Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session P17: Focus Session: Bulk Properties of Complex Oxides - 3d Oxides |
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Sponsoring Units: DMP GMAG Chair: Michelle Johannes, Naval Research Laboratory Room: D174 |
Wednesday, March 23, 2011 8:00AM - 8:36AM |
P17.00001: Surprises in low dimensional spin 1/2 magnets - from crystal chemistry to microscopic magnetic models of complex oxides Invited Speaker: A microscopic understanding of the structure-properties relation in crystalline materials is a main goal of modern solid state chemistry and physics. Due to their peculiar magnetism, low dimensional spin 1/2 systems are often highly sensitive to structural details. Seemingly unimportant structural details can be crucial for the magnetic ground state of a compound, especially in the case of competing interactions, frustration and near-degeneracy. Here, we present for selected, complex Cu$^{2+}$ systems that a first principles based approach can reliably provide the correct magnetic model, especially in cases where the interpretation of experimental data meets serious difficulties or fails. We demonstrate that the magnetism of low dimensional insulators crucially depends on the magnetically active orbitals which are determined by details of the ligand field of the magnetic cation. Our theoretical results are in very good agreement with thermodynamic and spectroscopic data and provide deep microscopic insight into topical low dimensional magnets. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P17.00002: Spin Waves and Magnetic Interaction in the Multiferroic Antiferromagnet MnWO4 J.A. Fernandez-Baca, Feng Ye, R.S. Fishman, A. Podlesnyak, G. Ehlers, H.A. Mook, Y.Q. Wang, B. Lorenz, C.W. Chu The spin wave excitations of the multiferroic MnWO4 have been measured in the low-temperature collinear commensurate phase using high-resolution inelastic scattering. The spin excitations can be well described by a Heisenberg model with competing exchange interactions up to 11th nearest neighbors. We find the magnetic exchange couplings are highly frustrated within each zigzag spin chain along c-axis and between chains along the a-axis. However, the magnetic interactions are much weaker between chains along the b-axis. Our measurements suggest that the delicate balance of long range magnetic couplings is subject to small perturbations that can lead to a complex magnetic configuration exhibiting magnetoelectric behavior. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P17.00003: Field-induced slow spin relaxation in monoclinic Nd$_{2}$Ti$_{2}$O$_{7}$ single crystals Hui Xing, Gen Long, Hanjie Guo, Chunmu Feng, Guanghan Cao, Hao Zeng, Zhuan Xu We report the ac susceptibility measurement in the paramagnetic state of the monoclinic Nd$_{2}$Ti$_{2}$O$_{7}$ single crystals. An unexpected slow spin relaxation is observed in the presence of a nonzero magnetic field. Such behavior is absent in zero field. Distinct features of the relaxation, including the intrinsic frequency on the order of 1 Hz, the field-, temperature- and spin dilution dependence, and its evolution under positive and negative pressures, indicate that the relaxation is associated with an unusual cooperative behavior involving spin correlations. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P17.00004: The d-band manifold in SrTiO$_{3}$: high mobility Shubnikov--de Haas effect in magnetic fields to the quantum limit. S. James Allen, Bharat Jalan, Guru Khalsa, Allan MacDonald, Jan Jaroszynski, Susanne Stemmer The molecular beam epitaxial growth of high mobility ($>$ 30,000 cm$^{2}$/volt.sec), low electron density ( $\sim $ 10$^{17}$ - 10$^{18}$cm$^{-3})$ La doped SrTiO$_{3}$ has provided an opportunity to explore the lowest conduction band states, which are derived from the Ti d-band. Despite the long history of experiments on these d-band states, including magneto transport, we are left without a firm quantitative model of the manifold at the conduction band minimum. But, these states form the basis of quantum confined 2D electron systems at oxide interfaces with SrTiO$_{3}$ and delta doped layers, both the subject of current interest. To remedy this, we have performed magneto transport at temperatures down to $\sim $ 0.4 K, in magnetic fields to 31 Tesla, which is sufficient to reach the quantum limit, in high mobility samples and with carrier densities that tune the Fermi energy through the energy splitting caused by the low temperature tetragonal distortion. In close analogy to hole states in conventional semiconductors, we use 5 ``Luttinger'' parameters and the splitting energy, to describe these results and compare with the current understanding of the SrTiO$_{3}$ d-band structure. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P17.00005: Low Energy Conduction Band Structure of SrTiO3 Guru Khalsa, Bharat Jalan, S. James Allen, Susanne Stemmer, Allan MacDonald The recent observation of a high mobility 2DEG, and truly two-dimensional superconductor, at oxide interfaces with SrTiO3 (STO) and in delta-doped layers of STO have thus far gone without a clear theoretical description. The starting point for any quantitative theory of these systems is a reliable low energy parameterization of the Ti d-band in the bulk parent compound. Here we present a five parameter symmetry constrained model of the t2g band in STO near the conduction band minimum. We use this model to describe a recent high field (up to 31 Tesla), low temperature, angular magneto-transport study of lightly La doped STO and compare our results with other available experimental data. We will also discuss the relation between orbital density and matrix element effects in photoemission experiments of d0 Perovskites. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P17.00006: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P17.00007: Unconventional Electronic Transport in Doped SrTiO$_{3}$ C. Leighton, A. Spinelli, M.A. Torija, C. Liu, C. Jan Resistivity, Hall effect, and magnetoresistance are reported on a large set of semiconducting SrTiO$_{3-\delta }$ single crystals doped n-type (by reduction or Nb substitution) over a broad range of carrier density (10$^{15}$ to $>$10$^{20}$ cm$^{-3})$. Temperature-independent densities, strongly temperature-dependent mobilities (up to 22,000 cm$^{2}$V$^{-1}$s$^{-1}$ at 4.2 K), and a remarkably low critical carrier density for the metal-insulator transition are observed, and interpreted in terms of the quantum paraelectricity of the host. We argue that an unusual, high mobility, low density, metallic state is thus established at carrier densities at least as low as 8.5 x 10$^{15}$ cm$^{-3}$. At low temperatures the temperature dependence of the mobility and resistivity exhibit a non-monotonic carrier density dependence and an abrupt change in character near 2 x 10$^{16}$ cm$^{-3}$, indicating a distinct crossover in conduction mechanism, perhaps associated with a transition from impurity band to conduction band transport. The results provide a simple framework for the understanding of the global transport behavior, and suggest some potential applications. Work supported by NSF. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P17.00008: Pseudogap in metallic layered nickelate $R_{2-x}$Sr$_x$NiO$_4$ Masaki Uchida, K. Ishizaka, Y. Ishida, Y. Onose, R. Arita, S. Shin, Y. Tokura, P. Hansmann, A. Toschi, K. Held, Y. Kaneko, X. Yang, O.K. Andersen, R. Kumai We have investigated charge dynamics and electronic structures for single crystals of metallic layered nickelates $R_{2-x}$Sr$_x$NiO$_4$. Angle-resolved photoemission spectroscopy (ARPES) on the barely-metallic Eu$_{0.9}$Sr$_{1.1}$NiO$_4$ has revealed a large hole surface of $x^2-y^2$ character with a high-energy pseudogap of the same symmetry and comparable magnitude with those of underdoped cuprates, although the antiferromagnetic interactions are one order of magnitude smaller. Our findings strongly indicate that the high-energy momentum-dependent pseudogap (or Fermi arc) is not unique to the high-$T_{\mathrm{c}}$ cuprates but commonly develops in the anomalous quasi-two-dimensional metallic state near the Mott transition reflecting the real-space charge correlation. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P17.00009: Density wave driven metal-insulator transition in nickelates SungBin Lee, Ru Chen, Leon Balents The Mott transition in nickelates, RNiO3, shows unusual magnetic ordering and charge ordering in the insulating phase. For the more itinerant nickelates, one may argue that these unusual density waves are actually driven by Fermi-surface nesting, originated from the large flat regions of Fermi surfaces. Using a tight-binding model of the band derived from doubly degenerate eg orbitals, we obtain the density wave induced metal- insulator transition phase diagram in the presence of on-site Coulomb interaction and Hund's coupling, treated in Hartree-Fock approximation. Furthermore, motivated by recent success in layer by layer growth of nickelates, the thin film effects in nickelates are also studied. Finally we calculate the optical conductivity for the various states in our phase diagram, suggesting experimental measurements to check the theory. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P17.00010: Molecular correlated insulating state in low-valence layered nickelates Victor Pardo, Warren E. Pickett In recent years, there has been an effort on artificially creating Fermi surfaces that resemble those of the superconducting cuprates. A Ni$^{3+}$:d$^7$ (one e$_g$ electron) configuration can be made into the electron-like analog of the d$^9$ (one e$_g$ hole) cuprate electronic structure.[1] Another cleaner alternative would be to grow low-valence Ni$^+$:d$^9$ compounds, which have recently become available through synthesis[2] of members of the series La$_{n+1}$Ni$_{n}$O$_{2n+2}$. We present LDA+U calculations on the layered compounds La$_4$Ni$_3$O$_8$ [3] and La$_3$Ni$_2$O$_6$, with three and two NiO$_2$ layers, respectively. Electron count implies very low Ni formal valencies: 1.33+ and 1.5+, respectively. If charge order is present, Ni$^+$:d$^9$ could occur in a geometry similar to that of the cuprates. However, this is not the case. Both compounds are insulators, which we can attribute to quantum confinement in the NiO$_2$ tri/bi-layers. The only states close to the Fermi level are Ni d$_{3z^2-r^2}$, which couple along the c-axis (Ni trimers or dimers). The insulating behavior must be viewed from a molecular orbital viewpoint, after AFM order within layers has narrowed the bands. Insulating behavior is that of a ``molecular" Mott insulator rather than a charge-ordered insulator. \newline [1] J. Chaloupka and G. Khaliullin, \emph{PRL} \textbf{100}, 016404 (2008). \newline [2] V. V. Poltavets \textsl{et al.}, \emph{Phys. Rev. Lett.} \textbf{102}, 046405 (2009). \newline [3] V. Pardo and W.E. Pickett, arXiv:1008.2707. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P17.00011: Search for structural fluctuations in the disordered stripe state of Nd$_{1:67}$Sr$_{0:33}$NiO$_{4}$ A.M. Milinda Abeykoon, Emil Bozin, Genda Gu, John Hill, John Tranquada, Simon Billinge We present a temperature series PDF and a Rietveld analysis of Nd$_{1:67}$Sr$_{0:33}$NiO$_{4}$ system to study the local structural response in the state above the charge-ordered state that has not been characterized in detail to date. We observed NiO$_{6}$ octahedral tilting patterns of different magnitude for short and long-range structure of the system. A sequential Rietveld refinement, and a T-series PDF analysis on the length scale (5-20){\AA} were carried out to characterize the long-range order of the system. A PDF analysis on the length scale (0-4.2) {\AA} revealed a different magnitude local octahedral tilt pattern as a function of temperature. The correlation length of short-range ordered charge stripes existing above T$_{co}$ was estimated using a Box-Car type PDF model. Combining this information with the refined isothermal atomic displacement parameters (ADPs) yields a much more complete picture of the nature of both atomic displacements and how they are correlated with each other in the system. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P17.00012: Lattice normal modes and electronic properties of the correlated metal LaNiO$_3$ Gaoyang Gou, James Rondinelli, Ilya Grinberg, Andrew Rappe We present results from density functional calculations of lattice vibrations and electronic properties of the correlated metal LaNiO$_3$. Using the landau theory of phase transitions and {\it ab initio} derived phenomenological coefficients obtained from local-spin density approximation (LSDA) calculation, we examine the evolution of the Raman-active phonon modes with temperature and find that the LSDA results give excellent agreement with experiments. To study the electronic structure of LaNiO$_3$, we extend to the post-LSDA functional methods, including the local spin density+Hubbard U (LSDA+U) method, and two hybrid exchange-correlation functionals, PBE0 and HSE. By comparing the results obtained from the various functionals with the experimental photoelectron spectroscopy (PES) and X-ray photoelectron spectroscopy (XPS) data, we argue that the screening effect coming from the delocalized O-2p and Ni-t$_{2g}$ electrons will be strong enough to reduce the electron correlation of LaNiO$_3$. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P17.00013: Time-Dependent Recovery of Charge and Spin Order in Stripe-Ordered Nickelates Y.F. Kung, A.F. Kemper, W.-S. Lee, B. Moritz, A.P. Sorini, Z.-X. Shen, T.P. Devereaux Using time-dependent Ginzburg-Landau theory, we study the melting and recovery of charge and spin order in striped nickelates (La$_{2-x}$Sr$_x$NiO$_4$) in response to an ultrashort pump pulse that destroys the order. We find that the critical temperature for onset of spin order varies with increasing coupling between charge and spin order. Solving the Gross-Pitaevskii equations to model the time evolution, we explore the temporal dynamics of charge and spin order parameters, to be compared to experimental observations at LCLS. [Preview Abstract] |
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