Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session J25: Focus Session: Novel and Complex Oxides: Ruthenates and Osmiumates |
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Sponsoring Units: DMP Chair: Chris Leighton, University of Minnesota Room: LACC 501A |
Tuesday, March 22, 2005 11:15AM - 11:51AM |
J25.00001: Superconductivity in the Osmium-based Beta-Pyrochlore Oxides Invited Speaker: Superconductivity is reported in recently discovered \textit{$\beta $}${\rm t}$pyrochlore oxides AOs$_{2}$O$_{6}$. The $T_{c}$ is 3.3 K, 6.3 K, and 9.6 K for A = Cs, Rb, and K, respectively. The highest $T_{c}$ of KOs$_{2}$O$_{6}$ is almost one order higher than the $T_{c}$ = 1.0 K of previously reported \textit{$\alpha $}-pyrochlore oxide superconductor Cd$_{2}$Re$_{2}$O$_{7}$ which is believed to be a conventional $s$-wave superconductor. Moreover, the upper critical field $H_{c2}$ of KOs$_{2}$O$_{6}$ is estimated to be 38 T, which seems to exceed Pauli's limit expected for conventional superconductivity. This is again in contrast to the case of Cd$_{2}$Re$_{2}$O$_{7}$, in which the $H_{c2}$ is 0.29 T, much smaller than the corresponding Pauli's limit. These distinct contrasts strongly suggest that the mechanism of superconductivity is essentially different between the two pyrochlore oxides. It is to be noted that the $T_{c}$ of these \textit{$\beta $}${\rm t}$pyrochlore oxides decreases with increasing the ionic radius of the alkaline metal ions, imposing negative chemical pressure upon the Os pyrochlore lattice. I believe that interesting physics is involved on the basis of strong electron correlations on the highly frustrated pyrochlore lattice. [Preview Abstract] |
Tuesday, March 22, 2005 11:51AM - 12:03PM |
J25.00002: Magnetic excitations of Sr$_{3}$Ru$_{2}$O$_{7}$ Matthew Stone, Mark Lumsden, Stephen Nagler, Brian Sales, Rongying Jin, David Mandurs Although transport measurements of the layered perovskite Sr$_{3}$Ru$_{2}$O$_{7}$ abound, a clear understanding of the underlying magnetic excitation spectrum is far from complete. Knowledge of the details of the magnetic fluctuations in this material has implications for both cuprate superconductors as well as other doped and undoped ruthenate compounds. We present a series of inelastic neutron scattering measurements as a function of temperature and wave-vector transfer in the (H 0 L) scattering plane. The magnetic response is clearly visible in constant E scans at the wavevector (0.75,0,L) measured carefully for T = 3.8 K up to T = 100 K up to $\hbar \omega $= 14 meV. ORNL is managed by UT-Battelle for the US DOE under contract DE-AC05-00OR22725. [Preview Abstract] |
Tuesday, March 22, 2005 12:03PM - 12:15PM |
J25.00003: Magnetic Excitation Spectrum of Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ M.D. Lumsden, S.E. Nagler, R. Jin, D. Mandrus, S. Wilson, P. Dai We have studied the concentration dependence of the magnetic excitation spectrum in single crystal samples of the layered perovskite ruthenates, Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ for 2$>$x$>$0.4. For large x, the spectrum is similar to that observed in pure Sr$_{2}$RuO$_{4}$ with incommensurate excitations strongly peaked in Q at ($\pm $0.3, $\pm $0.3, q$_{z})$, consistent with Fermi-surface nesting wavevectors. As the concentration approaches the x=0.5 quantum critical point, the spectrum becomes broadly distributed in \textbf{Q} with a sharp upturn at $\pm $0.3 in both $h$ and $k$ and a flat distribution of scattering across the 2d ferromagnetic zone center. Possible interpretations of this scattering and qualitative similarity to the excitation spectrum of Sr$_{3}$Ru$_{2}$O$_{7}$, also in close proximity to a quantum critical point, will be discussed. ORNL is managed by UT-Battelle for the US DOE under contract DE-AC05-00OR22725. [Preview Abstract] |
Tuesday, March 22, 2005 12:15PM - 12:27PM |
J25.00004: Low Temperature Structural Phase Transitions in Novel Oxides Chris Kendziora, Ivan A. Sergienko, D. G. Mandrus, B. C. Sales, R. Jin, P. Khalifah, Jian He Analysis of the Raman active phonon modes offers a symmetry dependent determination of structural phase transitions. We have performed polarized Raman scattering measurements on oriented single crystals of the superconducting pyrochlore Cd$_{2}$Re$_{2}$O$_{7}$ and the layered ruthenate La$_{4}$Ru$_{2}$O$_{10}$ as a function of temperature. In Cd$_{2}$Re$_{2}$O$_{7}$ we resolve and assign each of the six Raman-active (A$_{1g}$ + E$_{g}$ + 4F$_{2g}$) modes of the room temperature cubic phase. Below the structural phase transition at 200K (and 120K) we observe new symmetry dependent Raman-active vibrations associated with a cubic-tetragonal (tetragonal-tetragonal) phase transition. We identify two ``soft" modes and discuss a structural order parameter with E$_{u}$ symmetry. We measure La$_{4}$Ru$_{2}$O$_{10}$ through the monoclinic-triclinic phase transition @ 150K and compare the symmetry dependent results with expectations based on x-ray structural analysis. [Preview Abstract] |
Tuesday, March 22, 2005 12:27PM - 1:03PM |
J25.00005: Quasi-Two-Dimensional Metallic Ground State of Ca$_3$Ru$_2$O$_7$ Invited Speaker: Ca$_3$Ru$_2$O$_7$ is a three-dimensional antiferromagnetic metal between a first-order metal to nonmetal transition at 48~K and the antiferromagnetic ordering temperature, $T_{\rm N}$=56~K[1]. The crystal structure is the double layered Ruddlesden-Popper type with the $Bb2_1m$ space group, which has both the rotation and tiling of RuO$_6$ octahedra. We have succeeded in growing single crystals of Ca$_3$Ru$_2$O$_7$ using a floating-zone method for the first time. The temperature dependence of the electrical resistivity establishes that Ca$_3$Ru$_2$O$_7$ develops a quasi-two-dimensional metallic ground state below 30 K, from which the observed quantum oscillation derives. The specific heat measurement reveals the electronic specific-heat coefficient $\gamma$ to be as small as 1.7 mJ/Ru mol K$^2$[2]. From the results of powder neutron diffractions, we proposed the most possible magnetic structure with an antiferromagnetic ordering. The field dependence of the resistivity at the metamagnetic transition around 6 T can be explained by the tunneling magnetoresistance. This work was done in collaboration with S. I. Ikeda, N. Shirakawa, C. H. Lee, M. Kosaka, and S. Katano. [1] G. Cao et al., Phys. Rev. Lett. 78 (1997) 1751. [2] Y. Yoshida et al., Phys. Rev. B 69 (2004) R220411. [Preview Abstract] |
Tuesday, March 22, 2005 1:03PM - 1:15PM |
J25.00006: Phonon Instabilities in Ca(1.4)Sr(0.6)RuO4 Rob G. Moore, M.D. Lumsden, R. Jin, J. Zhang, D. Mandrus, E.W. Plummer Phonon instabilities in Ca$_{1.4}$Sr$_{0.6}$ RuO$_{4}$ are investigated by inelastic neutron scattering techniques. Sr$_{2}$RuO$_{4}$ is an unconventional p-wave superconductor with the same structure as La$_{2}$CuO$_{4}$, the parent compound of the high T$_{c}$ superconductor La$_{2-x}$Sr$_{x}$CuO$_{4}$. By partial substitution of Ca$^{+2}$ for Sr$^{+2}$, induced structural stresses create a complex phase diagram with exotic phases. La$_{2}$CuO$_{4}$ has a temperature dependent $\Sigma _{4}$ phonon instability, correlated with the tetragonal to orthorhombic structural transition. It is anticipated that Ca$_{1.4}$Sr$_{0.6}$RuO$_{4}$ will exhibit similar behavior as a precursor to its tetragonal to orthorhombic phase transition. Indeed the instability exists, but two anomalies appear in the spectra. A new phonon mode appears mimicking the dispersion of the $\Sigma _{4}$ phonon and a new Bragg peak appears incommensurate with the tetragonal unit cell. The origins and implications of the anomalies will be discussed. Work supported by NSF- DMR 0105232, NSF-DMR0346826, and DOE DE-FG02-04ER46125. ORNL, managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contract DE-AC05-00OR22725 [Preview Abstract] |
Tuesday, March 22, 2005 1:15PM - 1:27PM |
J25.00007: Low temperature magneto-transport measurements on Ca$_{1.5}$Sr$_{0.5}$RuO$_4$ H.C. Choi, J.-H. Park, S.-B. Cho, M.W. Meisel, Y. Lee, R. Jin, D. Mandrus In-plane electrical transport measurements were performed on Ca$_{1.5}$Sr$_{0.5}$RuO$_{4 }$in the presence of magnetic fields up to 8 T applied in the direction perpendicular to the plane. Upon substituting Sr with isovalent Ca, Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ shows an intriguing phase diagram ranging from p-wave superconductor at x = 2 to Mott insulator at x $\le $ 0.2. The x = 0.5 system investigated in this work is reported to be at the boundary between the magnetic metal (x $<$ 0.5) and the paramagnetic metallic phase. A small but distinct increase in resistance was observed at T* $\approx $ 450 mK on warming. In addition, T* decreases with the applied magnetic field, and the feature in resistance disappears around 500 G. Our detailed magneto-resistance measurements reveal unusual behavior in the low temperature and low magnetic field region that, we believe, is directly related to the resistance anomaly observed near 450 mK in zero magnetic field. [Preview Abstract] |
Tuesday, March 22, 2005 1:27PM - 1:39PM |
J25.00008: Three-Dimensional Band Structure of Sr$_4$Ru$_3$O$_{10}$ F. Wang, J.W. Allen, J.D. Denlinger, X.N. Lin, Gang Cao The electronic structure and Fermi Surface (FS) of the triple-layer ruthenate Sr$_4$Ru$_3$O$_{10}$ is probed with angle resolved photoemission. Angle-dependent FS maps show distinctly different sized FS orbits as compared to the measured single- and double-layer ruthenate FS topologies. In addition, photon-dependent FS maps reveal a distinct k$_{\perp}$ variation with a periodicity corresponding to the inter-layer spacing of the triple-layer stack, indicating a three-dimensionality of the band structure. Comparison of the FS topology is made to available band structure calculations. [Preview Abstract] |
Tuesday, March 22, 2005 1:39PM - 1:51PM |
J25.00009: Temperature and frequency dependence of the mid-infrared Hall effect in Ca$_x$Sr$_{1-x}$RuO$_3$ J. Cerne, T. Kiessling, A. Markelz, I. Ohkubo, P. Khalifah, H. Christen, D. Mandrus, Z. Fang Ca$_x$Sr$_{1-x}$RuO$_3$ compounds exhibit unusual properties, such as metamagnetism, quantum criticality, non-Fermi liquid behavior and an anomalous Hall effect that continue to challenge the condensed matter community. The mid-infrared (115-238 meV) complex Faraday, Kerr, and Hall angles are studied in Ca$_x$Sr$_{1-x}$RuO$_3$ films. The magneto-optical signals in transmission are up to an order of magnitude larger than those obtained in reflection for the same sample. The frequency dependence of the low-temperature magneto-optical signals in SrRuO$_3$ is in good qualitative and quantitative agreement with first-principles band calculations [Z. Fang et al., Science 2003]. Striking qualitative similarities and differences are observed in the temperature dependence of the Hall angle at three probe energies (dc, 120~meV and 224~meV) in CaRuO$_3$ and SrRuO$_3$. [Preview Abstract] |
Tuesday, March 22, 2005 1:51PM - 2:03PM |
J25.00010: Magnetoresistance Caused by Spin-polarized Variable Range Hopping--Perovskite Ruthenates I-W. Chen, F. Huang We report a detailed study of A-site and B-site substitution on magnetoreistance of perovskite ruthenates, which have broad implications. By progressively disrupting the conducting pathway in the ferromagnetic SrRuO$_{3}$, we found it first undergoes Anderson localization, then exhibits very large negative magnetoresistance (exceeding -60{\%} compared to the zero-field resistance). The implication is that any ferromagnetic metal should acquire a large magnetoresistance when it is rendered insulating by way of disorder, regardless of whether the disorder is caused by a magnetic or nonmagnetic impurity. This pathway is especially feasible in strongly correlated metals such as SrRuO$_{3}$. A model based on variable range hopping of spin-polarized electrons can quantitatively explain the data. [Preview Abstract] |
Tuesday, March 22, 2005 2:03PM - 2:15PM |
J25.00011: Charges play musical chairs on the pyrochlore lattice Doron Bergman, Gregory Fiete, Leon Balents We study a highly idealized model for quantum ``charge order'' phase transitions on a pyrochlore lattice, loosely motivated by observations of a ``valence skipping'' structure in vanadium spinels (K. Matsuno et al, PRL 90, 096404, 2003). The model maps onto a particular 3+1-dimensional compact quantum electrodynamics. We describe the transition in terms of a dual theory of monopole defect proliferation. The model gives rise to a number of degenerate low energy excitations which can condense in various patterns. Latest results will be presented. [Preview Abstract] |
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