Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session L16: Correlated Electrons: General I |
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Sponsoring Units: GMAG Chair: William Ratcliff, National Institute of Standards and Technology Room: Colorado Convention Center Korbel 4F |
Tuesday, March 6, 2007 2:30PM - 2:42PM |
L16.00001: Super-exchange in transition-metal oxides Walter Harrison Using contemporary tight-binding theory and parameters[1]. Anderson's perturbation approach [2] gives a qualitatively correct energy difference (a factor 2.3 too high) between ferromagnetic and antiferromagnetic configurations for MnO, It corresponds to a Heisenberg model with $J_2/J_1 = 11/7$. Perturbation theory fails as the energy denominator gets smaller for FeO and CoO, and changes sign for NiO. Use of the special- points method to treat exchange-split bands gives smaller values not well characterized by a $J_1$ and $J_2$. Carrying it out self-consistently reorders the NiO levels and leads to still smaller energy differences near experiment for all four oxides, as estimated from the experimental N\'eel temperature TN , The theory predicts a variation with pressure corresponding to $(d/ TN)\partial TN/\partial d = -12.2$ for MnO , near experiment, dropping to -9.1 for NiO. The theory is applicable also to the paramagnetic susceptibility. \newline \newline [1] Walter A. Harrison, Elementary Electronic Structure, World Scientific (Singapore, 1999), revised edition (2004). \newline [2] P. W. Anderson, Phys. Rev. 115, 2 (1959). [Preview Abstract] |
Tuesday, March 6, 2007 2:42PM - 2:54PM |
L16.00002: Electronic specific heat enhancement in the half-metallic ferromagnet $Cro_2$ explained by Fermi Liquid Theory Raul Chura, Kevin Bedell Available data on the electronic specific heat of the half-metallic ferromagnet (HMF) $CrO_2$, show that the obtained experimental values are systematically greater than the corresponding theoretical ones calculated through various band theory methods. This discrepancy is due to the presence of many-electron correlation effects (spin fluctuations, strong electron-magnon scattering) which are not taken into account in the band theory calculations. A renormalization of the band theory results is therefore needed to account for the observed enhancement in the value of the specific heat. A microscopic many-electron approach has been proposed and explains the referred enhancement in terms of non-quasiparticle effects. It has been argued that Fermi liquid theory is not sufficient to provide the appropriate renormalization able to explain the observed enhancement in the electronic specific heat of HMFs. Contrary to this statement, we have shown that the introduction of a spin-dependent density of states, in the framework of the Fermi liquid theory for spin polarized systems, gives place to a renormalization which, indeed, provides a reasonable account of the observed enhancement in the electronic specific heat of the HMF $CrO_2$. [Preview Abstract] |
Tuesday, March 6, 2007 2:54PM - 3:06PM |
L16.00003: Effect of Magnetic Field on the Induced Magnetic Moment System Pr$_{3}$In. Victor Fanelli, Andrew Christianson, Marcelo Jaime, Joe Thompson, Cristian Batista, Jon Lawrence Pr$_{3}$In is a singlet-triplet system similar to the classic induced moment system Pr$_{3}$Tl, with identical crystal structure. Both materials order magnetically at similar temperatures (T$_{C}$=11.6 K for Pr$_{3}$Tl and T$_{N}$=11.4 K for Pr$_{3}$In). The magnetic order is antiferromagnetic (AF) in Pr$_{3}$In as opposed to ferromagnetic in Pr$_{3}$Tl. The exchange interaction between Pr sites causes admixture of the crystal field triplet excited state into the singlet ground state, resulting in induced moment magnetic order below T$_{N}$. Application of a magnetic field can change the energies of the singlet and triplet in such a manner as to alter the admixture. We have measured magnetization, magnetoresistance, specific heat and magneto-caloric effect in the range 0 to 15 T. We observed a phase transition below 11 K and at magnetic field of order 1.9 T. At present, whether this is a spin rearrangement or a spin polarized phase remains an open question. It would be surprising for the 1.9 T transition to be to a spin polarized state, given that T$_{N}$ is around 11.4 K. In addition, at high fields, we observe a strong reduction of the specific heat as the AF interactions are suppressed and the system reverts to a crystal-field-only behavior. [Preview Abstract] |
Tuesday, March 6, 2007 3:06PM - 3:18PM |
L16.00004: de Haas-van Alphen study of the Fermi Surface of Ce$_{x}$La$_{1-x}$B$_{6}$ as a function of composition: the evolution of field-dependent quasiparticle effective masses Izabela Mihut, John Singleton, Albert Migliori, Long Pham, Cigdem Capan, Zachary Fisk The de Haas-van Alphen effect has been studied in single crystals of Ce$_{x}$La$_{1-x}$B$_{6 }$ (0 $\le \quad x \quad \le $ 0.075) using pulsed magnetic fields of up to 60 T and temperatures 0.38 K $\le \quad T \quad \le $ 4.0~K. The low-field effective mass grows smoothly with increasing $x. $Moreover, for $x \quad >$ 0, the effective mass becomes a function of magnetic field, decreasing as the field rises. These results may be fitted using the extended Lifshitz-Kosevich formalism due to Wasserman, the decrease in mass reflecting the suppression of spin fluctuations by the field. The data also show that a previously-observed effect, attributed to complete spin polarization of one of the Fermi-surface sheets for $x \ge $ 0.05, is in fact an artifact of the field-dependent mass, ignored in previous works. [Preview Abstract] |
Tuesday, March 6, 2007 3:18PM - 3:30PM |
L16.00005: Angle-resolved Resonant Inelastic X-ray Scattering in NaV$_2$O$_5$ Guoping Zhang, T.A. Callcott Angle-dependent resonant inelastic x-ray scattering spectrum at the V-$L_3$ edge is analyzed to determine the origin of the V-$dd$ peak in NaV2O5 [1]. Experiment shows that as the incident photon polarization is rotated from the $b$ to $c$ axis, the V-$dd$ peak grows relative to the V-d/O-p peak and its energy loss becomes larger [2]. Our first-principles calculations demonstrate that such growth must involve both the unoccupied dxy and dxz/dyz bands. Neither the dxz/dyz nor dxy excitation alone can reproduce the ratio change. For the $bc$ scan, the light first samples the dxy orbital and then the dxz/dyz orbital. Slightly detuning the incident energy away from the resonant edge reveals that the dxy band is slightly lower in energy and much narrower than the dxz/dyz band. The results suggest that our previous analysis of the correlation splitting of the dxy band is valid [3]. [1] G. P. Zhang, T. A. Callcott, G. T. Woods, {\it et al} Phys. Rev. Lett. {\bf 88}, 077401 (2002); [2] G. P. Zhang {\it et al.}, Phys. Rev. B {\bf 65}, 165107 (2002); [3] G. P. Zhang and T. A. Callcott, Phys. Rev. B {\bf 73}, 125102 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 3:30PM - 3:42PM |
L16.00006: Self-organized Electronic Extended van Hove Singularity as Electron- lattice Dynamic Confinement Effect Sergei Mukhin A mechanism of self-organized one-dimensionality in correlated electron systems is proposed. It is found that unidirectional dynamic confinement of electron motion by quantum lattice vibrations may cause transition into ordered state with extended electronic van Hove singularities. This may explain observed enhancement of the ordering instability in the anti-nodal regions of the ``Fermi surface'' in the under- and optimally doped high-T$_{c}$ cuprates. It is shown that ordered electrons in the anti-nodal regions bind with quantum lattice vibrations that obey ``selection rules'': $Q/\Omega =z_s /g$, where$_{ }z_s $ are zeros of the Bessel function $J_0 (z)$, and Q is amplitude of lattice vibration with frequency $\Omega $, g is electron-lattice coupling strength in the force units. Confining potential of these vibrations creates one-dimensional ``nesting'' of the Floquet indices of electronic states, provoking electronic ordering transition. The transition is destructed by external magnetic field with the Larmour frequency $\Omega _L \ge \Delta ^2/\hbar t_\bot $, here $\Delta $ is ordered electrons energy-gap; $t_\bot /t_\parallel \ll 1$ are bare hopping integrals of the anisotropic electron tight-binding model. [Preview Abstract] |
Tuesday, March 6, 2007 3:42PM - 3:54PM |
L16.00007: ARPES kink from strong electron correlations Shiladitya Chakraborty, Dimitrios Galanakis, Philip Phillips Recent ARPES experiments have found a `kink' in the energy dispersion ($\omega$ vs $k$) which has been attributed to electron-phonon interactions. In this study, we compute the energy dispersion defining the maximum in spectral function using the 2D Hubbard model to see if such kinks can be explained simply from strong correlations. To treat the strong correlations, we employ the cellular dynamical mean-field theory method and compute $\omega$ vs $k$ for various hole dopings, temperatures and ratios of $U/t$. The computed dispersions show `kinks' similar to those seen in ARPES experiments. The energy scale for the kink is $t^2/U$ and arises from local spin correlations. [Preview Abstract] |
Tuesday, March 6, 2007 3:54PM - 4:06PM |
L16.00008: Unique ground state in Ce$_{3}$Au$_{3}$Sb$_{4}$. Han-Oh Lee, Youn-Jung Jo, Luis Balicas, Pedro Schlottmann, Andrew Christianson, Jon Lawrence, Jason Gardner, Vladmir Sidorov, Cathie Condron, Susan Kauzlarich, Peter Klavins, Joe Thompson, Zachary Fisk Ce$_{3}$Au$_{3}$Sb$_{4 }$has diverging specific heat coefficient at low temperature in a semiconducting state, a property which can be approached differently within the Kondo and band insulator viewpoints. Sample sensitivity here presents difficulty for determining the intrinsic behavior of this system. We will present pressure and powder neutron scattering experiment data in addition to its basic physical properties in order to discuss the underlying physics. This work has been supported by NSF-DMR-0503360. [Preview Abstract] |
Tuesday, March 6, 2007 4:06PM - 4:18PM |
L16.00009: Strongly correlated electron behavior in RFe$_2$Zn$_{20}$ (R=rare earth elements, Zr, Hf) S. Jia, J. Frederick, Ni Ni, A.S. Sefat, S.L. Bud'ko, P.C. Canfield GdFe$_2$Zn$_{20}$ has a remarkably high ferromagnetic ordering temperature ($T_C$=86K), which can be explained as a result of submerging large local moments into a nearly ferromagnetic Fermi liquid YFe$_2$Zn$_{20}$. Thermodynamic and transport properties of pseudoquaternary compounds Y$_{1-x}$Gd$_x$Fe$_2 $Zn$_{20}$ show ferromagnetic ground state for $x>0.02$, and reveal the polarization of correlated electrons related to the concentration of Gd. Comparing with YFe$_2$Zn$_{20}$, stronger itinerant electron magnetism was observed in ScFe$_2$Zn$_{20}$, ZrFe$_2$Zn$_{20}$ and HfFe$_2$Zn$_{20}$, whose properties place them even closer to the Stoner limit than YFe$_2$Zn$_{20}$. [Preview Abstract] |
Tuesday, March 6, 2007 4:18PM - 4:30PM |
L16.00010: Magnetic Structure and Crystal Field Potential of PrOs$_4$As$_{12}$ Songxue Chi, Pengcheng Dai, H.J. Kang, J.W. Lynn, F. Ye, Z. Henkie, A. Pietraszko, M.B. Maple Neutron powder diffraction and elastic neutron scattering have been used to determine the magnetic structure of the Filled Skutterudite compound PrOs$_4$As$_{12}$. The system becomes antiferromagnetically ordered with a Neel temperature ($T_N$) at 2.28K, which has A-type magnetic structure with spins lying along the doubled axis of the magnetic unit cell. The crystal field potential of PrOs$_4$As$_{12}$ has been studied by inelastic neutron scattering (INS). The ground state in the $T_h$ point group symmetry is determined to be a $\Gamma_5$ triplet. This is confirmed by Zeeman effect exhibited at low temperatures under high magnetic fields. [Preview Abstract] |
Tuesday, March 6, 2007 4:30PM - 4:42PM |
L16.00011: Unusual transport properties in the orbitally-ordered system Lu$_{2}$V$_{2}$O$_{7}$ H.D. Zhou, B. Conner, B.W. Vogt, C.R. Wiebe, L.L. Lumata, J.S. Brooks, E.S. Choi, Y. Xin DC susceptibility ($\chi )$, AC and DC resistivity ($\rho )$, specific heat ($C_{p})$, and thermoconductivity ($k)$ measurements on single crystalline Lu$_{2}$V$_{2}$O$_{7}$ with the pyrochlore structure reveal two transitions: (1) a short-range magnetic ordering transition at $T_{s}$ = 175 K, which is identified by the slope change of 1/ $\chi $ and 1/$k$, an anomaly in the AC resistivity, and a change in the activation energy (2) an orbital ordering transition at $T_{o}$ = 70 K, which is confirmed by the sharp transition on $\chi $, $k$, and $C_{p}$. At $T_{o}$, the resistivity shows an unusual insulator-metal transition which will be discussed in relation to the orbital ordering transition. [Preview Abstract] |
Tuesday, March 6, 2007 4:42PM - 4:54PM |
L16.00012: Magnetic and Magnetoelastic Properties of Substituted Cobalt Ferrites David Jiles, Seong Jae Lee, John Snyder, Eugene Melikhov We report recent results on a family of compounds based on cobalt ferrite with various chemical additions that can be used to dramatically alter the properties. These have high magnetostriction, high sensitivity of magnetic induction to applied stress and are chemically very stable, making them attractive for use in magnetoelastic sensors. For practical applications a family of materials is needed. The magnetic properties, magnetoelastic response, and temperature dependences can be controlled by selecting the chemical composition and adjusting the site occupancies of cations. A series of Mn-, Cr-, and Ga-substituted cobalt ferrite compounds, CoMn$_{x}$Fe$_{2-x}$O$_{4}$, CoCr$_{x}$Fe$_{2-x}$O$_{4}$, and CoGa$_{x}$Fe$_{2-x}$O$_{4 }$(where x=0.0 to 0.8) have recently been investigated and these showed dramatic changes in properties including reductions of over 350K in Curie temperature. Another significant result was that the effects of the substituted contents (x) on magnetic and magnetoelastic properties were significantly different for each substituted cation due to the differences in cation site occupancies of the elements within the spinel crystal structure. [Preview Abstract] |
Tuesday, March 6, 2007 4:54PM - 5:06PM |
L16.00013: Momentum dependent light scattering in insulating cuprates F. Vernay, M. J. P. Gingras, T. P. Devereaux We investigate the problem of inelastic x-ray scattering in the spin$-\frac{1}{2}$ Heisenberg model on the square lattice. We first derive a momentum dependent scattering operator for the $A_{1g}$ and $B_{1g}$ polarization geometries. On the basis of a spin-wave analysis, including magnon-magnon interactions and exact-diagonalizations, we determine the qualitative shape of the spectra. We argue that our results may be relevant to interpret inelastic x-ray scattering experiments in the antiferromagnetic state of copper oxide materials. [Preview Abstract] |
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