.W. Noh, H. Jin, J. Yu, Y.S. Lee, V. Durairaj, G. Cao, A. Sumi, H. Funakubo We investigated the electronic structures of Ruddlesden-Popper series Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$ (n=1, 2, and $\infty )$ compounds with optical spectroscopy and first-principles calculation. Among Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$, while SrIrO$_{3}$ is a metal, Sr$_{2}$IrO$_{4}$ and Sr$_{3}$Ir$_{2}$O$_{7}$ are insulators. In optical conductivity spectra \textit{$\sigma $}(\textit{$\omega $}), we found unique bandwidth-driven changes of the electronic structures which were quite different from those of 3$d $or 4$d \quad S$=1/2 systems. From the comparison between \textit{$\sigma $}(\textit{$\omega $}) and the results of first-principles calculation, we found that the intriguing changes of the electronic structures can be realized by the cooperative interaction between the SO coupling and the electron correlation. These results clearly demonstrate that Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$ should be considered as a $J_{eff}$=1/2 single band system. [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W31.00003: Electrical Resistance of Quasi-1D Li$_{0.9}$Mo$_{6}$O$_{17}$ at Very High Magnetic Field Carlos A.M. dos Santos, J. Moreno, B.D. White, J.J. Neumeier, L. Balicas Recently, photoemission experiments, band structure calculations, tunneling, and the description of the electrical resistivity by two power-law terms suggest that Li$_{0.9} $Mo$_{6}$O$_{17}$ is an excellent example of a metallic Luttinger-liquid (LL) [a,b]. The crossover from metallic to insulating-like behavior near $T_M$ = 28 K was addressed by thermal expansion experiments which suggest that a dimensional crossover sets the stage for superconductivity [b]. To obtain more information about the crossover at $T_M$, magnetoresistance measurements were performed under very high magnetic field (0 $< H <$ 23 tesla). The results show that the minimum at $T_M$ increases with increasing $H$. The power-law temperature dependence of the electrical resistance at $T_M (H)$ is also evaluated. [a] C. A. M. dos Santos, M. S. da Luz, Yi-Kuo Yu, J. J. Neumeier, J. Moreno, and B. D. White. Submitted to Phys. Rev. Let. (2007). [b] C. A. M. dos Santos, B. D. White, Yi-Kuo Yu, J. J. Neumeier, and J. A. Souza, Phys. Rev. Let. {\bf98}, 266405 (2007). [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:42PM |
W31.00004: Unconventional magneto-transport in novel layered cobalt oxides Invited Speaker: Among strongly correlated transition-metal oxides, cobalt oxides are known to have unique features arising from the spin-state degree of freedom tightly coupled with Co valence. The Co$^{4+}$ ion in the low spin-state is responsible for anomalous metallic states such as large thermopower in Na$_{x}$CoO$_{2}$ and unconventional superconductivity in hydrated Na$_{x}$CoO$_{2}$. The Co$^{2+}$ ion favors the high-spin state, which makes magnetic insulators. The Co$^{3+}$ ion is most interesting in the sense that the low-, intermediate- and high-spin states are nearly degenerate, where a spin-state crossover/transition occurs with temperature or pressure. Recently we have discovered two complex layered cobalt oxides, which exhibit unprecedented transport originated from interplay between charge, orbital and spin-states. The first one is SrCo$_{6}$O$_{11}$, in which the Co-O Kagome lattice and two-types of Co-O pillars are stacked along the c axis [1]. The conduction electrons in the Kagome lattice interact with Ising spins in the pillars, and shows two-step plateau in the magnetoresistance along the c axis. The second one is Sr$_{3}$YCo$_{4}$O$_{10.5}$, which exhibits a ferromagnetic insulating state below 340 K. Various substitutions of Sr, Y and Co sites dramatically suppress this ferromagnetic state, and concomitantly modify the magneto- and thermoelectric transport. We will discuss the structure-property relationship based on structure analyses. The main part of this work was done in collaboration with S. Ishiwata, W. Kobayashi, and M. Takano. \newline [1] S. Ishiwata et al., Chem. Mater. 17, 2789 (2005)~; Phys. Rev. Lett. 98, 217201 (2007) \newline [2] W. Kobayashi et al. Phys. Rev. B 72, 104408 (2005)~; S. Ishiwata et al. Phys. Rev. B75, 220406(R) (2002) [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W31.00005: Search for Half-Metallic Antiferromagnetism in Double Perovskites V. Pardo, W. E. Pickett The wide class of double perovskite oxides was proposed earlier (PRB 57, 10613 [1998]) as promising for producing a half-metallic antiferromagnet [HMAFM] (more correctly, a spin-compensated half metal). Here we present examples of the affects of structural distortions on the electronic and magnetic properties in selected members. For La$_2$CrNiO$_6$ the idealized cubic perovskite structure had led to a spin-antiparallel state with net moment of 0.6 $\mu_B$, but a ferromagnetic half-metallic state (4 $\mu_B$) was 150 meV per metal atom lower in energy (within local density approximation). Starting with experimental information on LaCrO$_3$ and LaNiO$_3$ and their alloys, we have relaxed the volume and the (seven) internal coordinates within the orthorhombic Pnma space group. The charge states can be characterized by Cr$^{4+}$ and Ni$^{2+}$. The ferromagnetic state is lower by 50 meV within the generalized gradient approximation. Using LDA+U (U=3 eV on each transition metal ion) opens a gap of 0.6 eV (FM insulator) and is favored by 120 meV over the antialigned state. Although no HMAFM state is obtained, these results show that structural relaxation must be taken into account, and that in some cases (as here) it may make the antialigned state more favorable. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W31.00006: Electrical transport and thermodynamic properties of SrNbO$_{3.41}$ Ariana de Campos, Ann Deml, B.D. White, C.A.M. dos Santos, M.S. da Luz, J.J. Neumeier In 1991, Lichtenberg et al.$^1$ reported the electric conductivity of SrNbO$_{3.41}$ revealing quasi-1D behavior. This system offers many possibilities to vary the compositional, structural, chemical, and physical properties.$^1$ Depending upon the temperature range and crystallographic direction, it exhibits metallic behavior or a metal-semiconductor transition. In this work, the properties of SrNbO$_{3.41}$ single crystals are revisited. The single crystals were grown by the floating zone method and characterized by x-ray diffraction. Electrical resistivity as a function of temperature was measured with four-probe and Montgomery methods. We will also report results of heat capacity and thermal expansion measurements. $^1$F. Lichtenberg et al., Z. Phys. B {\bf 84}, 369 (1991); F. Lichtenberg et. al., Prog. Solid State Chem. {\bf 29}, 1-70 (2001). [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W31.00007: Extreme electron-phonon coupling in magnetic rubidium sesquioxide Robert de Groot, Jisk Attema, S. Riyadi, Greame Blake, Gilles de Wijs, Thomas Palstra Rb$_2$O$_3$ is a black, opaque oxide. Early work suggests that the stability range of the sesquioxide phase in the rubidium-oxygen phase diagram is rather broad. Rb$_2$O$_3$ remains cubic down to the lowest temperature measured (5~K). The oxygens form dumbbells with interatomic distances in between those of peroxide and superoxide anions, and strong athermal motion persists down to low temperatures. [1] Electronic-structure calculations show that the dynamics at low temperature is caused by 6 phonon modes of zero frequency, which induce a very strong electron-phonon interaction. The softness of half of these modes is suppressed by the application of pressure. Calculated using the average oxygen positions, rubidium sesquioxide is a half-metallic ferromagnet. [2] \newline [1] CR CHIM (11-13): 591-594 NOV 1999\newline [2] JACS 127 (46): 16325-16328 NOV 23 2005 [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W31.00008: Gigantic optical magneto-electric effect in CuB$_{2}$O$_{4}$ Mitsuru Saito, Kouji Taniguchi, Takahisa Arima It has been recognized since 1960s that magneto-electric (ME) materials may also show an optical magneto-electric (OME) effect showing up as a change in optical absorption with reversal of the propagating direction of light. The OME effect is an interesting object of scientific research and provides possibilities for applications. However, the changes in absorption coefficient ever discovered were very small (less than 0.2 {\%}). We present a gigantic OME effect in a noncentrosymmetric weak ferromagnet CuB$_{2}$O$_{4}$, in which the absorption coefficient changes by a factor of three with reversal of a very weak magnetic field of 300 Oe. This magnitude of OME effect enables us to observe it by a CCD camera with linearly polarized near-infrared and visible light. Spectroscopic study and comparison of OME effect with magnetization indicate an important role of canted antiferromagnetic spin ordering and local symmetry of a square Cu$^{2+}$ site. The gigantic OME effect can be applicable to optical devices like magnetic switching of color in the future. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W31.00009: Resonant inelastic X-ray scattering study of quasi-zero-dimensional copper metaborate Jason Hancock, Guillaume Chabot-Couture, Martin Greven, Guerman Petrakovskii, Kenji Ishii, Jun'ichiro Mizuki CuB$_2$O$_4$ consists of many CuO$_4$ plaquettes separated by B ions. We report a study of the electronic excitation spectra of this system in order to explore the relationship between excitation symmetry and the resonant inelastic X-ray scattering (RIXS) technique. We find a small number of well separated features in the experimentally accessible range of 0.5-15 eV energy transfer, and weak dispersion is suggestive of the quasi-zero-dimensional nature of this system. Systematic trends in the data are suggestive of a composite nature to one of the observed features. Using a cluster model, we describe these unexpected trends and clarify how the choice of experimental geometry selectively influences the sensitivity to particular excitation symmetries in the RIXS experimental technique. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W31.00010: Powder neutron diffraction study of quasi-one-dimensional Li$_{0.9}$Mo$_{6}$O$_{17}$ Mario S. da Luz, C.A.M. dos Santos, B.D. White, J.J. Neumeier, Q. Huang, J.B. Leao, J.W. Lynn The crystallographic structure of quasi-one-dimensional Li$_{0.9}$Mo$_6$O$_{17}$ was investigated by Rietveld refinement of powder neutron diffraction data at temperatures in the range 5 K $< T <$ 295 K. Structural parameters, atomic positions, occupation numbers, and isotropic thermal parameter $B_{iso}$ will be reported. The occupancy was refined revealing a Li occupancy greater than 0.9. Bond valences sums will also be reported for various Li and Mo sites. At room temperature, the crystal was found to exhibit monoclinic symmetry with space group P21/m and lattice parameters $a$ =12.7506(1) \AA, $b$ = 5.5242(1) \AA, $c$ = 9.4913(2) \AA \, and $\beta$ = 90.593(1)$^o$. Good agreement between the temperature dependence of lattice parameters and high resolution thermal expansion results$^*$ was obtained. $^*$C. A. M. dos Santos, B. D. White, Yi-Kuo Yu, J. J. Neumeier, and J.A. Souza, Phys. Rev. Lett. {\bf98}, 266405 (2007). [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W31.00011: Unusual Physical Properties of Ca$_{3}$Co$_{4}$O$_{9}$ Rongying Jin, Larry Allard, Doug Blom, Sriparna Bhattacharya, Veerle Keppens, Brian Sales, David Mandrus We have investigated the structural and physical properties of Ca$_{3}$Co$_{4}$O$_{9}$ single crystals including electrical and thermal conductivity, thermopower, specific heat, magnetic susceptibility, and electron diffraction. The study reveals many interesting features that are unique to Ca$_{3}$Co$_{4}$O$_{9}$. In addition to high thermopower and low thermal conductivity, the low-temperature specific heat yields large electronic specific heat coefficient, suggesting strong electron-electron correlation. However, the electronic specific heat coefficient is dramatically decreased under magnetic field, implying the modification of electronic density of states by magnetic field. The magnetic susceptibility data indicate that there are several magnetic transitions above room temperature. The possible correlation between charge, spin, and lattice will be explored. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W31.00012: X-ray absorption and x-ray magnetic dichroism study on Ca$_3$CoRhO$_6$ and Ca$_3$FeRhO$_6$ Tobias Burnus, Zhiwei Hu, Julio C. Cezar, Seiji Niitaka, Hua Wu, Hidenori Takagi, Chun Fu Chang, Nicholas B. Brookes, Ling-Yun Jang, Keng S. Liang, L. Hao Tjeng The valence-state of the transition-metal ions in the chain-like compounds Ca$_3$CoRhO$_6$ and Ca$_3$FeRhO$_6$ is currently an issue under debate. Using numerical simulations and x-ray absorption spectroscopy at the Rh-$L_{2,3}$, the Co-$L_{2,3}$, and the Fe-$L_{2,3}$ edges we reveal a Co$^{2+}$/Rh$^{4+}$ configuration in Ca$_3$CoRhO$_6$ and Fe$^{3+}$/Rh$^{3+}$ in Ca$_3$FeRhO$_6$. X-ray magnetic circular dichroism at the Co-$L_{2,3}$ edge shows that the Co$^{2+}$ ions carry a giant orbital moment of about $1.7\mu_B$. We attribute this to a $d_1^0d_1^2$ ground state for the high-spin Co $3d^7$ configuration in trigonal prismatic coordination. The intrachain-ferromagnetic coupling of two neighboring Co ions is mediated by a low-spin Rh$^{4+}$ ion ($S = 1/2$) in between. The results agree with our recent ab-initio study [Hua Wu {\it et al.}, Phys. Rev. B {\bf 75}, 245118 (2007)]. [Preview Abstract] |
Thursday, March 13, 2008 5:18PM - 5:30PM |
W31.00013: Anisotropy in magnetic properties of single crystal LiFePO$_{4}$ Gan Liang, Keeseong Park, John Markert, Jiying Li, David Vaknin We report the experimental and theoretical results on the anisotropies in the magnetic properties and x-ray absorption spectra of single crystal LiFePO$_{4}$. A mean-field theory is developed to explain the observed strong anisotropies in Lande g-factor, paramagnetic Curie temperature, and effective moment for LiFePO$_{4}$ single crystals. The values of the in-plane nearest- and next-nearest-neighbor spin-exchange ($J_{1}$ and $J_{2})$, inter-plane spin-exchange ($J_{\bot })$, and single-ion anisotropy ($D)$, obtained recently from neutron scattering measurements, are used for calculating the Curie temperatures with the formulas derived from the mean-field Hamiltonian. It is found that the calculated Curie temperatures match well with that obtained by fitting the magnetic susceptibility curves to the modified Curie-Weiss law. [Preview Abstract] |
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