Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session H20: Focus Session: Properties of Complex Oxides and Interfaces I |
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Sponsoring Units: DMP Chair: Marco Buongiorno Nardelli, NCSU Room: LACC 407 |
Tuesday, March 22, 2005 8:00AM - 8:12AM |
H20.00001: Dielectric and vibrational properties of crystalline and amorphous high-k lanthanum aluminate Pietro Delugas, Vincenzo Fiorentini, Alessio Filippetti We present ab-initio density-functional and self-interaction-corrected calculations on the structure, long-wavelength IR and Raman vibrational modes, and static dielectric properties of the high-k oxide LaAlO$_{3}$, including the recently-observed dielectric anisotropy. The orientation-averaged dielectric constant k=23.3 in the crystal is in good agreement with experiment, and it is dominated by lattice response. We then evaluate by finite-field linear-response calculations the dielectric and polarization properties of an amorphous sample generated by ab initio molecular dynamics. The dielectric response is largely analogous to that of the crystal, due to the conservation of the main IR active modes and La anomalous charges due to a quite sharp Al-O and La-O short-range order. We find it important to calculate the dynamical charges by a self-interaction-corrected scheme, bringing about a $\sim $15{\%} improvement over DFT. [Preview Abstract] |
Tuesday, March 22, 2005 8:12AM - 8:48AM |
H20.00002: Dielectric Anomalies in Cuprates and Nickelates Invited Speaker: Low frequency measurements of the dielectric response of hole-doped insulators La$_{2}$Cu$_{1-x}$Li$_{x}$O$_{4}$ and La$_{2-x}$Sr$_{x}$NiO$_{4}$ find a two order of magnitude decrease in the real part of the dielectric constant $\varepsilon $' as these crystals are cooled below a material- and frequency-dependent temperature T$_{f}$. The real and imaginary parts of $\varepsilon $ exhibit universal scaling in Cole-Cole plots, which are used commonly to analyze classical structural glasses. The relaxation rate $\tau $, obtained from the frequency dependence of T$_{f}$, is best described by $\tau $(T) = a(T-T$_{0})^{-n}$, where T$_{0}$ is the zero-frequency limit of T$_{f}$. This power-law form, consistent with critical scaling, is followed over four decades in frequency. Together, these observations are consistent with glassy charge dynamics arising from collective behavior of doped holes in both the cuprate and nickelate. Intrinsic non-uniformities (stripes in the nickelate and stripes/potential non-stripe patterns in the cuprate) are common to both systems and drive the glassy dielectric behaviors observed. The temperature scale on which charge glassiness develops correlates with the onset of glassy spin dynamics found in NMR, $\mu $SR and susceptibility measurements, which suggests their interrelationship. [Preview Abstract] |
Tuesday, March 22, 2005 8:48AM - 9:00AM |
H20.00003: Infrared properties of the bismuth pyrochlores Minghan Chen, David Tanner, Juan Nino We measured the temperature dependence of the reflectivity spectra of cubic bismuth pyrochlores Bi$_{3/2}$ZnTa$_{3/2}$O$_{7 }$(BZT), Bi$_{3/2}$MgNb$_{3/2}$O$_{7}$ (BMN), Bi$_{3/2}$MgTa$_{3/2}$O$_{7} $ (BMT) and Bi$_{3/2}$(Zn$_{0.92}$Nb$_{1.5})$O$_{6.92}$ (BZN) by infrared spectroscopy from 30 to 3300 cm$^{-1}$ between 50 and 300K. The optical constants were estimated by Kramers-Kronig analysis and classical dispersion theory. Infrared-active phonon modes were assigned to specific bending and stretching vibrational modes. Of particular interest is a previously unassigned IR mode at $\sim $850 cm$^{-1 }$which may suggest a complex structure. Splitting of the B-O and O-B-O stretching phonons is attributed to mixed cation occupancy. The temperature dependences of the phonon frequencies and the damping coefficients indicate a decrease of lattice constant and orientation disorder at low temperatures. [Preview Abstract] |
Tuesday, March 22, 2005 9:00AM - 9:12AM |
H20.00004: Structural Phase Transition in Zircon and Hafnon studied by Perturbed Angular Correlation Spectroscopy Herbert Jaeger, Sean McBride We have used perturbed angular correlation spectroscopy (PAC) to determine the electric field gradient (EFG) at the \textit{Zr}-site in zircon (\textit{ZrSiO}$_{4})$ between room temperature and 1400~K. The EFG is axially symmetric and the quadrupole interaction frequency $v_{Q}$ decreases linearly with increasing temperature. For very pure zircon samples the slope of the $v_{Q}$\textit{ vs. T} increases above $1000 K$; this is consistent with a displacive structural transition reported in the literature.$^{1} \quad ^{ }$However, zircon samples with a moderate impurity concentration do not show this behavior. In order to learn more about this structure we have begun performing PAC experiments on isostructural hafnon (\textit{HfSiO}$_{4})$, which was synthesized in our laboratory. PAC spectra of hafnon are very similar to those of zircon but show a small second-site interaction, which we believe to be due to residual \textit{HfO}$_{2}$ from the sample preparation. The temperature dependence of the quadrupole interaction frequency will be discussed in context of a displaceive phase transition in these materials. $^{1 }$Z. Mursic, T. Vogt, and F. Frey, Acta Cryst. \textbf{B48} (1992) 584. [Preview Abstract] |
Tuesday, March 22, 2005 9:12AM - 9:24AM |
H20.00005: Dependence of the band gap in the high-k dielectric HfO$_2$ on crystalline phase K. Nishitani, P. Rinke, Ph. Eggert, S.J. Hashemifar, P. Kratzer, M. Scheffler Recently HfO$_2$ has attracted considerable attention as a gate dielectric for (CMOS) CMOS technology, because it combines a wide band gap and good thermal stability with a low-frequency dielectric constant much higher than that of SiO$_2$. While the exact values of the band gap and band alignment with Si(001) are important for the application as gate dielectric, experimental data show some scatter possibly due to ill-defined crystallinity of the samples. We analyze the structural and electronic properties of both cubic and tetragonal HfO$_2$ employing density-functional theory within the local-density approximation (LDA) in a pseudopotentials, plane-wave approach. We find that cubic HfO$_2$ has a direct band gap, while the tetragonal phase exhibits a slightly larger indirect gap. Applying many-body perturbation theory in the $GW$ approximation corrects the underestimation of the LDA band gap and gives band gaps within the experimentally reported range. We find that the energy shifts introduced by the $GW$ self-energy are largely independent of the structure and show that the differences in band structure between the two phases arise mainly from internal relaxations of the oxygen atoms in the tetragonal phase, while the c/a value has little effect on the character of the band gap. [Preview Abstract] |
Tuesday, March 22, 2005 9:24AM - 9:36AM |
H20.00006: Density Functional Theory Simulations of Atomic Layer Deposition of HfO2 Joseph Han, Charles Musgrave We have used DFT to predict the growth mechanisms and kinetics of hafnium oxide ALD using metal chloride and metal alkylamide precursors. We have determined parameters for Arrhenius rate equations and compare with experimental results to propose explanations for the different observed film growth characteristics including submonolayer deposition, non-uniform growth, and impurity incorporation. We find that in the case of Hf tetrachloride, etching reactions are significant and lead to film defects. Furthermore, HfCl4 ALD reactions have relatively high barriers which necessitate the use of high ALD temperatures. However, in contrast to the metal chloride system, we predict kinetics that suggest that deposition process using alkylamine precursors should occur at lower temperatures with better growth characteristics, such as a more uniform film with less impurity incorporation. [Preview Abstract] |
Tuesday, March 22, 2005 9:36AM - 9:48AM |
H20.00007: ZrO2 high-K dielectric thin films for high-density field-effect doping studies Xin Zhou, Gertjan Koster, Malcolm Beasley ZrO2 is a well-known high-K dielectric material with a wide band gap. We have studied the PLD deposition and properties of thin films of ZrO2 for the purpose of high-electron-density field-effect doping applications. The observed dielectric constants 24 are larger than expected for the equilibrium monoclinic crystal structure of ZrO2, suggesting higher symmetry phases are present in our films. Breakdown fields as high as 14 MV/cm are sometimes observed. These films are being used for field-effect doping studies of superconducting amorphous In-oxide thin films. In this talk, both the properties of our ZrO2 films and their use in doping studies of our In-oxide thin films will be reported. Work supported by the U.S. DoE BES. [Preview Abstract] |
Tuesday, March 22, 2005 9:48AM - 10:00AM |
H20.00008: The Dynamical Structure Factor of NiO and CoO* B.C. Larson, J.Z Tischler, P. Zschack, K.D. Finkelstein, Wei Ku, O. Restrepo, A.G. Equiluz Non-resonant inelastic x-ray scattering (IXS) and \textit{ab initio }dynamical electronic response calculations have been used to investigate highly correlated transition metal monoxides NiO and CoO. Absolute IXS measurements were made as a function of the magnitude and orientation of momentum transfers, q, at the APS and CHESS using energy resolution ranging from 0.3 -- 1.1 eV. In addition to $\sim $4 eV energy gaps observed for all q, sharp excitonic peaks were observed below the gap of both NiO and CoO for momentum transfers higher than $\sim $2 A$^{-1}$. Comparisons of S(q,w) measurements with dynamical response calculations performed within LDA+U (including crystal field effects) show that the gap energy and the electronic response above the gap are described by U $\sim $8 eV within RPA for low q-values. However, the excitonic peaks are not described by LDA+U calculations, nor are the calculated S(q,w) spectra in agreement with the measured response for large q. The results will be compared with resonant x-ray emission and resonant electron energy loss spectra in the literature. *Work at the APS supported by the DOE Office of Science, DMS under contract with ORNL, managed by UT-Battelle, LLC; UNI-CAT is supported by UIUC, ORNL, NIST and UOP Res., Inc. The APS is supported by the DOE and CHESS is supported by the NSF. [Preview Abstract] |
Tuesday, March 22, 2005 10:00AM - 10:12AM |
H20.00009: Influence of Shallow Core Level Hybridization on the Electronic Structure of Metal Oxides R.G. Egdell, P-A Glans, T. Learmonth, K.E. Smith, J. Guo, C. McGuinness, A. Walsh, G. Watson The influence of shallow core level hybridization on the electronic structure of HgO, ZnO, CdO, and In$_{2}$O$_{3}$ has been investigated using high-resolution soft x-ray emission and absorption spectroscopies.$^{a,b}$ Synchrotron radiation excited O K$_{\alpha }$ emission spectra provide a direct measure of the O $2p$ partial density of states and shallow core level hybridization, and reveal significant mixing of O $2p$ and shallow-core metal $d$ states. The extent of shallow core hybridisation with the O $2p$ levels is much more pronounced in HgO than in ZnO or CdO and shows an inverse correlation with the binding energy of the core state. Bandstructure calculations confirm the importance of mixing between Hg $5d$ and O $2p$ states in HgO. However the assumption that direct intra-atomic mixing between Hg $6s$ and Hg $5d$ orbitals determines the linear stereochemistry in HgO is shown to be incorrect.$^{b}$ \newline a) Phys. Rev. B \textbf{68}, 165104 (2003); b) Chem. Phys. Lett. \textbf{399}, 98 (2004) [Preview Abstract] |
Tuesday, March 22, 2005 10:12AM - 10:24AM |
H20.00010: First Principles Investigation of 47Ti and 17O Nuclear Quadrupole Interactions in the Rutile and Anatase Phases of TiO2 S. Byahut, Sudha Srinivas, Lee Chow, Junho Jeong, R. H. Scheicher (*) , T. P. Das (**) Using the First Principles Hartree-Fock Cluster procedure, electronic structures of rutile and anatase phases of TiO$_{2}$ have been investigated and utilized to calculate the nuclear quadrupole interaction (NQI) parameters for $^{47}$Ti and $^{17}$O in both systems. For the rutile phase, the NQI coupling constants (e$^{2}$qQ) and asymmetry parameters $\eta $ are found to be in reasonable agreement with experiment for both nuclei, especially the sign of e$^{2}$qQ for $^{17}$O. Experimental results are awaited for the NQI parameters in anatase to compare with our predictions, $\eta $ for $^{47}$Ti vanishing due to local axial symmetry. (*) Current Address: Dept. of Physics, Uppsala University, Sweden (**) Also: Dept. of Physics, University of Central Florida, Orlando, Florida. [Preview Abstract] |
Tuesday, March 22, 2005 10:24AM - 10:36AM |
H20.00011: Dielectric properties of Ca- and Ba-doped SrTiO3 Li-Chun Tung, Tuson Park, John Sarrao, Alex Lacerda, Ward Beyermann Dielectric properties of high quality polycrystalline Ca- and Ba- doped SrTiO$_{3}$ perovskite were studied by means of dielectric constant, dielectric loss and ferroelectric hysteresis measurements. Low frequency dispersion of the dielectric constant was found to be very small and a simple relaxor model may not be able to explain its dielectric behavior. A few relaxation modes were found in these samples and they were all interpreted as thermally activated dipolar re-orientation across energy barriers. In Sr$_{1-x}$Ca$_{x}$TiO$_{3}$ (x=0-0.3), two modes were found and its concentration dependence implies a competition between these two relaxation processes. In Sr$_{1-x}$Ba$_{x}$TiO$_{3}$ (x=0-0.25), relaxation modes were found to be related to the structural transitions and the relaxation modes existed even at low doping levels (x$<$0.1) at which the structural ordering were not identified by previous Neutron scattering studies. Application of various models was discussed and both of the Ca- and Ba-doped samples can be understood qualitatively. [Preview Abstract] |
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H20.00012: Intrinsic Effect of a Nitrogen Atom on Hf-based High-k Gate Dielectrics -A First Principles Study Naoto Umezawa, Kenji Shiraishi, Takahisa Ohno, Heiji Watanabe, Toyohiro Chikyow, Kazuyoshi Torii, Kikuo Yamabe, Keisaku Yamada, Hiroshi Kitajima, Tsunetoshi Arikado We theoretically investigate the nitrogen incorporation effect in hafnia (HfO$_{2})$ from the first- principles calculations within GGA framework, especially focusing on the interaction between N atoms and O vacancies (Vo)s. Vo is known to be problematic for Hf-based high-k gate dielectrics because it causes unfavorable charge trap. Moreover, some experimental results suggest that Vo related gap states$_{ }$assist the electron leakage current, which deteriorates the electrical properties of Hf-based MOSFETs. Our results clearly show that N atoms selectively occupy nearby O sites of Vo and that Vo levels are completely eliminated by the N incorporation. These results clearly show that N atoms have intrinsic effects to decrease Vo related gap states, leading to the reduction in both the number of charge trap sites and the leakage current through HfO$_{2}$. [Preview Abstract] |
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H20.00013: The Magnetocaloric Effect in New Rare Earth Compounds A.L. Lima, M.F. Hundley, V. Fritsch, S. Bobev, J.D. Thompson, J.L. Sarrao The discovery of the giant magnetocaloric effect (MCE) [1] gave a new impulse towards the development of magnetic cooling and also heating technology [2], thereby renewing the interest in these compounds. Many intermetallic compounds have been revisited in the last few years in order to evaluate their magnetocaloric properties which are observed as a change in the temperature of the material under an applied magnetic field. Mixed compounds such as RE$_{1-x}^a RE_x^b Al_2$ and \textit{RE}$_{2}$\textit{Al}$_{3}$\textit{Si}$_{2}$ (where RE = rare earth and $a\ne b)$ were experimentally and theoretically studied in this work to extract the maximum magnetocaloric potential. We measure the magnetic, magnetocaloric and transport properties and compared them to our theoretical predictions. To carry out this calculation, we have used an extended Hamiltonian [3] that takes into account crystalline eletric field (when present), exchange interactions and the Zeeman effect in a composed space for the rare earth with different angular moments. The Hamiltonian is solved exact and self-consistently. Our experimental and theoretical results indicate that the specific-heat anomalies exhibited by the Er-rich compounds show evidence for magnetoelastic interactions. [1] V.K Pecharsky and K.A. Gschneidner Jr., Phys. Rev. Lett., 78, 4494 (1997). [2] K.A. Gschneidner Jr. and V.K Pecharsky, Ann.Rev. Mater Sci. 30, 387 (2000) and references therein. [3] A.L. Lima, I.S. Oliveira, A.M. Gomes and P.J. von Ranke Phys. Rev. B, 65, 172411 (2002) [Preview Abstract] |
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