Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session H39: Ferroelectric Phase Transitions |
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Sponsoring Units: DCMP Chair: Clifford M. Krowne, NRL Room: Conference Center 514 |
Tuesday, March 22, 2005 8:00AM - 8:12AM |
H39.00001: Microscopic Origin of Polarity in Quasi-Amorphous BaTiO3. A.I. Frenkel, Y. Feldman, V. Lyahovitskaya, E. Wachtel, I. Lubomirsky The recent observation of pyroelectricity in quasi-amorphous thin films of BaTiO$_{3}$ introduced a previously unreported type of polar ionic solid where the appearance of a macroscopic dipole moment is not accompanied by long-range crystal-like order. This poses a question regarding the mechanism of polarity in non-crystalline ionic systems and the nature of their local dipoles. By combining X-ray diffraction and X-ray absorption fine structure spectroscopy techniques we have identified the local dipoles as stable but distorted TiO$_{6}$ octahedra. The magnitude of the off-center displacement of the Ti ion and the concomitant dipole moment in both quasi-amorphous (polar) and amorphous (non-polar) BaTiO$_{3}$ were found to be nearly twice as large as those in bulk BaTiO$_{3}$. We propose that the mechanism of macroscopic polarity in quasi-amorphous BaTiO$_{3}$ is in a weak orientational ordering of the TiO$_{6}$ bonding units. In this view, one may expect that other amorphous ionic oxides containing stable local bonding units, for example NbO$_{6}$, TiO$_{6}$ or VO$_{6}$, may also form non-crystalline polar phases. [Preview Abstract] |
Tuesday, March 22, 2005 8:12AM - 8:24AM |
H39.00002: Influence of Phase Coexistence on Crystal Structure of 6\%-La Series of Plzt Solid Solutions V.M. Ishchuk, V.N. Baumer, Z.A. Samoyilenkolenko, V.L. Sobolev La modified lead titanate zirconate (PLZT) solid solutions with 6 percent of La are characterized by a wide interval of Ti concentrations (from y = 0.16 to about y = 0.3) in which coexistence of ferroelectric (FE) and antiferroelectric (AFE) states is possible. Presence of coexisting domains of FE and AFE phases influences the crystal structure of these compounds noticeably. Detailed studies of position and shape of the (200) and (222) X-ray diffraction lines, which are the most characteristic lines for cubic perovskites, were carried out for solid solutions within and outside the above mentioned interval of Ti concentrations. It is demonstrated that the crystal structure of solid solutions in this interval of Ti content is non-cubic at the temperatures below the Curie temperature. The complex structure of the (200) and (222) diffraction lines is caused by the simultaneous presence of domains of coexisting FE and AFE phases. The shape of the X-ray lines undergoes modifications with the change of Ti content in solid solution and returns to the standard shape when the compositions of the solid solution correspond to both pure FE (rhombohedral) and pure AFE (tetragonal) phases. [Preview Abstract] |
Tuesday, March 22, 2005 8:24AM - 8:36AM |
H39.00003: Redistribution of electronic charges in the spin-Peierls state in (TMTTF)$_2$AsF$_6$ observed by $^{13}$C NMR Shigeki Fujiyama, Toshikazu Nakamura We report he results of $^{13}$C NMR for a quasi-one-dimensional quarter-filled organic material (TMTTF)$_{2}$AsF$_{6}$, which undergoes charge ordering ($T_{\textrm{CO}}$ = 102 K) and spin-Peierls phase transitions ($T_{\textrm{SP}}$ = 12 K). The ratio of two $1/T_1$ for the charge accepting and donating sites which grows from $T_\textrm{CO}$ finally saturates in approaching $T_{\textrm{SP}}$, that indicates an opening of single gap for the spin excitation spectra. At $T_{\textrm{SP}}$, however, doubly split NMR lines from inequivalently charged molecules merge originated from the variation in \textit{charge} densities. This shows a rearrangement of the charge configuration in the spin-Peierls state. [Preview Abstract] |
Tuesday, March 22, 2005 8:36AM - 8:48AM |
H39.00004: Auxilliary Field Quantum Monte Carlo Calculations of Ferroelectric Instability Hendra Kwee, Henry Krakauer, Shiwei Zhang The perovskite structure alloys (BiScO$_{3}$)$_{1-x}$ -(PbTiO$_{3}$)$_{x}$ have strong ferroelectric instabilities. The ferroelectric double-well depths of BiScO$_{3}$, calculated by density functional theory (DFT), are an order of magnitude larger than for PbTiO$_{3}$ and PbZrO$_{3}$\footnote{J. \'{I}\~{n}iquez, D. Vanderbilt, L. Bellaiche, {\it Phys. Rev B}{\bf 67}, 224107 (2003)}. We carry out quantum Monte Carlo (QMC) calculations on BiScO$_{3}$ to help assess the accuracy of DFT. We use a recently developed auxilliary field QMC method \footnote{S. Zhang, and H. Krakauer, {\it Phys. Rev. Lett} {\bf 90}, 136401 (2003)}. The two-body electronic Coulomb interactions are decoupled using a Hubbard-Stratonovich transformation. The method iteratively projects out the ground state from an initial trial wave function by random walks in the space of Slater determinants. The trial wave function is a single Slater determinant constructed from a DFT calculation using ABINIT \footnote{http://www.abinit.org}. A plane wave basis and periodic boundary conditions were used in the calculations. The ions are represented by norm-conserving Kleinman-Bylander non-local pseudopotentials. Applications of the method on simpler systems gave very encouraging results. [Preview Abstract] |
Tuesday, March 22, 2005 8:48AM - 9:00AM |
H39.00005: Electric-field poling effect on thermal stability of monoclinic phase in a (110)-oriented Pb(Mg$_{1/3}$Nb$_{2/3})_{0.74}$Ti$_{0.26}$O$_{3}$ crystal V. Hugo Schmidt, R. R. Chien, C.-L. Tsai, Chi-Shun Tu, F.-T. Wang Temperature-dependent phase transformations in unpoled and poled samples cut from a (110)-cut Pb(Mg$_{1/3}$Nb$_{2/3})_{0.74}$Ti$_{0.26}$O$_{3}$ (PMNT26{\%}) single crystal have been investigated by polarizing microscopy and dielectric permittivity. X-ray diffraction (XRD) at room temperature (RT) was also measured. Poling was done at RT at various fields below and above the coercive field ($\sim $3 kV/cm). XRD and polarizing microscopy at RT indicate that the unpoled sample has a dominant rhombohedral ($R)$ phase coexisting with some monoclinic ($M)$ phase, i.e. $R(M)$, whereas the poled sample has coexistence of $R$ and$ M$ phases, i.e. $R/M$, in which the fraction of the induced $M$ phase increases with poling strength. In both unpoled and poled samples, continuous polarization rotation with increasing temperature via a monoclinic phase was revealed by polarizing microscopy from $\sim $360 K to 375 K, the same range in which a dielectric anomaly occurs in the poled sample. The crystal phase becomes cubic ($C)$ at $\sim $395 K. Thus, the phase transition sequence is $R(M)\to M\to C$ in the unpoled sample and $R$/$M\to M\to C$ in the poled sample. [Preview Abstract] |
Tuesday, March 22, 2005 9:00AM - 9:12AM |
H39.00006: Aging and Slow Dynamics in the Relaxor Ferroelectrics SBN and SBN:La Lambert K. Chao, Eugene V. Colla, M. B. Weissman, Dwight D. Viehland The uniaxial relaxor ferroelectrics Sr$_{x}$Ba$_{1-x}$Nb$_2$O$_6$ (SBN) and SBN doped with 1\% La (SBN:La) is found to show a crossover from spinglass-like aging (exhibiting hole-like memory effects) to cumulative aging upon cooling into the relaxor regime. This is in contrast to the hole-like aging seen in the cubic perovskite relaxors.\footnote{E.V. Colla \emph{et al.}, Phys. Rev. B {\bf 88} 134107 (2001).} It lends support to our proposed picture of the glassy behavior in the cubic relaxors as arising from the freezing of canted moments (not present in uniaxial SBN) around the local polarization, similar to the x-y spins in a reentrant spinglass. We will discuss aging behavior of SBN and in particular the behavior of the dielectric polarization under DC-bias in this ``cumulative-aging regime''. Barkhausen noise experiments underway, in comparison with previous experiments on the cubic relaxors, may also shed more light on the dynamic cooperativity in this regime. [Preview Abstract] |
Tuesday, March 22, 2005 9:12AM - 9:24AM |
H39.00007: [111] Electric Field Enhancement of Polar Nano-Regions in a Ferroelectric Relaxor Guangyong Xu, Peter Gehring, Gen Shirane The response of polar nanoregions (PNR) to a [111]-oriented electric field has been studied by measuring the effect on the diffuse scattering observed in the relaxor compound PZN-8PT. In contrast to the behavior exhibited by normal ferroelectric domains, the diffuse scattering associated with the PNR in this relaxor system is partially enhanced by cooling in the presence of a field. The effect of the field is retained by the PNR after the field is removed. Remarkably, the "memory" of the field reappears even after heating the system above Tc and cooling in zero field. [Preview Abstract] |
Tuesday, March 22, 2005 9:24AM - 9:36AM |
H39.00008: Nonlinear optical evidence for broken inversion symmetry in Cd$_2$Re$_2$O$_7$ Jesse Petersen, J. Steven Dodge, Michael Steger, Jian He, David Mandrus Cd$_2$Re$_2$O$_7$ is a superconducting metallic pyrochlore that exhibits a second-order phase transition at 200 K, from a cubic to a tetragonal structure. According to Landau theory, such a phase transition must be accompanied by a loss of inversion symmetry; to date, however, the precise symmetry of the tetragonal phase remains undetermined. We observe second-order optical nonlinearity in this phase, confirming the loss of inversion symmetry. We will discuss efforts to characterize the low-temperature point group via second-harmonic polarimetry. [Preview Abstract] |
Tuesday, March 22, 2005 9:36AM - 9:48AM |
H39.00009: Study of Ferroelectricity in Monodisperse BaTiO3 Nanoparticles by Optical Second-Harmonic Generation Daohua Song, Limin Huang, Zhuoying Chen, Michael Loy, Stephan O`Brien, Tony Heinz The influence of finite-size effects on the ferroelectric response of materials has been a subject of intense interest for both fundamental reasons and because of its implications for applications. In particular, the role of film thickness and grain size on the ferroelectric properties of the well-known BaTiO$_{3}$ system has attracted considerable attention. In this work we present the results of studies on controlled monodispersive BaTiO$_{3}$ nanoparticles. The samples, prepared using a solution-phase method with a single bimetallic alkoxide molecular precursor,$^{1 }$were studied in thin film form.$^{ }$An organic ligand attached to the surface of the nanoparticles ensured that the nanoparticles remained well separated from one another and that they did not sinter. The ferroelectric response was probed using optical second-harmonic generation for monodisperse nanoparticles with a range of diameters between 4 - 12 nm. The second-harmonic generation process was chosen as a non-contact method of examining the ferroelectric-paraelectric phase transition, since this nonlinear optical process is allowed only for the non-centrosymmetric ferroelectric phase. Results on the dependence of ferroelectric response on the nanoparticle size will be reported. $^{1}$ S. O'Brien, L. Brus, and C. B. Murray, J. Am. Chem. Soc. \textbf{123}, 12085 (2001). [Preview Abstract] |
Tuesday, March 22, 2005 9:48AM - 10:00AM |
H39.00010: Proton Tunneling Relaxation in the Dielectric glass RADP Yejun Feng, Carlos E. Ancona-Torres, Thomas F. Rosenbaum, Eric Courtens Tunneling plays an important role in glassy relaxation at low temperature, but quantitative modeling of data is rare. We relate the dielectric susceptibility over 7 decades of frequency to the microscopic hydrogen free energy landscape obtained in neutron Compton experiments for the proton glass Rb$_{0.28}$(NH4)$_{0.72}$H$_{2}$PO$_{4}$ (RADP:72). RADP is a dipolar structural glass with spatial frustration between local ferroelectric RDP and antiferroelectric ADP ordering. We find a well-defined attempt frequency and a logarithmic relaxation over decades of frequency, with a classical to quantum crossover analogous to the vortex tunneling dynamics in high T$_{c}$ superconductors. [Preview Abstract] |
Tuesday, March 22, 2005 10:00AM - 10:12AM |
H39.00011: Response of Polar Nanoregions in the Relaxor Ferroelectric PZN-8\%PT to an Electric Field Peter Gehring, Guangyong Xu, Gen Shirane We report measurements of the neutron diffuse scattering from a single crystal of the relaxor ferroelectric PbZn$_{1/3}$Nb$_{2/3}$O$_3$ doped with 8\% PbTiO$_3$ (PZN-8PT) for temperatures 100K $\le T \le$ 550K and electric fields 0kV/cm $\le E \le$ 10kV/cm oriented along [001]. The diffuse scattering near the (300) and (003) Bragg peaks, which is strong for this system, was measured in both zero field cooled and field cooled conditions [1]. In all cases we find the surprising result that the diffuse scattering is not eliminated by electric fields up to 10 kV/cm, but is instead approximately preserved. However the reciprocal space geometry, or ``shape," of the diffuse scattering is modified. Thus field-cooling PZN-8\%PT into the tetragonal (ferroelectric) phase does not produce a uniformly polarized state. Measurements were also made to confirm the interesting memory effect in PZN-8PT reported by Xu {\it et al.} under a [111]-oriented field [2]. [1] P.M. Gehring, K. Ohwada, and G. Shirane, Phys. Rev. B {\bf 70}, 014110 (2004). [2] Guangyong Xu, P.M. Gehring, and G. Shirane, submitted to Science. [Preview Abstract] |
Tuesday, March 22, 2005 10:12AM - 10:24AM |
H39.00012: Electronic Asymmetry by Compositionally Braking Inversion Symmetry Maitri Warusawithana, H. Chen, J. Zuo, M. B. Weissman, J. N. Eckstein By stacking molecular layers of 3 different perovskite titanate phases, BaTiO3, SrTiO3 and CaTiO3 with atomic layer control, we construct nanostructures where global inversion symmetry is broken. With the structures clamped to the substrate, the stacking order gives rise to asymmetric strain fields. The dielectric response show asymmetric field tuning consistent with the symmetry of the stacking order. By analyzing the temperature and frequency dependence of the complex dielectric constant, we show that the response comes from activated switching of dipoles between two asymmetric states separated by an energy barrier. We find the size of average dipole units from the temperature dependence of the linewidth of field tuning curves to be around 10 unit cells in all the different nanostructures we investigate. At low temperatures we observe a deviation from the kinetic response suggesting a further growth in correlations. Pyrocurrent measurements confirm this observation indicating a phase transition to a ferro-like state. We explain the high temperature dipoles as single unit cell cross sectional columns correlated via the strain fields in the stacking direction, with the height somewhat short of the film thickness possibly due to some form of weak disorder. [Preview Abstract] |
Tuesday, March 22, 2005 10:24AM - 10:36AM |
H39.00013: Spontaneous Emergence of Free Carrier at Phase Transitions of Insulating BaTiO$_{3}$ Single Crystals Yukio Watanabe, Hirotaka Yoshioka, Yosuke Urakami A novel phenomenon that is probably universal in a wide range of ferroelectric oxides is found. We have measured \textit{DC} current-voltage characteristics (\textit{IVC}) of undoped insulating and doped BaTiO$_{3}$ single crystals grown by different methods. We find that a small but finite \textit{dc} conductance that is not due to impurities or thermal excitations appears at all three ferroelectric phase transitions. The conductance of the crystals is typically 1 n$\Omega ^{-1}$cm$^{-1 }$at the peak of the paraelectric-ferroelectric transition. The conductance is mostly evident in undoped insulating crystals that posses the spontaneous polarization perpendicular to the surface. However, we confirmed that the pyroelectric current or the dielectric relaxation, or the partial polarization reversal is excluded as origin of the observed conductance. [Preview Abstract] |
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