Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session L20: Focus Session: Properties of Complex Oxides and Interfaces II |
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Sponsoring Units: DMP FIAP Chair: Marco Fornari, Central Michigan University Room: LACC 407 |
Tuesday, March 22, 2005 2:30PM - 2:42PM |
L20.00001: Direct observation of the formation of polar nanoregions in Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$ using neutron pair distribution function analysis Il-Kyoung Jeong, Tim Darling, J.K. Lee, Thomas Proffen, Robert Heffner, J.S. Park, K.S. Hong, Wojtek Dmowski, Takeshi Egami Using neutron pair distribution function (PDF) analysis over the temperature range from 1000~K to 15~K, we demonstrate the existence of local polarization and the formation of medium-range, rhombohedrally ordered polar nanoregions (PNRs) in a prototypical relaxor ferroelectric Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$. We estimate the volume fraction of the PNRs as a function of temperature and show that this fraction steadily increases from 0 \% to a maximum of $\sim$ 30\% as the temperature decreases from 650~K to 15~K. Below T$\sim$200~K the PNRs start to overlap as their volume fraction reaches the percolation threshold. We propose that percolating PNRs and their concomitant overlap play a significant role in the relaxor behavior of Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$. [Preview Abstract] |
Tuesday, March 22, 2005 2:42PM - 2:54PM |
L20.00002: Electrical and Dielectric Properties of ACu$_3$Ti$_4$O$_{12}$ Compounds Jianjun Liu, Chun-Gang Duan, Wai-Ning Mei, John Hardy, Robert Smith We studied frequency and temperature dependences of impedance and permittivity of several compounds in the system ACu$_3$Ti$_4$O$_{12}$ (A=Ca, Bi$_{2/3}$, Y$_{2/3}$, La$_{2/3}$) in the ranges of 10$^{-1}$$\sim$10$^{6}$ Hz and -150$\sim$200 $^\circ$C, respectively. We found all compounds investigated displayed similar electrical and dielectric properties, namely, they all have two electrical responses in the impedance formalism and a Debye-like peak in the permittivity formalism, besides, their dielectric constants are independent of frequency and temperature in a wide range. We explained the experimental results in terms of a two-layer model with conducting grains partitioned from each other by poorly conducting grain boundaries. We attributed the two electrical responses in the impedance formalism to the grain and grain boundary effects, respectively, while the Debye-like peak in the permittivity formalism to a Maxwell-Wagner relaxation. [Preview Abstract] |
Tuesday, March 22, 2005 2:54PM - 3:30PM |
L20.00003: The Dielectric Properties of Complex Oxides: CaCu$_{3}$Ti$_{4}$O$_{12}$ and Other Perovskites Invited Speaker: Detailed studies of the properties of ceramic CaCu$_{3}$Ti$_{4} $O$_{12}$ (CCTO) have clarified the physics of this interesting material. The unusual dielectric relaxational properties of CCTO are explained in terms of a capacitive-layer model, as for an inhomogeneous semiconductor consisting of semiconducting grains and insulating grain boundaries. The kinetics of the main (low T) relaxation reveal that two different thermally- activated processes in the CCTO grains control the dynamics. A higher T relaxation is determined by grain boundary conduction. Both Nb and Fe doping lower both the dielectric constant, $\varepsilon \prime $, and loss, but Fe doping leads to the more dramatic effects; 3 at.{\%} Fe removes the anomalous $\varepsilon \prime $ (T) response making CCTO an intrinsic dielectric. The intrinsic $\varepsilon \prime $ ( $\underline{\sim }$75) and its T dependence are shown to be largely determined by a low-lying soft TO phonon. At low T, cubic CCTO transforms into an antiferromagnetic phase at T$_{N }$= 25 K. T$_{N}$ decreases significantly with Fe doping. Analysis of the high T dependence of the magnetic susceptibility provides insight into the role of Fe. Finally, an $\varepsilon \prime $ (T) anomaly associated with the onset of antiferromagnetic order has been discovered providing evidence for coupling between the polarization and sublattice magnetization. Possible origin of this coupling is discussed. We have also observed large dielectric constants and relaxations similar to those in CCTO in doped single crystals of KTaO$_{3}$. [Preview Abstract] |
Tuesday, March 22, 2005 3:30PM - 3:42PM |
L20.00004: Optical Homogeneous Linewidths and Spectral Diffusion at 1.5 microns in Mixed Er$^{3+}$:Eu$^{3+}$:Y$_{2}$SiO$_{5}$ Studied by Photon Echo R.L. Cone, T. B\"{o}ttger, C.W. Thiel, Y. Sun Er$^{3+}$-doped materials are important for spectral hole burning applications at 1.5 micron communication wavelengths, including analog signal processing and laser frequency stabilization. Doping Er$^{3+}$:Y$_{2}$SiO$_{5}$ with Eu$^{3+}$ is shown to broaden the inhomogeneous linewidth of the $^{4}$I$_ {15/2}$ - $^{4}$I$_{13/2}$ transition without significantly broadening the homogeneous linewidth. This maximizes bandwidth in real-time analog signal processing applications without compromising resolution. Photon echo and stimulated photon echo decays between 1.5 K and 5.5 K were measured along with angle- dependent Zeeman spectra and site-selective absorption and emission. Detailed modeling of observed spectral diffusion induced by spin dynamics considered Er$^{3+}$-Er$^{3+}$ dipole interactions driven by direct-phonon processes. The model describes and explains observed behavior and predicts behavior vs. magnetic field, crystal temperature, Er$^{3+}$ dopant concentration, and crystal orientation. $^{*}$ Currently at University of San Francisco $^{**}$ Currently at University of South Dakota [Preview Abstract] |
Tuesday, March 22, 2005 3:42PM - 3:54PM |
L20.00005: In-situ Spectroscopy on Erbium doped Lithium Niobate during Domain Inversion Christian Sandmann, S. Tafon Penn, Volkmar Dierolf Lithium niobate (LiNbO$_{3}$) has found wide application due to its favorable acousto-optical, electro-optical, and nonlinear optical properties. For many applications it is crucial to create ferroelectric domain patterns. With the most common technique, using electrodes patterned by optical lithography, periodically poled components with period lengths down to 3${\mu}$m could be fabricated. Smaller structures, however, are difficult to achieve with present techniques. For the investigation of novel domain inversion techniques, which potentially offer the feasibility to create smaller structures, it is vital to probe in-situ if domain inversion is occurring with high spatial resolution. In an earlier work we have shown that the emission of the Er$^{3+}$ ion is different for the as grown and domain inverted part of the sample this. In this work we will show that it is possible to probe the Er$^{3+}$ defect in-situ during domain inversion offering the potential to study the spatial extension of a moving domain wall and to develop an active feedback system for electron beam or light-induced domain inversion. We found significant differences between a moving domain wall and a static domain wall in terms of their spatial extension and their Raman spectra. [Preview Abstract] |
Tuesday, March 22, 2005 3:54PM - 4:06PM |
L20.00006: Atomic Scale Catalysis: Structure and Reactions of Chromium Oxide Species on Transition Aluminas Sanwu Wang, A.Y. Borisevich, S.N. Rashkeev, M.V. Glazoff, K. Sohlberg, S.J. Pennycook, S.T. Pantelides Chromium-supported transition aluminas are widely used for de-hydrogenation of alkanes, yet an understanding of the structure and relevant chemical reactions on atomic scale is lacking. With first-principles calculations and Z-contrast scanning transmission electron microscopy observations, we show that dispersed CrO$_x$ species supported on transition aluminas are critical for catalytic activities. The atomic-scale species induce dissociation of alkanes whereas crystalline chromium oxide inhibits such dissociation. Contrary to prior models, chromium atoms do not participate in the catalytic reactions, but act as binding centers to which active oxygen atoms are bound. We also show that $\eta$-alumina is more efficient to support the CrO$_x$ species while nucleation of crystalline Cr$_2$O$_3$ is facilitated on $\gamma$-alumina. The difference is attributed to the different vacancy distributions in $\eta$- and $\gamma$-aluminas. [Preview Abstract] |
Tuesday, March 22, 2005 4:06PM - 4:18PM |
L20.00007: Growth and spectroscopic characterization of epitaxial dielectrics on SiC J. Choi, C. Tanner, R. Puthenkovilakam, J.P. Chang SiC has attracted much attention as a promising wide-bandgap semiconductor material due to its excellent physical and electrical properties. In SiC-based devises, inclusion of an AlN buffer layer improves the performance due to its good lattice match with SiC substrate and relatively low misfit with the III- nitride layers. In this work, we demonstrate the deposition of HfO$_2$, a well known high-k gate dielectric material for silicon based devices, by an atomic layer deposition process on SiC and AlN/SiC stacks. During the thin-film growth, the crystallinity of the epitaxial layers is monitored by in-situ reflection high-energy electron diffraction (RHEED) and the thin film composition is characterized by in-situ x-ray photoelectron spectroscopy (XPS). To examine the characteristic electrical properties, capacitance-voltage and current-voltage measurements are performed on the metal-insulator-semiconductor (MIS) capacitors. The correlation between growth conditions, stoichiometry and crystallinity of the eptaxial layer, and device performance will be discussed. Finally, we present first- principle calculations of the valence band structures of AlN and SiC as well as band alignment at the interface, in comparison with the experimentally determined band alignment by XPS. [Preview Abstract] |
Tuesday, March 22, 2005 4:18PM - 4:30PM |
L20.00008: Layered Tunnel Barrier Lowering in High-k Heterostructures using Bias-dependent Internal Photoemission Spectroscopy Douglas Bell, Julie Brewer, Harry Atwater Layered tunnel barriers have been proposed as replacements for SiO$_{2}$ for use as injection barriers in nanocrystal floating-gate memories. Due to the predicted larger change in conductance as a function of injection voltage, such barriers are expected to enable nanosecond programming and erase times with archival data retention times. We have utilized internal photoemission (IPE) spectroscopy to study barrier height lowering of Al$_{2}$O$_{3}$/HfO$_{2}$/SiO$_{2}$/Si layered tunnel barrier structures over a wide range of applied biases. 15 nm layers of Al$_{2}$O$_{3}$ and HfO$_{2}$ were grown on n-Si substrates by atomic layer deposition. The IPE results are analyzed using a simple electrostatic model to yield effective barrier heights and overall band alignments across the entire voltage range. Using this technique we demonstrate substantial barrier lowering ($\sim $0.75 eV) for Si-compatible dielectric heterostructures, and we discuss the application of these barriers to improved speed and reliability of floating gate nonvolatile memory devices. [Preview Abstract] |
Tuesday, March 22, 2005 4:30PM - 4:42PM |
L20.00009: Energies of 4f$^{N}$ and 4f$^{N-1}$5d States Relative to Host Bands in Rare-earth-doped Fluorides C.W. Thiel, A. Collombet, R.L. Cone, M.-F. Joubert, A. Tkachuk Energies of 4f$^{N}$ states relative to crystal band states were measured for rare-earth ions in the optical host materials LiYF$_{4}$, Na$_{0.4}$Y$_{0.6}$F$_{2.2}$, and LaF$_{3}$ using x-ray photoemission spectroscopy. Spectra were modeled to determine the valence band maximum and 4f$^{ }$electron binding energies in each material. These results were combined with 4f$^{N}$ to 4f$^{N-1}$5d transition energies to determine 5d binding energies for the lowest levels of excited 4f$^{N-1}$5d configurations. While 4f$^{N}$ ground-state energies vary within several eV of the valence band maximum for different rare-earth ions in each host, the lowest 4f$^{N-1}$5d states have similar energies and are several eV below the bottom of the conduction band. A simple model accurately described 4f$^{N}$ and 4f$^{N-1}$5d binding energies across the entire series of rare-earth ions. These results improve the understanding of optical materials for lasers, phosphors, and spectral hole burning applications for optical signal processing and data storage. [Preview Abstract] |
Tuesday, March 22, 2005 4:42PM - 4:54PM |
L20.00010: Theoretical study of orbital and lattice structure of $MnF_3$: the origin of orbital ordering D. Volja, Wei-Guo Yin, Wei Ku Orbital ordering in $MnF_3$ is studied with first-principles theory. Mathematical description of $e_g$ states within pseudospin formalism shows the importance of electron-electron interactions that oppose conventional electron-phonon interactions (e.g.: Jahn-Teller effects). Results obtained with LDA+U give stable ground state with experimentally observed orbital ordering, that cannot be explained solely in terms of electron-phonon interactions. Instead, the resulting orbital ordering is a consequence of competition between electron-phonon and electron-electron interactions. Our quantitative conclusion can be directly verified in future experiments. [Preview Abstract] |
Tuesday, March 22, 2005 4:54PM - 5:06PM |
L20.00011: Hydrogen-Bond Nature in Isolated Hydrogen-Bonded Material h-MeHPLN and h-BrHPLN Studied by Neutron and X-ray Diffraction Ryoji Kiyanagi, Akiko Kojima, Hiroyuki Kimura, Masashi Watanabe, Yukio N, Tomoyuki Mochida, Tadashi Sugawara MeHPLN and BrHPLN are hydrogen-bonded dielectric materials. Though their crystal structures are essentially the same, their phase transition schemes are significantly different. MeHPLN undergoes a phase transition at 41 K, while h-BrHPLN does not show any phase transitions. When the hydrogen atom in the hydrogen-bond of BrHPLN is replaced with a deuterium atom (d-BrHPLN), successive phase transitions occur. In this study neutron and X-ray diffraction experiments were performed on h-MeHPLN and h-BrHPLN to elucidate the difference about the phase transition from the structural aspect. From the experimentally obtained electron and nuclear distributions, a local disordered electronic dipole moment was found in the hydrogen bond region at room temperature. The phase transition of h-MeHPLN was found to occur with the ordering of the moment. Meanwhile, the moment in h-BrHPLN remains disordered until very low temperature, suggesting a tunneling motion of the hydrogen atom. [Preview Abstract] |
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