Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session N41: Correlated Electrons: Magnetism in Cobaltates, Ruthenates and Skutterudites |
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Sponsoring Units: GMAG Chair: Joe Feldman, NRL Room: LACC 150A |
Wednesday, March 23, 2005 8:00AM - 8:12AM |
N41.00001: Doping dependence of electron-electron scattering in Na$_x$CoO$_2$ S. Y. Li, Louis Taillefer, Fangcheng Chou, Y. S. Lee The in-plane resistivity $\rho$ was measured down to 40 mK for a single crystal of Na$_x$CoO$_2$ ($x$ = 0.75), which has spin-density-wave order below $T_c$ = 22 K. We show its Fermi-liquid ground state by observing a $T^2$ dependence of $\rho$ at low temperature, $\Delta \rho = AT^2$. The measured value of coefficient $A$ = 2.60 $\mu \Omega$ cm K$^{-2}$ is about 100 and 3 times of that in Na$_{0.31}$CoO$_2$ and Na$_ {0.70}$CoO$_2$, respectively, indicating an increase of electron-electron scattering upon Na doping. The enormous $A$, and the moderate electron specific heat coefficient $\gamma$, gives an even larger Kadowaki-Woods ratio $A/\gamma^2$ than that previously reported in Na$_{0.70}$CoO$_2$ (Li {\it et al.}, Phys. Rev. Lett. {\bf 93}, 056401 (2004)). [Preview Abstract] |
Wednesday, March 23, 2005 8:12AM - 8:24AM |
N41.00002: Magnetic-field-induced Fermi surface reconstruction in Na$_{0.5}$CoO$_2$ Luis Balicas, Majed Abdel-Jawad, Nigel Hussey, Fangcheng Chou, Patrick A. Lee We have performed an electrical transport study in Na$_{0.5} $CoO$_{2}$ at high fields $B$ and low temperatures $T$. We find that the charge ordered state observed below $T_{CO}$ = 53 K can be suppressed by large in- plane magnetic fields, but not by fields applied along the inter-plane direction. For $B$ rotating within the conductive CoO$_{2}$ layers we observe angular magnetoresistance oscillations of essentially two-fold periodicity consistent with the reported orthorhombic symmetry of Na$_{0.5} $CoO$_{2}$. As $B$ increases however, a new 6-fold periodicity emerges indicating the stabilization of a hexagonal FS as reported by the ARPES measurements. This observation suggests on the one hand, that the Na superstructure defines the geometry of the FS at low temperatures, and on the other, that the charge order in the conducting plane is suppressed by high in-plane fields. At low temperatures Shubnikov de Haas oscillations (SdH) of very small frequencies are observed for $B$ // c-axis, indicating that almost the entire FS reported for $x$ = 0.6 and 0.7 disappears below $T_{CO}$ for $x$ = 0.5. Our results strongly indicate that the charge ordering involves the coupling with the Na order and involves the large hole pocket. [Preview Abstract] |
Wednesday, March 23, 2005 8:24AM - 8:36AM |
N41.00003: Counter-Nagaoka-Thouless Problem On the Triangular Lattice Jan Haerter, Sriram Shastry On the frustrated triangular lattice, the Nagaoka -Thouless theorem predicts ferromagnetism in the case of a single hole in the $U=\infty$ Hubbard model for a particular sign of hopping ( $t>0$), while for the counter case ($t<0$), nothing is known exactly or rigorously. We study the counter case for the triangular lattice motivated by the connection with the physics of sodium cobaltate $Na_x Co O_2$, which realizes this physical situation. We investigate the competition between charge ordered states, N\'{e}el ordered states and spin liquid states, by studying small clusters variationally as well as using exact diagonalization. While the ground state always seems to be a singlet for clusters with an even number of electrons, the full spectrum gives insights into the possibility of long ranged order. Comparing the spin configurations for specific locations of the hole with those of an {\em odd site} Heisenberg model followed by spin projection, enable us to examine as well as evaluate the notion of the charge carriers being solitons or holons in a well prepared spin background. [Preview Abstract] |
Wednesday, March 23, 2005 8:36AM - 8:48AM |
N41.00004: Large anisotropy in the paramagnetic susceptibility of SrRuO$_{3}$ films Lior Klein, Yevgeny Kats, Isaschar Genish, James W. Reiner, M. R. Beasley By using the extraordinary Hall effect in SrRuO$_{3}$ films we performed measurements of the paramagnetic susceptibility in this itinerant ferromagnet, from $T_{c}$ ($\sim 150$ K) to $300$ K. These measurements, combined with measurements of magnetoresistance, reveal that the susceptibility, which is almost isotropic at $300$ K, becomes highly anisotropic as the temperature is lowered, diverging along a single crystallographic direction in the vicinity of $T_{c}$. The results provide a manifestation of the effect of large magnetocrystalline anisotropy in the paramagnetic state of a \textit{4d} itinerant ferromagnet [e-print: cond-mat/0311341]. They also shed light on the controversy regarding the nature of the universality class of the phase transition in SrRuO$_{3}$ [D. Kim \textit{et al}, Phys. Rev. B \textbf{67}, 100406(R) (2003)]. [Preview Abstract] |
Wednesday, March 23, 2005 8:48AM - 9:00AM |
N41.00005: Comparing the Effects of Varying Carrier Density and Isoelectronic Doping on Magnetic Properties of SrRuO_3 Michael De Marco, Ryan Heary, Dermot Coffey, Piotr Klamut, Michael Maxwell, Peter Mattocks, Peter Khalifah, Steve Toorongian, Michael Haka We change the carrier density by introducing Ru vacancies to form SrRu$_{1-v}$O$_3$ and introduce lattice distortions with the substitution of Ca to form Sr$_{1-x}$Ca$_x$RuO$_3$. The hyperfine field falls with increasing $x$ in Sr$_{1-x}$Ca$_x$RuO$_3$ and is less than 1T at x=0.8. For all values of x$<0.8$ the isomer shift and the quadrupole splitting remain unchanged. This fall in the value of the hyperfine field is consistent with what others have found for T$_c$. $T_c$ rapidly drops with increasing number of Ru vacancies so that T$_c$=45K when $v$=0.12 suggesting that carrier density has a stronger effect on T$_c$ than lattice distortions. The hyperfine field remains unchanged which may be explained by assuming that the polarization of the s-electrons responsible for the hyperfine field is due to localized d-electrons which give rise to the local moment present above T$_c$. [Preview Abstract] |
Wednesday, March 23, 2005 9:00AM - 9:12AM |
N41.00006: Magnetic and Transport Study of Single Crystal SrRu$_{1-x}$Cr$_{x}$O$_{3}$ with Increased Curie Temperature G. Cao, V. Durairaj, E. Elhami, X.N. Lin, S. Chikara We report results of a structural, magnetic and transport study of single crystal SrRu$_{1-x}$Cr$_{x}$O$_{3}$ (0$\le $ x$<$0.15), i.e., Cr doped SrRuO$_{3}$. Unlike other impurity doping (such as Ca, Mn, Fe), the Cr doping systematically enhances the Curie temperature from Tc=165 K for x=0 to Tc=190 K for x=0.13. In the meantime, the Cr doping also significantly increases the saturation moment from 1.1 for x=0 to as large as 1.5 Bohr magneton/Ru for x$>$0. The system stays itinerant for the entire doping range with resistivity showing the Fisher-Langer behavior at Tc. Magnetic anisotropy and negative magnetoresistance are also observed. All results will be presented and discussed along with comparisons with other related systems. [Preview Abstract] |
Wednesday, March 23, 2005 9:12AM - 9:24AM |
N41.00007: Magnetic ordering transition in the spin-gap compound Sr$_2$Cu(BO$_3$)$_2$ I.R. Fisher, Suchitra E. Sebastian, P. Tanedo, P.A. Goddard, M. Jaime, N. Harrison, S.A. Zvyagin Sr$_2$Cu(BO$_3$)$_2$ is a weakly coupled dimer compound with an accessible spin gap between the singlet ground state and the lowest triplet excited state. The critical field (H$_{c} \simeq 56T$) above which the compound magnetically orders is significantly smaller than the intradimer coupling (J$ \simeq 100K$), indicating triplet delocalisation due to the effect of interdimer exchange. Here, we present results of anisotropic high field magnetisation measurements that reveal the effects of Dzyaloshinskii-Moriya exchange interactions in determining the nature of the ordered state for H $>$ H$_c$. We also present results that demonstrate how chemical pressure (the substitution of Ba for Sr) can be used to tune the spin gap in this material. [Preview Abstract] |
Wednesday, March 23, 2005 9:24AM - 9:36AM |
N41.00008: Oxygen Anneal Effect on Single Crystalline Sr$_2$RhO$4-x$ Shin-Ichi Ikeda, Ichiro Nagai, Yoshiyuki Yoshida, Norio Umeyama, Naoki Shirakawa Two dimensional perovskite-type transition metal oxides provide rich issues which originate from strong electron-electron correlation such as a spin triplet superconductivity in Sr2RuO4, quantum criticality at around the metamagnetic transition in Sr3Ru2O7 and antiferromagnetic metal phase in Ca3Ru2O7. We regard Rh oxides as another intriguing materials because of the similarity. Especially, Sr2RhO4 has been studied using single crystals grown by a floating-zone method. The importance of the oxygen content in Sr2RhO4 will be discussed based upon the results of electrical resistivity, magnetic susceptibility and specific heat. [Preview Abstract] |
Wednesday, March 23, 2005 9:36AM - 9:48AM |
N41.00009: Magnetic Susceptibility, Electrical Resistivity, and Specific Heat Measurements of the filled Skutterudite PrOs$_4$As$_{12}$ Todd Sayles, William Yuhasz, Neil Frederick, Nicholas Butch, Pei-Chun Ho, M. Brian Maple, Zygmunt Henkie Single crystals of PrOs$_{4}$As$_{12}$ were grown by a flux method. Measurements on single crystals of PrOs$_{4}$As$_{12}$ reveal antiferromagnetic ordering at a N\'{e}el temperature near 2.2 K. Low-temperature electrical resistivity, $\rho $(T), measurements indicate that T$_{N}$ is completely suppressed by fields of 1.5 T, while ac magnetic susceptibility and magnetization measurements reveal a metamagnetic transition at temperatures near the N\'{e}el temperature for low fields. High temperature Curie-Weiss fits yield an effective moment, $\mu _{eff}$ = 3.8 $\mu _{B}$, higher than the Pr$^{3+}$ free ion value $\mu $ = 3.58 $\mu _{B}$ derived from by Hund's rules. Fits to specific heat data above T$_{N}$ reveal a moderately enhanced electronic specific heat coefficient, $\gamma \quad \sim $ 100 mJ/mol K$^{2}$. The temperature dependence of $\rho $ and the negative magnetoresistance are indicative of Kondo lattice behavior. Research at UCSD supported by the U.S. DOE and NSF. [Preview Abstract] |
Wednesday, March 23, 2005 9:48AM - 10:00AM |
N41.00010: Temperature dependence of crystal field transitions in PrOs$_4$Sb$_{12}$ Raymond Osborn, Eugene Goremychkin, M. Brian Maple, Eric D. Bauer, Brian D. Rainford PrOs$_4$Sb$_{12}$ has attracted considerable attention as a heavy fermion superconductor, in which quadrupolar fluctuations play an important role in the pairing mechanism. Recently, we determined the crystal field potential using inelastic neutron scattering [Goremychkin \textit{et al} Phys. Rev. Lett. \textbf{93}, 157003 (2004)], and determined that the ground state is a a $\Gamma_1$ singlet. Although this does not favor quadrupolar Kondo models of the superconducting pairing, we have proposed that inelastic quadrupolar, or aspherical Coulomb, scattering of the conduction electrons by the 4f CF levels plays a significant role in enhancing T$_c$ compared to the isoelectronic lanthanum compound. Comparison with the crystal field level scheme in PrRu$_4$Sb$_{12}$ [Goremychkin \textit{et al} Physica B, in press], where T$_c$ has been reduced by praseodymium substitution, is consistent with this model. We report on recent high resolution neutron measurements of the temperature dependence of the transition to the $\Gamma_5$ triplet at 0.63 meV, whose linewidth provides a probe of \textit{s-f} interactions. [Preview Abstract] |
Wednesday, March 23, 2005 10:00AM - 10:12AM |
N41.00011: Ordered Magnetic State in PrFe$_{4}$Sb$_{12}$ Single Crystals N.P. Butch, W.M. Yuhasz, P.-C. Ho, J.R. Jeffries, N.A. Frederick, T.A. Sayles, X.G. Zheng, M.B. Maple, J.B. Betts, A.H. Lacerda, F.M. Woodward, J.W. Lynn, P. Rogl, G. Giester Single crystals of the filled skutterudite compound PrFe$_{4}$Sb$_{12}$ were prepared and characterized via X-ray and neutron diffraction, specific heat, electrical resistivity, and magnetization measurements. Long range magnetic ordering occurs at $T_{\mathrm{c}} \approx 4.1$~K. The magnetic structure consists of ordered moments on both Pr and Fe sites and may be ferrimagnetic. The electrical resistivity exhibits a very weak dependence on both applied magnetic field and pressure. Specific heat measurements indicate an enhanced effective mass. This work was supported by the DOE, NSF, and NNSA through SSAA. NHMFL is supported by the DOE, NSF, and State of Florida. [Preview Abstract] |
Wednesday, March 23, 2005 10:12AM - 10:24AM |
N41.00012: Impurity and magnetic field investigation of PrOs$_{4}$Sb$_{12}$ Costel Rotundu, Bohdan Andraka \\ We have performed a low level doping study of PrOs$_{4}$Sb$_{12}$. Samples with small amounts of La and Nd substituted for Pr were investigated by magnetic susceptibility and specific heat in magnetic fields to 14 T. The goal of this study was to search for correlations between superconductivity, effective electron mass, crystalline electric fields, and field-induced antiferro-quadropolar order. In agreement with our previous study $^{1}$, we find T$_{c}$ to be only weakly affected by the alloying while other characteristics, such as the specific heat discontinuity at T$_{c}$ and the width of the transition, show a dramatic dependence on the concentration of impurities. Results will be discussed in relation to considered models of superconductivity and heavy fermion behavior. Supported by Department of Energy, grant No. DE-FG02-99ER45748. \\ \\ $^{1}$ C. R. Rotundu, P. Kumar, B. Andraka, cond-mat/0402599 [Preview Abstract] |
Wednesday, March 23, 2005 10:24AM - 10:36AM |
N41.00013: Large Magnetoelastic Coupling in Pr$_{2}$CuO$_{4}$ Single Crystals David Mandrus, Rongying Jin, Brian Sales The in-plane thermal conductivity, $\kappa _{a}$, of insulating Pr$_{2}$CuO$_{4}$ was measured at low temperatures and in magnetic fields of up to 8 Tesla. At 5 K and 8 Tesla $\kappa _{a}$ increases by 50{\%} for H // \textbf{a}, and 300{\%} for H // \textbf{c}, relative to its value in zero magnetic field. This increase is most likely due to the ability of the magnetic field to mix the non-magnetic ground state of the Pr 4f crystal field level with the first excited crystal field level at 18 meV. The magnetic field produces a substantial magnetic moment at each Pr site as well as a significant change in the 4$f$ charge distribution. Either effect could significantly alter the heat conducted by acoustic phonons. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contract DE-AC05-00OR22725. [Preview Abstract] |
Wednesday, March 23, 2005 10:36AM - 10:48AM |
N41.00014: Thermodynamic Properties of Pr3RuO7 M. Freamat, X.N. Lin, V. Durairaj, S. Chikara, G. Cao, J.W. Brill We have measured the thermal and magnetic properties of single crystals of Pr$_{3}$RuO$_{7}$. The magnetic measurements indicate an antiferromagnetic transition with spins aligned along the RuO chains (i.e. c-axis) at T$_{N}$ = 54 K at which there is a large step anomaly in the specific heat, $\Delta $c $\sim $ R, but the estimated entropy change is small, $\Delta $s $\sim $ R/10, suggesting only incomplete spin ordering in zero field. On the other hand, there is a metamagnetic transition at low temperature at B$_{c} \quad \sim $ 3T with a large saturated moment of $\sim $ 5 $\mu _{B}$. Surprisingly, there is a large linear contribution to the specific heat at low temperatures, $\gamma \quad \sim $ 0.16 J/mol{\textbullet}K$^{2}$ and large temperature independent contribution to the susceptibility, with a Wilson ratio $\chi _{0}$/$\gamma \quad >$ 1, suggesting that, despite its insulating behavior, there is a finite density of states of strongly correlated electrons at the Fermi surface. [Preview Abstract] |
Wednesday, March 23, 2005 10:48AM - 11:00AM |
N41.00015: Magnetic properties of iron doped magneto-electric LiNiPO$_4$ single crystals Jiying Li, Jerel L. Zarestky, David Vaknin Neutron scattering and magnetic susceptibility studies of pure and iron-doped magneto-electric LiNiPO$_4$ single crystals are compared. Elastic neutron scattering of single-crystal LiNiPO$_4$ reveal a spontaneous first-order commensurate- incommensurate magnetic phase transition (at $T_{C-IC}$ = 20.8 K) accompanied by a second transition ($T_{IC}$ = 21.7 K) from long- to short-range IC structure. These transitions are also identified in the magnetic susceptibility measurements. The modulated structure has a predominant antiferromagnetic component giving rise to satellite peaks in the vicinity of the fundamental antiferromagnetic peak, and a ferromagnetic component giving rise to peaks at small momentum transfers around the origin $(0,\pm Q,0)$. The wavelength of the modulated magnetic structure varies continuously with temperature. Magnetic susceptibilities of the iron doped crystals show magentic-field dependent properties characteristic of spin-glass systems. [Preview Abstract] |
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