Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session S13: Focus Session: 4d & 5d Transition-Metal Oxides |
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Sponsoring Units: DMP GMAG Chair: Warren Pickett, University of California, Davis Room: Colorado Convention Center Korbel 4C |
Wednesday, March 7, 2007 2:30PM - 3:06PM |
S13.00001: Superconductivity and Unusual Lattice Dynamics in the $\beta $-Pyrochlore Oxides Invited Speaker: Recently two families of pyrochlore oxide superconductors were found: one is $\alpha $ -pyrochlore oxide Cd$_{2}$Re$_{2}$O$_{7}$ with $T_{c}$ = 1.0 K$^{1}$ and the other is $\beta $ -pyrochlore oxides AOs$_{2}$O$_{6}$, where A = Cs, Rb and K, with $T_{c}$ = 3.3 K, 6.3 K and 9.6 K, respectively.$^{2}$ The superconductivity of the former compound is of weak-coupling BCS type, while, in the latter compouds, the superconductivity changes from conventional weak-coupling to extremely strong-coupling from Cs to K. In particular, KOs$_{2}$O$_{6}$ with the highest $T_{c}$ exhibits various unconventional features, which may be ascribed to anomalous electron-phonon couplings arising from the heavy rattling of the K ions.$^{3}$ Possibly related to this, a first-order phase transition at $T_{p}$ = 7.6 K below $T_{c}$ has been found only for KOs$_{2}$O$_{6}$. I will discuss on what is the rattling and how it affects the surrounding conduction electrons in the $\beta $-pyrochlores. \newline \newline $^{1}$M. Hanawa\textit{ et al.}, Phys. Rev. Lett. \textbf{87}, 187001 (2001). \newline $^{2}$S. Yonezawa, Y. Muraoka, Y. Matsushita, and Z. Hiroi, J. Phys.: Condens. Matter \textbf{16}, L9 (2004). \newline $^{3}$Z. Hiroi, S. Yonezawa, Y. Nagao, and J. Yamaura, submitted to PRB. [Preview Abstract] |
Wednesday, March 7, 2007 3:06PM - 3:18PM |
S13.00002: Structural Studies of Technetium and Rhenium Oxides Efrain E. Rodriguez , Frederic Poineau, Anna Llobet, Alfred P. Sattelberger, Ken Czerwinski, Anthony K. Cheetham The oxide chemistry of technetium$-99$ (t$1/2 = 2.12\times~10^{5}$ y) has not been investigated to any significant extent and presents an opportunity to explore new structural, electronic and magnetic regimes. The chemistry of rhenium broadly resembles that of technetium, and although ReO$_{3}$ is a widely studied material with a prototypic structure, it exhibits an unusual thermal expansion evolution that we have studied using neutron powder diffraction. The results of the thermal expansion study will be presented. The structural study of TcO$_{2}$ and of a new ternary oxide containing technetium and bismuth Bi$_{2}$Tc$_{2}$O$_{7}$ will be also presented and discussed. Some new reactions of TcO$_{2}$ and other oxides of heavy elements will be described. [Preview Abstract] |
Wednesday, March 7, 2007 3:18PM - 3:30PM |
S13.00003: Manifestation of on-site Coulomb and spin-orbit interactions in the ground state electronic structure of Sr$_2$IrO$_4$ Hosub Jin, Jaejun Yu In contrast to the superconducting and metallic ground states in Sr$_2$RuO$_4$ and Sr$_2$RhO$_4$, the ground state of Sr$_2 $IrO$_4$ has been reported to be a magnetic insulator. Such an insulating character of Sr$_2$IrO$_4$ is rather surprising and unexpected when the extended nature of Ir 5$d$ state is considered. To investigate the electronic structure of Sr$_2 $IrO$_4$, we performed LDA+U calculations taking account of spin-orbit interactions, where both on-site Coulomb interactions and spin-orbit couplings in the description of Ir 5 $d$ states are expected to play a significant role. From the results, it is shown that neither the on-site U nor the spin- orbit term only can explain the insulating feature of Sr$_2 $IrO$_4$. An interesting interplay between the two competing interactions is found to determine the spin and orbital configuration, leading to a novel insulating ground state. To understand the nature of the ground state, we suggest a minimal model for the $t_{2g}$ manifold based on the tight binding Hamiltonian. [Preview Abstract] |
Wednesday, March 7, 2007 3:30PM - 3:42PM |
S13.00004: Spin-Orbit Coupling Assisted Mott Insulator Sr$_{2}$IrO$_{4}$ S. J. Moon, J. S. Lee, M. W. Kim, T. W. Noh, H. Jin, B. J. Kim, J. Yu, S.-J. Oh, J.-H. Park, C. Kim, G. Cao We have systematically investigated the effect of spin-orbit coupling to the optical conductivity spectra \textit{$\sigma $}(\textit{$\omega $}) of Sr$_{2}$IrO$_{4}$. Both Sr$_{2}$RhO$_{4}$ and Sr$_{2}$IrO$_{4}$ have five $d$ electrons and similar crystal structures. However, Sr$_{2}$RhO$_{4}$ and Sr$_{2}$IrO$_{4}$ are metallic and insulating, respectively. The insulating ground state of Sr$_{2}$IrO$_{4}$ is rather surprising, since it has 5$d$ electrons, which are commonly thought to have extended orbitals. We observed a sharp absorption at about 0.5 eV in \textit{$\sigma $}(\textit{$\omega $}). This spectral feature cannot be explained in terms of orbital degeneracy and/or density wave. Note that Ir has 5$d$ electrons, so that its spin-orbit coupling should be larger than that of 4$d$ Rh ions. With the aid of the first principles calculation based on the LDA+$U$ scheme, we took into account of the effect of spin-orbit coupling. Our results clearly demonstrate that spin-orbit coupling plays a crucial role to the Mott insulating ground state of Sr$_{2}$IrO$_{4}$. [Preview Abstract] |
Wednesday, March 7, 2007 3:42PM - 3:54PM |
S13.00005: Elastic properties of the Mott transition system Ca$_{2-x}$Sr$_{x}$RuO$_{4}$. Veerle Keppens, Yanbing Luan, Sriparna Bhattacharya, Rongying Jin, David Mandrus Layered perovskite ruthenates have attracted considerable interest since the discovery of superconductivity in Sr$_{2}$RuO$_{4}$, which remains the only copper-free superconductor isostructural to the cuprates. Among the doped varieties of Sr2RuO$_{4}$, the Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ series is heavily studied, as it connects the Mott insulator Ca$_{2}$RuO$_{4}$ with the superconductor Sr$_{2}$RuO$_{4}$ and exhibits a variety of physical properties. The current work focuses on the elastic properties of Ca$_{2-x}$Sr$_{x}$RuO$_{4}$. Resonant Ultrasound Spectroscopy (RUS) has been used to measure the resonant frequencies of the samples, which are directly related to the elastic moduli, and results are presented for single crystal samples with x = 0.5, 1.9 and 2.0. The temperature-dependence of the frequencies is found to be quite unusual and reflects the rich phase diagram that sets these materials apart. [Preview Abstract] |
Wednesday, March 7, 2007 3:54PM - 4:06PM |
S13.00006: Strong orbital-dependent $d$-band hybridization and Fermi surface reconstruction in metallic Ca$_{2-x}$Sr$_{x}$RuO$_4$ Eunjung Ko, B. J. Kim, C. Kim, Hyoung Joon Choi The layered ruthenate Ca$_{2-x}$Sr$_x$RuO$_4$ displays diverse ground states ranging from a superconductor ($x$$=$2) to a Mott insulator ($x$$=$0), accompanied by structural distortions.We investigate the effects of RuO$_6$ rotation on Ru 4$d$ band structures in the metallic phase (0.5$<$$x$$<$2) by first-principles electronic structure calculations. Our study reveals that the symmetry lowering due to RuO$_6$ rotation induces a strong orbital-dependent t$_ {2g}$- e$_g$ hybridization. As a result, only the d$_{xy}$ band among three t$_{2g} $ bands is severely affected and thereby the Fermi surface is reconstructed, forming a new electron-like d$_{xy}$ Fermi surface near $\Gamma$ and nested sections near $x$$=$0.5. These findings should provide a new insight on the electron correlation in the material. [Preview Abstract] |
Wednesday, March 7, 2007 4:06PM - 4:18PM |
S13.00007: Surface Structural Phases of Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ R. G. Moore, V. B. Nascimento, Jiandi Zhang, Ismail NLS, R. Jin, D. Mandrus, E. W. Plummer Surface structural phases of Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ (CSRO) are investigated using Low Energy Electron Diffraction (LEED-IV). The surface structure and phases are compared to the bulk in this layered perovskite material. Bulk phases are generated from rotational and tilt distortions of the RuO$_{6}$ octahedral. While the surface emphasizes the system's instability against the rotational distortion, the tilt distortion is stabilized. Surface relaxation interferes with the RuO$_{6}$ tilt resulting in a lower metal-to-insulator transition (MIT) temperature (x = 0.1) and a lower tetragonal-to-orthorhombic phase transition temperature (0.2 $<$ x $<$ 0.5). In addition, structural transitions accompanying the bulk MIT are nonexistent on the surface for x = 0.1 and superstructure reflections are evident for x = 0.5 indicating a shift in the quantum critical point at the surface. [Preview Abstract] |
Wednesday, March 7, 2007 4:18PM - 4:30PM |
S13.00008: Dopant-Induced Nanoscale Electronic Inhomogeneity in Ca2-xSrxRuO4 Jiandi Zhang, Rob Moore, Shancai Wang, Hong Ding, Rongying Jin, David Mandrus, Ward Plummer Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ single crystals with 0.1 $\le \quad x \quad \le $ 2.0 have been studied systematically using scanning tunneling microscopy (STM) and spectroscopy (STS), low-energy electron diffraction (LEED), and angle resolved photoelectron spectroscopy (ARPES). In contrast to the well-ordered lattice structure, the local density of states (LDOS) at the surface clearly shows a strong doping dependent nanoscale electronic inhomogeneity, regardless of the fact of \textit{isovalent }substitution. Remarkably, the surface electronic roughness measured by STM and the inverse spectral weight of quasiparticle (QP) states determined by ARPES are found to vary with $x$ in the same manner as the bulk in-plane residual resistivity, following the Nordheim rule. For the first time, the surface measurements--especially those with STM--are shown to be in good agreement with the bulk transport results, all clearly indicating a doping induced electronic disorder in the system. [Preview Abstract] |
Wednesday, March 7, 2007 4:30PM - 4:42PM |
S13.00009: Antiferromagnetic metallic state: A transport and thermodynamic study of Ca$_{3}$(Ru$_{1-x}$Cr$_{x}$)$_{2}$O$_{7}$* V. Durairaj, S. Chikara, G. Cao, P. Schlottmann Among the variety of exciting physical properties, a signature feature of the bilayered Ca$_{3}$Ru$_{2}$O$_{7}$ is the antiferromagnetic metallic (AFM) state that lies between a Neel temperature, T$_{N}$=56 K and a Mott-like transition (MIT), T$_{MI}$=48 K. The results of our recent thermodynamic and transport study of single crystal Ca$_{3}$(Ru$_{1-x}$Cr$_{x})_{2}$O$_{7}$ (0$\le $x$\le $0.20) reveal that the temperature regime for the AFM state is significantly broadened with T$_{MI }$and T$_{N}$ being pushed to lower and higher temperatures, respectively, as Cr doping (x) increases. In addition, the magnetic easy axis for magnetization moves gradually away from \textbf{\textit{a}}-axis to \textbf{\textit{b}}-axis as x increases and at x=0.20, the magnetic anisotropy in the basal plane diminishes. This reduced spin polarization along the easy axis is promptly reflected in the less pronounced negative magnetoresistance as x increases. Furthermore, the DC current--voltage characteristics show the S-shaped negative differential resistivity for x$\le $0.17. As seen in the pure compound, observed non-ohmic behavior is restricted to the AF nonmetallic region. All results are presented along with comparisons drawn from related systems such as perovskite CaRu$_{1-x}$Cr$_{x}$O$_{3}$ where highly anisotropic magnetism is induced by Cr substitution. * This work was supported by NSF grants DMR-0240813 and DMR-0552267. [Preview Abstract] |
Wednesday, March 7, 2007 4:42PM - 4:54PM |
S13.00010: Effects of Doping on the Field-dependent Phases of Ca$_{3}$Ru$_{2}$O$_{7}$ J. F. Karpus, S. L. Cooper, G. CAO The double-layer ruthenate Ca$_{3}$Ru$_{2}$O$_{7}$ has been shown to possess a rich magnetic-field dependence for fields applied in the a-b plane. In its ground state, Ca$_{3}$Ru$_{2}$O$_{7}$ is an antiferromagnetic insulator, turning metallic above T$_{MI}$ = 48 K and paramagnetic above T$_{N}$ = 56 K. Light doping with Sr (r = 1.18 \r{A} vs. r = 1.00 \r{A} for Ca) causes a reduction in the anisotropy field due to the change in lattice parameter along the c-axis, thereby giving us a bandwidth control, while doping with trivalent La (r = 1.03 \r{A}) not only alters the bandwidth but also adds an extra electron for filling control. Using Raman scattering we map out the orbital, magnetic, and conducting phases by studying the field- and temperature-dependence of the magnetic and vibrational spectra. In this talk we describe the role Sr- and La- doping has on the field dependent phases of (Ca,Sr,La)$_{3}$Ru$_{2}$O$_{7}$. Work supported by NSF DMR02-44502, NSF DMR02-40813, and DOE DEFG02-91ER45439. [Preview Abstract] |
Wednesday, March 7, 2007 4:54PM - 5:06PM |
S13.00011: Measurement of the Mott insulating gap in Ca$_{3}$Ru$_{2}$O$_{7 }$by tunneling spectroscopy Anthony Bautista, V. Durairaj, S. Chikara, G. Cao, K.-W. Ng The bilayered Ca$_{3}$Ru$_{2}$O$_{7}$ adopts an orthorhombic structure with the tilting and rotation of RuO6 octahdera in the basal plane. It is thus highly anisotropic magnetically and electronically. The extensive d-electron orbitals generate strong coupling between spin-orbit and lattice, resulting in an exotic ground state and lead to interesting properties at low temperatures. For instance, it undergoes a transition from a paramagnetic metallic state to an antiferromagnetic metallic state as the temperature is lowered to 56K, and subsequently another transition, a Mott transition, at a lower temperature of 48K. We have prepared Ca$_{3}$Ru$_{2}$O$_{7}$/Al$_{2}$O$_{3}$/Ag tunnel junctions to measure the density of states of Ca$_{3}$Ru$_{2}$O$_{7}$ at low temperatures. The Mott insulating gap is determined to be $\sim $0.07eV when it first opens up at 48K, but the gap continues to grow with decreasing temperature. In some junctions, the density of states displays unusual activities in the temperature range of the antiferromagnetic metallic state, between 48K and 56K. Results of this study, in particular the temperature dependence of the gap structure in the density of states will be presented and discussed. [Preview Abstract] |
Wednesday, March 7, 2007 5:06PM - 5:18PM |
S13.00012: Quantum criticality and coexistence of spontaneous ferromagnetism and field- induced metamagnetism in triple-layered Sr$_{4}$Ru$_{3}$O$_{10}$ Shalinee Chikara, Vinobalan Durairaj, Gang Cao, Joseph W. Brill, Pedro Schlottmann Results of a thermodynamic and transport study of Sr$_{4}$Ru$_{3}$O$_{10}$ as a function of temperature and magnetic field are presented. The central results of this work include growing specific heat C with increasing field B, divergent magnetic contribution to C at low temperatures, an abrupt jump and a peak in C/T at B=2.90 T and =7 T for B$\vert \vert $ab-plane and B$\vert \vert $c-axis, respectively, and corresponding changes in the power law of resistivity. All results provide not only strong evidence for metamagnetic quantum criticality but also quantum fluctuations in a \textit{spontaneously} ferromagnetic state. The novelty of this work lies in the fact that the quantum criticality occurs in a system that shows both intralayer metamagnetism and interlayer \textit{spontaneous} ferromagnetism, a feature characteristically different from all other relevant systems involving quantum criticality. [Preview Abstract] |
Wednesday, March 7, 2007 5:18PM - 5:30PM |
S13.00013: Evidence of strong disorder in both the ferromagnetic and antoferromagnetic phases of SrRu$_{1-x}$Mn$_x$O$_3$ using the M\"{o}ssbauer Effect Michael DeMarco, B. Graves, G. Harmon, N. Miller, D. Coffey, B. Dabrowski, S. Kolesnik, M. Maxwell, S. Toorongian, M. Haka We investigate the magnetism and disorder in powder samples of SrRu$_{1-x}$Mn$_x$O$_3$ and find significant changes in the M\"{o}ssbauer spectra even at very low values of $x$. At $x=0.1$, although the width of the spectrum is consistent with a hyperfine field $\simeq 32 T$, which is similar to that of SrRuO$_3$(33T), a single site fit fails, suggesting that there is a range of RuO$_6$ octahedra distortions. This sensitivity to doping is also seen in the spectrum of CaRu$_{0.8}$Cr$_{0.2}$O$_3$ where there is a $\sim$30T wide distribution of hyperfine fields. At $x=0.9$ the hyperfine field is due to antiferromagnetic order and is much larger, $50T$, than at $x=0.9$. The charge state of the Ru atom has also changed from $+4$ to close to $+5$. Again the spectrum is not that of a single Ru site in spite of the low density of Ru atoms in the sample. [Preview Abstract] |
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