Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session U13: Focus Session: Maganites, Titanates, & Vanadates |
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Sponsoring Units: DMP GMAG Chair: Yoichi Horibe, Rutgers University Room: Colorado Convention Center Korbel 4C |
Thursday, March 8, 2007 8:00AM - 8:36AM |
U13.00001: Combining DFT and Many-body Methods to Understand Correlated Materials Invited Speaker: Electronic and magnetic properties of strongly-correlated systems are typically controlled by a limited number of electronic states, located near the Fermi level and well isolated from the rest of the spectrum. This opens a formal way for combining the first-principles methods of electronic structure calculations, based on the density-functional theory (DFT), with model many- body methods, formulated in a restricted Hilbert space of states near the Fermi level. The core of this project is the construction of ``\textit{ab initio} model Hamiltonians'', which would incorporate the physics of on-site Coulomb correlations and provide a transparent physical picture for the low-energy properties of strongly- correlated systems. First, I will describe a systematic procedure for constructing such an effective Hubbard-type model, which consists of three major steps, starting from the electronic structure in the local- density approximation.$^1$ (i) Construction of the kinetic-energy part using an exact version of the downfolding method;$^{1,2}$ (ii) Construction of the Wannier functions; (iii) Calculation of screened Coulomb interactions using a hybrid approach, which combines the random phase approximation with the constraint DFT.$^{1,3}$ Then, I will illustrate abilities of this method for resolving a number of controversial issues, related with the interplay of the experimental lattice distortion and magnetic properties of four narrow $t_{2g}$ band perovskite oxides (YTiO$_3$, LaTiO$_3$, YVO$_3$, and LaVO$_3$), for which the obtained Hamiltonian was solved using a number of techniques, including the Hartree-Fock (HF) approximation,$^4$ the second-order perturbation theory and the $t$-matrix approach for the correlation energy,$^ {4,5}$ and a variational superexchange theory, which takes into account the multiplet structure of the atomic states.$^4$ I will argue that the crystal distortion imposes a severe constraint on the form of the possible orbital states, which favors the formation of experimental magnetic structures in YTiO$_3$, YVO$_3$, and LaVO$_3$, even at the level of HF approximation. The correlation effects systematically improve the agreement with the experimental data and additionally stabilize the experimentally observed G- and C-type antiferromagnetic states in YVO$_3$ and LaVO$_3$. The role of relativistic spin-orbit interaction will be also discussed. \newline $^1$ I.~V.~Solovyev, Phys.~Rev.~B~\textbf{73}, 155117 (2006). \newline $^2$ I.~V.~Solovyev, Z.~V.~Pchelkina, and V.~I.~Anisimov, cond- mat/0608528. \newline $^3$ I.~V.~Solovyev and M.~Imada, Phys.~Rev.~B~\textbf{71}, 045103 (2005). \newline $^4$ I.~V.~Solovyev, Phys.~Rev.~B~\textbf{74}, 054412 (2006). \newline $^5$ I.~V.~Solovyev, cond-mat/0608625. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U13.00002: Additional evidence for complex 2-site polarons in CMR manganites. Frank Bridges, Geza Kurczveil, Lisa Downward, John J. Neumeier Recently we have proposed a complex 2-site polaron model (which we call a dimeron) that exists for temperatures near and above the ferromagnetic transition temperature, T$_c$ [1]. The dimeron has a hole delocalized over two Mn sites (i.e. a hole and an electron share the two Mn sites) and the two Mn sites have a reduced distortion compared to the remaining Jahn-Teller distorted electron sites. Magnetic clusters just above T$_c$ are likely clusters of these dimeron quasiparticles. The average valance of the two Mn sites in the dimeron is 3.5 and the spin is 7/2. We show that the Mn K-absorption edge is much better described as a sum of a 3.5 valence edge (fraction 2x) plus a 3 valance edge (fraction 1-2x), compared to earlier simulations using x CaMnO$_3$ plus 1-x LaMnO$_3$. We also show that fitting the Mn-O peak to a sum of two experimental Mn-O standards leads to a similar result as in the earlier study - a fraction 2x of lower distorted Mn sites (dimerons) and a fraction 1-2x of more distorted sites with 1 e$_g$ electron. Both support the proposed complex - 2-site polaron model.\newline Supported under NSF grant DMR0301971.\newline [1] L. Downward et. al., Phys Rev Lett 95, 106401 (2005). [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U13.00003: Magnetic mapping of phase separated manganite films Casey Israel, Diana Sanchez, Takeshi Kasama, Rafal Dunin-Borkowski, Neil Mathur, Tien-Ming Chuang, Alex de Lozanne We summarize the recent results of efforts to image the different phases in phase-separated epitaxial thin manganite films. We present images acquired by three techniques, electron holography in a transmission electron microscope, magnetic force microscopy (MFM), and conducting atomic force microscopy (CAFM), all of which are applied to La$_{1-x}$Ca$_{x}$MnO$_{3}$ (LCMO) films grown on NdGaO$_{3}$ (001) substrates. Electron holography images of a focused ion beam milled LCMO ($x=0.3$) film (uniformly ferromagnetic at low temperatures) demonstrate the feasibility of using this method to distinguish local phases with different magnetic properties and illustrate the dangers of ion implantation during focused ion beam processing. MFM and CAFM scans of an as-deposited LCMO ($x=0.40$) film (phase-separated at low temperatures) indicate the coexistence of ferromagnetic (metallic) and nonferromagnetic (insulating) regions characterized by length scales below roughly 100 nm. It appears that the ferromagnetic regions preferentially form conducting pathways aligned with the film's easy-axis for magnetization. This seems intrinsic, in that there are no topographical features linked with this phase anisotropy. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U13.00004: Observation of Nanoscale Electronic Phase Separation and Charge Density Wave in Manganites Jeehoon Kim, Junwei Huang, Alex de Lozanne, J. S. Zhou, J. B. Goodenough Scanning tunneling microscopy (STM) is used to image the surface topography and local density of states (LDOS) in the bilayer colossal magnetoresistance (CMR) material LSMO below the Curie temperature. While our STM is capable of atomic resolution, on this surface the smallest features are randomly distributed islands with a lateral size of $\sim $1 nm. We obtained conductance maps to investigate the local electronic structure associated with these islands. The 2-D cross-correlation map between the gap map and the topography suggests a random distribution of three different domains which are Mn$^{3+ }$rich, Mn$^{4+}$ rich or mixed valence Mn$^{3/4}$. The spectroscopic data show a large gap around 600 meV. We also observed some modulations in the conductance map. The Fourier transform of the conductance map shows two major modulations with 1.6 nm and 60 nm wavelength along the crystal axis. The 1.6 nm wavelength modulation can be explained by charge density wave (CDW) model resulting from a Fermi surface instability. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U13.00005: Susceptibility imaging of a weakly disordered manganite in high magnetic field Weida Wu, Casey Israel, Alex de Lozanne, Namjung Hur, Soonyong Park, Sang-Wook Cheong We report variable temperature magnetic force microscopy (VT-MFM) studies of (La,Pr)$_{0.625}$Ca$_{0.375}$MnO$_3$ in a 1 T magnetic field with temperature ranging from 10 K to 250 K. {\it In-situ} resistance measurements confirmed the bulk phase transitions between paramagnetic, anti-ferromagnetic charge ordering and ferromagnetic (FM) phases. Above the FM transition ($\sim$80K), MFM images show $\mu$m-scale magnetic contrast which originates from the local susceptibility variations of the different phases.\footnote{C. Israel, W. Wu and A. de Lozanne, APL, {\bf 89}, 032502 (2006).} By comparing the MFM images with polarized optical microscopy images taken at room temperature, it becomes apparent that the magnetic phase inhomogeneity observed at elevated temperature is linked to local strain variations at the surface of the sample. The high field susceptibility imaging technique could be extended to the study of other magnetic phase separated systems. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U13.00006: Evidences for Metal-Insulator Phase Coexistence Below Tc in Self-doped Manganite Films Peng Gao, Trevor A. Tyson, Michael Deleon, Catherine Dubourdieu, Zhenxian Liu We present evidence for the existence of a significant insulating phase component more that 100 K below the magnetic ordering temperature in the self-doped system La$_{0.80}$MnO3. Reflectivity measurements were made over the range 100 to 8000 cm-1. Using the Drude-Lorenz Model, we convert reflectivity spectra into optical conductivity spectra. The insulating phase was evident as sharp resonances corresponding to atomic vibrational modes which disappeared deep in the metallic phase at low temperature. A significant component of the insulating phase exists down to 200 K below Tc. Between 200 K and 150 K, the metallic phase becomes the dominant phase. Comparisons are made with the La0.7Ca0.3MnO3 samples measured under the same conditions. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U13.00007: Phase-segregated Glass Formation Linked to Long-Range Strain using Resonant Ultrasound Spectroscopy Peter Sharma, S. El-Khatib, I. Mihut, J. B. Betts, A. Migliori, S. B. Kim, S. Guha, S-W. Cheong We have observed a very large damping of ultrasonic waves in the magnetically/electronically/structurally phase segregated state of a CMR manganite that suddenly disappears upon the formation of a glassy state. A subtle stiffening of the shear modulus accompanies the putative glass transition. Our observations most explicitly link strain to the proposed formation of an unusual glass state composed of coexisting, macroscopic structural domains found in this material. These results may implicate strain as the determining factor in the formation of non-equilibrium mixed-phase states in other systems that display widely hysteretic first order magneto-structural transitions. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U13.00008: Raman scattering studies of field- and temperature-dependent melting of charge order in La$_{0.25}$Pr$_{0.375}$Ca$_{0.375}$MnO$_{3}$ and La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ Minjung Kim, H. Barath, S.L. Cooper, M. Rubhausen, S.W. Cheong The La$_{0.25}$Pr$_{0.375}$Ca$_{0.375}$MnO$_{3}$ system provides an interesting opportunity to study the effects of chemical disorder---introduced by replacing La$^{3+}$ with Pr$^{3+}$ having a smaller ionic radius---on the charge-ordered (CO) state and the complex field- and temperature-dependent phase behavior observed in the manganese perovskite La$_{1-x}$Ca$_{x}$MnO$_{3}$. We report field- and temperature-dependent Raman studies of La$_{0.25}$Pr$_{0.375}$Ca$_{0.375}$MnO$_{3}$ and La$_{0.5}$Ca$_{0.5}$MnO$_{3}$, in which we are able to carefully study the effects of disorder on both field-induced melting of CO and on the field-induced evolution of novel structural and magnetic phases in the (La,Pr,Ca)MnO$_{3}$ system. Among other results we will discuss, La$_{0.25}$Pr$_{0.375}$Ca$_{0.375}$MnO$_{3}$ exhibits a quantum melting transition from a CO state to a ferromagnetic metal phase at fields less than 7 T, which is a much lower value than that observed in La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ ($\sim $20T). [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U13.00009: Optical study of Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ thin films on SrTiO$_{3}$ Seung Yup Jang, M.W. Kim, K.W. Kim, T.W. Noh, N. Nakagawa, H.Y. Hwang Doped rare earth manganites show interesting phase diagrams depending on strong coupling among spin, charge, and orbital degrees of freedom. Especially, Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ (NSMO) undergoes two phase transitions of para-magnetic insulating state to ferro-magnetic metallic state (FMM) and FMM to charge/orbital ordered insulating state as temperature decreases. When NSMO is grown as a thin film on SrTiO$_{3}$(STO) (001), it has no phase transition. Recently, however, thin films grown on STO (110) substrates were found to retain the phase transitions of bulk NSMO. This implies that the charge/orbital state could be modulated by controlling lattice strain. We have investigated the optical conductivity spectra of NSMO epitaxial thin films grown on STO substrates. Optical spectra of NSMO grown on STO (110) show drastic change, depending on temperature and polarization. Based on the careful comparison between the films grown on differently oriented STO substrates, we demonstrate that the substrates strain can cause crucial effect on the ground state of NSMO to result in a novel insulating state which is rarely known. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U13.00010: Phase diagram for Bi1-xCaxMnO3 (x $<$ 0.5) Yuhai Qin, Trevor Tyson, Sang-Wook Cheong, Xiaonong Xu The multiferroic BiMnO3 system, in which ferroelectronic and ferromagnetic orders can coexist, has attracted much research work in the past years for its potential technological applications. For the more general system Bi1-xCaxMnO3, the phase diagram for the Ca rich region (x $>$ 0.4) has been established [1]. In order to understand the multiferroic behavior near the x=0 system, the hole-doped region (0$<$x$<$0.5) was investigated. We have completed the magnetic, transport, and structural phase diagram of Bi1-xCaxMnO3, by performing detailed structural (XRD and XAFS), magnetization (ZFC/FC) and electrical measurements on Bi1-xCaxMnO3 (0$<$x$<$0.5), showing the transition form the highly distorted monoclinic phase to the orthorhombic phase. This work is supported by NSF DMR-0512196. \newline \newline [1] H. Woo, T. A. Tyson, M. Croft, S. W. Cheong, and J. C. Woicik, Physical Review B: Condensed Matter and Materials Physics 63, 134412/1 (2001). [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U13.00011: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U13.00012: Investigation of the local Mn-O distortions in the concentration-induced Metal/Insulator transition materials, La$_{1-x}$ Ca$_{x}$MnO$_{3}$ (x= 0.16-0.22) using EXAFS. Yu Jiang, Lisa Downward, Frank Bridges, John Neumeier The pseudo-cubic manganites La$_{1-x}$ Ca$_{x}$MnO$_{3}$ (LCMO) are CMR (Colossal Magnetoresistance) materials for x between approximately 0.2 and 0.5. They have a temperature driven Metal/Insulator (MI) transition at the ferromagnetic transition temperature T$_{c}$ which is caused by Jahn-Teller-like distortions which form around Mn sites as T increases through T$_{c}$. Near x=0.2 the MI is driven by concentration. To investigate the concentration and temperature dependent local distortions for low doped powder samples (LCMO x = 16{\%}, 18{\%}, 20{\%}, 22{\%}), we carried out EXAFS (Extended X-ray Absorption Fine Structure) experiment at SSRL (Stanford Synchrotron Radiation Laboratory). The results of our preliminary analysis show the following: (1) a sharp, step increase in $\sigma ^{2}$ near T$_{c}$ ($\sigma $ is the width of the Mn-O pair distribution function (PDF)), (2) the magnitude of the step decreases rapidly with decreasing x, (3) the value of $\sigma ^{2}$ at low T is close to the value for zero-point motion for x = 0.22, but increases substantially for lower x, indicating that in the magnetized state a large J-T distortions remain at low T for low x. Also the magnetization at low T is small for lower x, indicating that not all of the sample is ferromagnetic. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U13.00013: Temperature- and field-dependent far-infrared studies of (La$_{0.4}$Pr$_{0.6}$)$_{0.67}$Ca$_{0.33}$MnO$_{3}$ films Naveen Margankunte, Tara. P. Dhakal, Alexandre Zimmers, D.J. Arenas, Y.J. Wang, Amlan Biswas, D.B. Tanner We report far-infrared temperature (300 K-20 K) and magnetic field (0-18 T) dependent reflectance and transmittance studies of 1000 A-thick hole-doped LPCMO thin films. Temperature-dependent measurements show evidence of phase coexistence in the sense that the film still shows insulating behavior at very low temperatures. Magnetic field studies at 4.2 K observe an insulator to metal transition, seen as large changes in reflectance and transmittance. The spectra are analyzed by using standard multilayer film-fitting procedures to extract the optical conductivity and other optical constants. The results are discussed in the context of effective medium approximations. [Preview Abstract] |
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