Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session D45: Strongly Correlated Electrons on Triangular Lattices |
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Sponsoring Units: GMAG Chair: Cedomir Petrovic, Brookhaven National Laboratory Room: Baltimore Convention Center 348 |
Monday, March 13, 2006 2:30PM - 2:42PM |
D45.00001: Charge Localization and near zero in-plane thermal expansion in layered Na$_{x}$CoO$_{2}$, $x\sim\frac{1}{2}$ Dimitri Argyriou, C. Milne, O. Prokhnenko, L.C. Chapon, P.G. Radaelli We have used neutron powder diffraction and X-ray single crystal diffraction to investigate the lattice response to charge localization and magnetic ordering in samples with stochiometry close to Na$_{0.5}$CoO$_{2}$ over the temperature range of 2 to 600K. Our measurements show there presence of two crystallographical distinguishable Co sites but with essentially identical bond valences. This indicates only a marginal charge disproportionation between Co$^{3}$ and Co$^{4}$. Our measurements show Na-ordering achieved by de-intercalation imposes a constraint on the charge and spin-state degrees of freedom that acts as to reduce the in-plane thermal expansion to near zero values between 2 to 300K. At higher temperatures we find a phase non-reversable transition to a phase with a $3a \times\space 3b \times\space c$ cell, wtr to the orthorhombic P$nmm$ low temperature phase. This phase does not exhibit any of the anomalies observed at 52 and 87K in the magnetic susceptibility of the as made phase. [Preview Abstract] |
Monday, March 13, 2006 2:42PM - 2:54PM |
D45.00002: Spin Glass Statics and Dynamics in the Disordered Stacked Quantum Ferromagnet LiNiO2 J.P. Clancy, B.D. Gaulin, S.R. Dunsiger, S. Haravifard, J.R.D. Copley, Y. Qiu We have performed time-of-flight neutron scattering measurements on polycrystalline LiNiO2 using the Disk Chopper Spectrometer at the NIST Center for Neutron Research. Similar measurements performed on its sister compound, NaNiO2, show a simple magnetic structure below Tn$\sim $22K consisting of ferromagnetic sheets of s=1/2 moments stacked in an antiferromagnetic (AF) fashion [1]. Our measurements on LiNiO2 illustrate the absence of magnetic order down to 1.7K. Further they provide direct evidence for a spin glass transition at 9K through a narrowing of the inelastic magnetic scattering on approaching Tg from above, along with a concomitant increase in the elastic magnetic scattering, indicative of frozen disordered moments. The inelastic magnetic scattering in LiNiO2 resembles that in NaNiO2 above Tn, suggesting that the low Q spin response is due to short-lived spin wave modes below the lowest Q AF ordering wavevector. This reinforces the picture that the simple stacked ferromagnetic ordered state displayed by NaNiO2 is precluded by mixing between the Li and Ni sublattices, which occurs in LiNiO2 at the 1-3{\%} level. [1] M.J. Lewis et al., PRB 72, 014408 (2005). [Preview Abstract] |
Monday, March 13, 2006 2:54PM - 3:06PM |
D45.00003: Coexistence of f-wave superconductivity, charge order, and spin antiferromagnetism around nonmagnetic impurities in Na$_{0.33}$CoO$_{2}\cdot 1.3$H$_{2}$O Jin An, Hai-Qing Lin, Chang-De Gong To check whether charge dynamics is responsible for the superconductivity in Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O, we investigate local electronic and magnetic structure around a nonmagnetic impurities embedded in this material at $x=0.33$ in the vicinity of charge instability, by using a phenomenological model within the slave-boson framework including competitions among a $\sqrt{3}\times \sqrt{3}$ charge order, antiferromagnetism and f-wave superconductivity. Around the repulsive impurities, it is found that both local charge and spin orders are induced. Furthermore, the f-wave pairing order parameter is decreased on one sublattice but increased on another honeycomb sublattice. If the charge dynamics is responsible for the superconductivity, the predicted local electronic and magnetic structure could be observed by the STM and spatial resolved NMR experiments. [Preview Abstract] |
Monday, March 13, 2006 3:06PM - 3:18PM |
D45.00004: Thermoelectric properties of Na$_{0.68}$CoO$_{2}$ on a 2D triangular lattice Michael R. Peterson, Jan O. Haerter, B. Sriram Shastry Na$_{x}$CoO$_2$ at $x=0.68$ is a material with important and interesting thermopower. Using a new formalism for computing thermal response functions, via the response to dynamical temperature gradients, in the high frequency limit a nearly frequency independent term is identified for the thermopower $S^*$, the Lorentz ratio $L^*$, and the dimensionless figure of merit $Z^*T$. We calculate, via exact diagonalization for small systems, $S^*$, $L^*$, and $Z^*T$, at all temperatures for the $t$-$J$ model on a 2D triangular lattice exploring the model parameters relevant to the experiments by I. Terasaki {\em{et al}}, (PRB {\bf{56}}, R12685 (1997)) and Y. Wang {\em{et al}}, (Nature {\bf{423}}, 425 (2003)). Our objective is to understand the (large) magnitude of the thermopower, and its remarkable sensitivity to a magnetic field. We also consider the hypothetical and interesting case of the opposite sign of hopping $t\rightarrow -t$. [Preview Abstract] |
Monday, March 13, 2006 3:18PM - 3:30PM |
D45.00005: Jahn-Teller distortion and magnetic ordering in layered nickelates Michelle Johannes, Igor Mazin, Noam Bernstein One of the long-standing challenges in the theory of layered nickelates is that NaNiO$_2$ is antiferromagnetic and has a strong Jahn-Teller induced orbital ordering, while LiNiO$_2$ has none. A variety of hypothesis have been proposed to explain this mystery: a difference in Li/Na ionic radii, differences in the underlying electronic structure, the zero-point motion of Li ions, etc. One of the most viable propositions is that the difference is due to the presence of Na2$^+$ ions (1\% or more) on the Li sites. It remains unclear whether the magnetic interaction of Na2$^+$ with the in-plane Na3$^+$ is strong enough to cancel the Li-assisted antiferromagnetic superexchange even for 1\% of Na2$^+$, and what effect it has on orbital ordering. We provide quantitative arguments, based on first principles calculations and atomistic simulations, that spin and orbital orderings are unrelated, and that both are destroyed (the latter in a rather unusual way) by Na2$^+$ impurities, but for entirely separate reasons. [Preview Abstract] |
Monday, March 13, 2006 3:30PM - 3:42PM |
D45.00006: Pronounced increase of the thermopower in $\rm Na_xCoO_2$ in the large-$x$ regime ($x\ge$0.75) Minhyea Lee, Liliana Viciu, Lu Li, Yayu Wang, M. L. Foo, S. Watauchi, R. A. Pascal Jr., R. J. Cava, N. P. Ong We report for the first time the systematic transport study on high doped Na$_x$CoO$_2$ over series of samples which lie ($ 0.75 \leq x \leq 1$). We discovered two distinctive regions of phase diagram exist in high Na doping level : one is characterized by largely enhanced thermopower (200 -- 300 $\mu$V/K at $\sim$ 130K) yet highly metallic resistivity in $ 0.75 \alt x \alt 0.88$ (region I). The other (region II) is emerged in $0.89 \alt x \alt 0.97$ , in which the electronic phase is mixed with the region I phase and $x=1$ of non-conducting end compound and the mixture seems to occur mainly along the layer. We will discuss the newly revised phase diagram of the high Na part and speculate on a relation to the characteristics of Na layer. [Preview Abstract] |
Monday, March 13, 2006 3:42PM - 3:54PM |
D45.00007: Single-particle excitation-spectra in the Hubbard model on a kagom\'{e} lattice Wataru Koshibae, Nejat Bulut, Kenji Tsutsui, Sadamichi Maekawa The effects of frustration in spin systems have been studied for many years. Recently, the transport properties of the layered cobalt oxide Na$_{x}$CoO$_{2}$ and of related oxides have generated new interest in the frustrated systems. In the cobalt oxides, the Co ions form a triangular lattice. The hopping matrix element of electrons in the cobalt 3$d$ orbitals is not isotropic, and we have shown [PRL\textbf{91}, 257003] that the triangular CoO$_{2}$ lattice consists of four coupled kagom\'{e} sublattices. For this reason, here, we examine the single-particle excitation spectrum of the Hubbard model on the kagom\'{e} lattice, and study the motion of a carrier in this frustrated system. We use the quantum Monte Carlo and the exact-diagonalization methods. The dispersion relation of the tight-binding model on the kagom\'e lattice has a flat dispersion at the top or the bottom of the energy band depending on the sign of hopping-matrix element $t$. This causes a two-fold degeneracy at the $\Gamma$ point where the flat piece of the dispersion is located. However, in the interacting system, we find that the lowest- lying states have a two-fold degeneracy at the $\Gamma$ point independent of the sign of $t$, when the Coulomb repulsion is sufficiently strong. In this talk, we will discuss these numerical results on the electronic structure of the Hubbard model on the kagom\'{e} lattice. [Preview Abstract] |
Monday, March 13, 2006 3:54PM - 4:06PM |
D45.00008: Squaring the Triangle: Insulating Ground State of $Na_{0.5} CoO_{2}$ Ting-Pong Choy, Dimitrios Galanakis, Philip Phillips We demonstrate that at a filling of $n=1.5$, an hexatic insulating state obtains in the extended Hubbard model on a triangular lattice. Composed of two tetragonal sublattices with fillings of $n=1$ and $n=2$, the insulating state is charge ordered and possesses an antiferromagnetic superlattice with dimension $a\times\sqrt{3}$. Two distinct energy scales arise in our model, a charge gap for the insulator and the effective exchange interaction in the antiferromagnet. Our model is capable of explaining the sign change of the Hall-coefficient as a function of temperature as well as the persistence of antiferromagnetism above the insulating state. [Preview Abstract] |
Monday, March 13, 2006 4:06PM - 4:18PM |
D45.00009: Itinerant correlated electrons on 2D and 3D triangular lattices M. Bruehwiler, S. M. Kazakov, J. Karpinski, B. Batlogg In insulating frustrated systems, where localized magnetic moments sit on the vortices of e.g. triangles or tetrahedra, competing interactions lead to a wealth of novel states of fundamental interest. Ground states of infinite degeneracy are a typical signature of such frustrated systems. While for local moments the intimate connection between spin and the lattice leads to the observed frustration effects, it is an open question to what degree itineracy modifies these features and how charge, spin, and lattice degrees of freedom are affected. In this context, the superconductivity recently discovered in 2D triangular Na$_{x}$CoO$_{2}-y$H$_{2}$O and 3D triangular $A$Os$_{2}$O$_{6}$ ($A$ = K, Rb, Cs) is of considerable interest. In order to shed light onto the above mentioned questions associated to these materials, we have performed thermodynamic and transport measurements on both the parent compound of the hydrated superconductor, Na$_{x}$CoO$_{2}$, and the pyrochlores KOs$_{2}$O$_{6}$ (T$_{c}$ = 9.5 K) and RbOs$_{2}$O$_{6}$ (T$_{c}$ = 6.4 K). We have mapped out parts of the phase diagram as a function of band filling of Na$_{x}$CoO$_{2}$ (varying Na content $x)$ and find peculiar low-energy excitations below about 10 K. In the 3D system, we also find a significant mass enhancement by factors up to about 8 compared to band structure calculations, part of which can be attributed to electron-phonon interaction ($\lambda _{ep}$ = 1 to 1.5). [Preview Abstract] |
Monday, March 13, 2006 4:18PM - 4:30PM |
D45.00010: Hall constant on the Icosahedral t-J model Jan Haerter, Michael Peterson, Sriram Shastry We investigate the Hall constant $R_H$ in the zero field limit through the exact diagonalization technique on small clusters within the triangular lattice t-J model. As a model system we study the icosahedron, a 12-site Platonic solid consisting solely of five fold coordinated triangles. We compare results with toroidal geometries up to 12 sites. While the high-temperature behavior of the high-frequency $R_H^*$ is quite well understood analytically, our objective is to explain the complete T-dependence of $R_H$ in $Na_{.68}CoO_2$ as observed in experiments (Ong et al), especially in the low-T regime. Furthermore, we investigate $R_H$ as function of frequency by explicit evaluation of Kubo formulae. We compare results with existing studies of $R_H$ on square lattices. [Preview Abstract] |
Monday, March 13, 2006 4:30PM - 4:42PM |
D45.00011: Ground-state valency and spin configuration of the nickelates. Leon Petit, George M. Stocks, Takeshi Egami, Zdzislawa Szotek, Walter M. Temmerman The ab initio self-interaction-corrected local-spin-density approximation is used to study the electronic structure of both stoichiometric and non-stoichiometric nickelates. From total energy considerations it emerges that, in their ground-state, both LiNiO$_2$, and NaNiO$_2$ are insulators, with the Ni ion in the Ni$^{3+}$ low spin state ($t_{2g}^6e_{g}^1$) configuration. We find that the substitution of Li/Na atoms by divalent impurities, drives an equivalent number of Ni ions in the NiO$_2$ layers from the JT-active trivalent low-spin state to the divalent JT-inactive state. We propose that an experimental study on Mg$_x$Na$_{1-x}$NiO$_2$ might clarify the role of Ni$^{2+}$ impurities with respect to the vanishing of long range orbital ordering in Li$_{1-x}$Ni$_{1+x}$O$_2$. (Work sponsored by the Laboratory Directed Research and Development Program (LDRD) program of ORNL (LP, GMS, TE), and by the DOE-OS through the Offices of Basic Energy Sciences (BES), Division of Materials Sciences and Engineering (LP, GMS, TE). Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the Department of Energy under Contract No. DE-AC05-00OR22725.) [Preview Abstract] |
Monday, March 13, 2006 4:42PM - 4:54PM |
D45.00012: Phase Diagram of the ${\rm Li_2O}$-${\rm V_2O_3}$-${\rm V_2O_5}$ System at 700 $^\circ$C: Correlations with Magnetic Defect Concentration in Heavy Fermion ${\rm LiV_2O_4}$ S. Das, X. Zong, X. Ma, A. Niazi, D.C. Johnston Polycrystalline samples of ${\rm LiV_2O_4}$ have been synthesized with magnetic defect concentrations $n$ ranging from 0.01 mol\% to 0.77 mol\% and average spins $S$ from 3/2 to 4. (1) Low-temperature $T$ magnetic susceptibility $\chi$ and magnetization $M$ versus applied magnetic field $H$ isotherms were measured to determine these $n$ and $S$ values. Here we report the phase diagram of the ${\rm Li_2O}$-${\rm V_2O_3}$-${\rm V_2O_5}$ ternary system at 700 $^\circ$C for compositions in equilibrium with ${\rm LiV_2O_4}$. This study provided a clarification of the synthesis conditions under which low and high magnetic defect concentrations can be obtained in ${\rm LiV_2O_4}$. The samples were prepared using conventional solid state reaction of appropriate amounts of ${\rm Li_2CO_3,\ V_2O_3}$ and ${\rm V_2O_5}$. $M(H)$ isotherms and $\chi(T)$ data were obtained on many of these samples. We confirmed that the ${\rm LiV_2O_4}$ phase can be obtained containing low (0.01 mol\%) to high ($\geq 0.5$ mol\%) magnetic defect concentrations and with consistently high defect $S$ values around 3 to 4. The highest $n$ values were obtained for ${\rm LiV_2O_4}$ samples in equilibrium with ${\rm V_2O_3}$ and the lowest values for ${\rm LiV_2O_4}$ samples in equilibrium with ${\rm V_3O_5}$, consistent with the results in Ref.\ (1).\\ (1) S. Kondo et al., PRL \textbf{78}, 3721 (1997); PRB \textbf {59}, 2609 (1999). [Preview Abstract] |
Monday, March 13, 2006 4:54PM - 5:06PM |
D45.00013: $^7$Li NMR Study of Magnetic Defects in Heavy Fermion LiV$_2$O$_4$ X. Zong, D.C. Johnston, F. Borsa, S. Das, J. Schmalian A small concentration ($n \approx 0.7$ mol\%) of magnetic defects within the normal spinel structure of heavy fermion ${\rm LiV_2O_4}$ was recently found to have a strong effect on the $^7 $Li NMR at low temperatures $T <$ 4.2 K\@.(1) The $^7$Li nuclear magnetization relaxation versus time after saturation, $M(t)$, changed from a pure exponential in a pure sample with $n$ = 0.01 mol\% to a stretched exponential for $n$ = 0.7 mol\%. Here we present a systematic study of the variations of the $^7$Li NMR versus $n$ for additional $n$ values from 0.05 to 0.8 mol\%. Non-exponential $M(t)$ recovery was consistently obtained for samples with $n \geq 0.2$ mol\% and the nuclear spin lattice relaxation rate versus temperature evolved monotonically with increasing $n$, consistent with the interpretation in Ref.\ (1). In addition, we obtained relaxation data for much shorter times than studied in Ref.\ (1), which indicate a previously unknown initial $M(t) \propto \sqrt{t}$ dependence for all samples with $n \geq 0.5$ mol\%, for times $t <$ 20 ms. At present, there exists no microscopic theory for the influence of magnetic defects on our $^7$Li NMR results. Phenomenological models that may help to understand the data will be discussed.\newline (1). D. C. Johnston et al., Phys.\ Rev.\ Lett.\ \textbf{95}, 176408 (2005). [Preview Abstract] |
Monday, March 13, 2006 5:06PM - 5:18PM |
D45.00014: Magnetic Excitations in the Orbitally Degenerate Triangular Lattice LiVO$_{2}$ Studied by Inelastic Neutron Scattering W. Tian, M.B. Stone, D.G. Mandrus, B.C. Sales, R. Jin, D.T. Adroja, T. Perring, S.E. Nagler Inelastic neutron scattering experiments were performed to study a two dimensional triangular lattice material LiVO$_{2}$ (S=1, V$^{3+}$ ions) with orbital ordering involving threefold degenerate t$_{2g}$ orbitals. At T$_{t} \quad \approx $ 500 K, LiVO$_{2}$ undergoes a first order phase transition, accompanied by a large reduction in the magnetic susceptibility in the low temperature phase. It has been proposed that this phase transition is associated with the formation of trimers of V$^{3+ }$ions yielding a spin-singlet ground state. We report inelastic neutron scattering measurements on powder and single crystal samples of LiVO$_{2}$ carried out using the HET and MAPS spectrometers at ISIS. At low temperature, several magnetic excitations with energy transfers as large as several hundred meV were observed. We propose that the observed multiple magnetic excitations arise from the orbital ordering. Model calculations for trimers including an orbital-ordering term showing qualitatively similar spectra suggesting that localized orbiton excitations have been observed in LiVO$_{2}$. [Preview Abstract] |
Monday, March 13, 2006 5:18PM - 5:30PM |
D45.00015: Strong Electron correlation in the Cobaltates: a CDMFT study Dimitrios Galanakis, Tudor Stanescu, Philip Phillips The cobaltates $Na_{x}CoO_{2}$ with $0 |
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