Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session P25: Focus Session: Novel and Complex Oxides: Multiferroic and Other |
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Sponsoring Units: DMP DCMP Chair: Michelle Johannes, NRL Room: LACC 501A |
Wednesday, March 23, 2005 11:15AM - 11:51AM |
P25.00001: Magnetoelectric Coupling in Multiferroic Materials Invited Speaker: There has been increasing recent interest in magnetoelectric multiferroics, which are materials that show spontaneous magnetic order and ferroelectricity in the same phase. In addition to the fascinating physics resulting from the independent existence of two or more ferroic order parameters in one material, the coupling between magnetic and electric degrees of freedom gives rise to additional phenomena. \par In this talk we will discuss possible coupling scenarios between the ferroelectric polarization and the magnetization in magnetoelectric multiferroics. As an example we present results for the magnetoelectric multiferroic bismuth ferrite. Using first-principles calculations in the framework of density functional theory we analyze the nature of the electric polarization, the magnetization, and the coupling between these two quantities. We show that weak ferromagnetism occurs in this material, and that the resulting magnetization is strongly coupled to the structural distortions. We explore the possibility of electric-field-induced magnetization reversal and show that, although it is unlikely to be realized in bismuth ferrite, it is not in general impossible. Finally we outline the conditions that must be fulfilled to achieve such a switching of the magnetization using an electric field. [Preview Abstract] |
Wednesday, March 23, 2005 11:51AM - 12:03PM |
P25.00002: Orbitally-driven Peierls state in correlated oxides Daniel Khomskii, Takashi Mizokawa In studying superstructures in transition metal oxides such as charge or orbital ordering, one usually considers site-centered superstructures. Hovewer there exist another possibility: bond-centered superstructures, such as e.g. the Peierls state in low-dimensional systems. In this talk we will consider the possibility of existence of site-centered and bond-centered structures on a few examples. We will show that in spinels and in some other frustrated systems a site-centered orbital ordering (ODW-Orbital Density Wave) may lead to the formation of bond-centered singlet Peierls-like states [1]. This picture gives a simple explanation of extremely strange superstructures observed recently below metal-insulator transitions in MgTi$_2$O$_4$ [2] and CuIr$_2$S$_4$ [3], and may be relevant for several other materials, such as NaTiO$_2$, La$_4$Ru$_2$O$_{10}$ [4] and some others. We will also give a general discussion in which cases bond-centered structures may be favourable, and discuss the role of orbital degrees of freedom for the insulator-metal transitions. [1] D.Khomskii and T.Mizokawa, cond-mat/0407458 [2] M.Schmidt et al., Phys.Rev.Lett. \textbf{92}, 056402 (2004) [3] P.G.Radaelli et al., Nature \textbf{416}, 155 (2002) [4] P.Khalifah et al., Science \textbf{297}, 2237 (2002) [Preview Abstract] |
Wednesday, March 23, 2005 12:03PM - 12:15PM |
P25.00003: Andreev Edge State on Semi-Infinite Triangular Lattice: Detecting the Pairing Symmetry in Na$_{0.35}$CoO$_{2}$.yH$_{2O}$ Tamar Pereg-Barnea, Hsiu-Hau Lin We study the Andreev edge state on the semi-infinite triangular lattice with different pairing symmetries and boundary topologies. We find a rich phase diagram of zero energy Andreev edge states that is a unique fingerprint of each of the possible pairing symmetries. We propose to pin down the pairing symmetry in recently discovered Na$_{x}$CoO$_{2}$ material by the Fourier- transformed scanning tunneling spectroscopy for the edge state. A surprisingly rich phase diagram is found and explained by a general gauge argument and mapping to 1D tight-binding model. Extensions of this work are discussed at the end. ref: cond-mat/0407187 [Preview Abstract] |
Wednesday, March 23, 2005 12:15PM - 12:27PM |
P25.00004: Finite Size Effects in Nanocrystals of Magnetoelectric Multiferroic Oxides. Prasenjit Guptasarma, Shishir Ray, Mark Williamsen, Vaijayanti Palkar The coexistence of coupled ground states of ferroelectricity and magnetism is an intriguing phenomenon, observed in some multiferroic materials. Among the many interests in such materials is the possibility of using an electric field as a means to control magnetic spin. The ability to maintain a small grain size is an important parameter in memory devices, and therefore it is important to study the effect of reduced crystal size on properties. We report studies of a select group of ABO3-type oxide materials with controlled particle size in the 20-100 nm size range, fabricated using solution techniques (samples $\sim $ 20nm, 33nm, 56nm and 80nm particle size). We have studied particle-size induced changes in the lattice structure, and in the electric and magnetic properties. We also find a change in the electronic band gap and the phonon spectrum. We compare these results with bulk single crystals and conclude that our size effects mostly arise from a size induced change in lattice symmetry. [Preview Abstract] |
Wednesday, March 23, 2005 12:27PM - 12:39PM |
P25.00005: Far-infrared phonon behavior in the undersconstrained negative thermal expansion system Zr(WO$_4$)$_2$ Zack Schlesinger, Chandra Turpen, Jason Hancock, Glen Kowach, Art Ramirez Zr(WO$_4$)$_2$ is an unusual material in that it contracts isotropically as it is heated over a very broad temperature range from about 10 to 1000 K. Temperature dependence of the lattice volume and specific heat indicate that the important energy range for the mechanism of this negative thermal expansion (NTE) phenomenon is about 2 to 12 meV. Using infrared spectroscopy, we have studied[1] the optic phonons as a function of temperature. Comparison of energy levels to the measured specific heat and neutron density-of-states indicates that anomalous features arise in precisely the spectral region where NTE phonons are believed to exist. In addition, lattice-dynamical modeling suggests that most of the low-energy modes involve a combination of twisting and translational motion of WO$_4$ tetrahedra. We will discuss the relationship of these data to the origins of negative thermal expansion and the possible role of geometrical frustration in sustaining the unusual environment that supports NTE in this under-constrained open-structured system. [1] J. N. Hancock et al, Phys. Rev. Lett. 93, 225501 (2004), cond-mat/ 0409533. [Preview Abstract] |
Wednesday, March 23, 2005 12:39PM - 12:51PM |
P25.00006: Eleectric Field Gradients and Born Effective Charges of PST Dandan Mao, Eric Walter, Henry Krakauer Relaxor behavior in some complex ferroelectrics oxides is thought to be related to the local chemical environment. High magnetic field MAS NMR measurements have recently shown great promise as a microscopic probe of local structure of relaxors \footnote{G. Hoatson et al., {\it Phys. Rev. B} {\bf 66} 224103, (2002).} by their ability to resolve electric field gradient (EFG) splittings. It is thus of considerable interest to be able to calculate EFGs in these materials. Here we present local density functional EFG calculations of Pb(Sc$\frac{1}{2}$Ta$\frac{1}{2}$) (PST) using the linear augmented planewave (LAPW) method. Our calculations focus on PST unit cells with different chemical ordering and ferroelectric phases. Trends of EFGs of PST and the correlation between EFGs and Born effective charges are discussed to better understand how local environments induce changes in EFGs. \newline \newline * Supported by ONR [Preview Abstract] |
Wednesday, March 23, 2005 12:51PM - 1:03PM |
P25.00007: High-yield growth of semiconducting tungsten oxide nanowires Christian Klinke, James B. Hannon, Phaedon Avouris We characterized the growth, composition, and electrical properties of crystalline WO$_3$ nanowires grown using a catalyst-free chemical vapor deposition method. We showed that growing the wires in a mixture of methane and hydrogen dramatically increases the yield compared to growing the wires in argon carrier gas. The high yield makes simple nanowire `harvesting' schemes feasible. Additionally, we demonstrated that field-effect transistors can be produced using single WO$_3$ nanowires as the transistor channel. Devices made by using Ni as source and drain contacts are $n$-type and have good ON-currents and reasonable ON/OFF ratios. Scanning tunnelling spectroscopy gives a bandgap of about 2.2~eV. [Preview Abstract] |
Wednesday, March 23, 2005 1:03PM - 1:15PM |
P25.00008: Synthesis and characterization of MgO and ZnO nanoparticle Mary Ross, M. Farinelli, S. Chinta, D. Beach, A. Rondinone , John Larese We will discuss recent synthetic, thermodynamic and electron microscopicy investigations that probe the topological, adsorption and chemical properties of MgO and ZnO nanometer sized particles. Using a novel, patented process we find that pure and doped particles of metal oxides can be produced in large quantities with well-defined crystal habitat. Our thermodynamic investigations indicate that at low temperatures several layering transitions of methane and hydrogen are easily observable. Our electron microscopy studies indicate that numerous shapes including cubes, rods, plates and tetrapods can be selectively produced. We will discuss the effectiveness of these materials as catalysts when small (20nm) sized metal clusters of Au and Pd are deposited. If time permits the optical properties of the ZnO materials will also be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 1:15PM - 1:27PM |
P25.00009: Non-integral spin moment and electron correlation effects in magnetite P. A. Montano, Yinwan Li, B. Barbiellini, P. E. Mijnarends, S. Kaprzyk, Hsin Lin, A. Bansil In order to directly probe the electronic ground state of magnetic electrons, we have carried out temperature dependent magnetic Compton scattering experiments on an oriented single crystal of magnetite (Fe$_3$O$_4$). First principles band theory computations using the conventional local density approximation (LDA) as well as computations treating correlation effects beyond the LDA are used to gain insight into these measurements. The magnetic moment associated with unpaired spins in magnetite is found to be insensitive to temperature over the range of 10-300K with a value of about 3.6 $\mu_B$/formula unit, including the region of the Verwey transition. The non-integral value of the spin moment implies that some majority spin states must be present at the Fermi energy ($E_F$) at all temperatures so that the polarization of electrons at the $E_F$ cannot be 100\%. Our analysis emphasizes the role of Fe$^{2+}$ ions on the octahedral sites in producing a correlated ground state of magnetite and gives insight into the nature of the order parameter for Verwey transition and the lack of quenching of the orbital magnetic moment in the system. Work supported by the USDOE. [Preview Abstract] |
Wednesday, March 23, 2005 1:27PM - 1:39PM |
P25.00010: Theoretical studies on strongly correlated systems: bulk and surfaces of magnetite Fe$_3$O$_4$ Henry Pinto, Simon Elliott Transition metal oxides (TMOs) are important materials because of their wide range of properties, the underlying physics and the tremendous implications for tomorrow's technology. Magnetite, Fe$_3$O$_4$ is one technologically important TMO that undergoes a first-order metal-insulator transition at $T_V$=120 K as reported first by E. Verwey. We use density functional theory adding a Hubbard-U parameter (DFT+U) to account for intra-atomic interactions for the strongly correlated Fe:3$d$ electrons of Fe$_3$O$_4$. Applying plane-wave DFT within the generalized gradient approximation and appropriate parameters, we examine the electron-phonon effects that cause a small structural distortion and lead to the insulating state with low symmetry. The electronic structure for this phase presents a sub-band of partially-delocalised minority spin electrons below the Fermi level. Here we investigate the competing roles of the screened coulomb repulsion, Fe-O hydridization and Fe-Fe overlap. We also study the Fe$_3$O$_4$ (001) surface: $(\sqrt{2}\times\sqrt{2})R45^{\circ}$ slab is used and we consider both tetrahedral and octahedral terminations.The surface reconstruction, electronic structure, magnetic properties and surface energies are computed. The calculated superficial density of states of our optimal slab is compared with scanning tunneling microscope data. Finally we analyze the effect of the electron-phonon interaction in the electronic structure of the surface and the possible existence of charge ordering. [Preview Abstract] |
Wednesday, March 23, 2005 1:39PM - 1:51PM |
P25.00011: Effect of the electron-phonon coupling on the magnetism in the Nickelatematerials Li$_{x}$Na$_{1-x}$NiO$_{2}$ H. Meskine, S. Satpathy The absence of magnetic and orbital ordering in the nickelate LiNiO$_{2}$ has long been a subject of speculation, especially in light of the fact that its sister compound NaNiO$_{2}$ exhibits both magnetic and orbital structure. Although this issue has attracted much attention in recent years from both the theoretical and experimental fronts, the unusual spin-glass state of lithium nickelate remains a mystery. We are able to account for the observed type A magnetic structure of NaNiO$_{2}$ by computing the intra- and inter-layer exchange couplings using a model Hamiltonian which includes electronic hopping, on- site energy, and Coulomb interaction. The electronic structure parameters are obtained via \emph{ab initio} density functional theory calculations using the linear muffin-tin orbitals method. The dynamical electron-phonon coupling is then introduced by quantization of the motion of the Na/Li ion. We compute the ground-state of the full Hamiltonian by exact diagonalization as well as using a Lang-Firsov unitary transformation. We find that the coupling of the electronic degrees of freedom to the motion of the metallic ion decreases the exchange coupling. [Preview Abstract] |
Wednesday, March 23, 2005 1:51PM - 2:03PM |
P25.00012: Hole dynamics in spin and orbital ordered vanadium perovskites Sumio Ishihara We present a theory of the doped perovskite vanadates with spin and orbital orders [1]. Two kinds of spin-orbital orders are found in the ground state: the G-type (three-dimensional (3D) staggered) spin order (SO) with the C-type (rod type) orbital order (OO) (the alternative $d_{xy}^1d_{yz}^1/d_{xy}^1d_{zx}^1$ configuration) termed (SG/OC) in YVO$_3$, and the C-type SO with the G-type OO termed (SC/OG) in LaVO$_3$. Mobile holes are strongly renormalized by spin excitations (magnons) in the spin G-type and orbital C-type (SG/OC) order, and orbital excitations (orbitons) in the spin C-type and orbital G-type (SC/OG) one. It is found that hole dynamics in a staggered $t_{2g}$ orbital array is distinct from that in a antiferromagnetic order as well as the $e_g$ orbital one. The anomalously fragile character of the (SG/OC) order observed in Y$_{1-x}$Ca$_x$VO$_3$ is attributed to the orbiton softening induced by a reduction of the spin order parameter. [1] S. Ishihara, cond-mat/0408395. [Preview Abstract] |
Wednesday, March 23, 2005 2:03PM - 2:15PM |
P25.00013: P-O and Al-O Bonding in Alumina-Calcia-Monazite Melts Studied by Raman Scattering and Ultra High-temperature NMR Robert Marzke, Susan Boucher, Jeremy Piwowarczyk, George Wolf Raman scattering and NMR of $^{27}$Al have been used to investigate the structure of molten samples of ceramics in the Al$_{2}$O$_{3}$-CaO-LaPO$_{4}$ system. Raman spectra of quenched samples indicate the presence of PO$_{4}$ structures similar to those of metaphosphate glasses, involving Q$_{2}$ tetrahedral$_{ }$units. NMR of $^{27}$Al in melts shows strong 4-fold coordination, but also 5- and 6-fold Al-O bonding and diffusivities far more rapid than those expected for networked AlO$_{4}$ tetrahedra. Evidence that Al cross links tetrahedral chains, in addition to forming these tetrahedra, will be discussed. [Preview Abstract] |
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