Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session L33: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides: Lattice Dynamics, Polarons, and Structure |
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Sponsoring Units: DMP DCOMP Chair: Matt Dawber, Stony Brook University Room: C143/149 |
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L33.00001: Ultrafast Polaron Dynamics in Multiferroic LuFe$_{2}$O$_{4}$* R.P. Prasankumar, J. Lee, D. Talbayev, A.J. Taylor, C.L. Zhang, S.-W. Cheong, X.S. Xu The multiferroic material LuFe$_{2}$O$_{4}$ has attracted much recent attention due to its strong magnetoelectric coupling. We used ultrafast optical spectroscopy to examine LuFe$_{2}$O$_{4}$ by employing a 1.55 eV probe pulse to examine the Fe$^{2+}->$Fe$^{3+}$ polaronic excitation as a function of temperature and pump photon energy. After 1.55 eV excitation, the photoinduced reflectivity change $\Delta $R/R decreases within $\sim $1 picosecond (ps), after which a $\sim $30 ps acoustic phonon oscillation is observed. The initial fast drop in $\Delta $R/R can be explained by Fe$^{2+}->$Fe$^{3+}$ polaron hopping, and the subsequent rapid recovery is due to polaron redressing; this is observed at all temperatures. Pumping the Fe$^{2+}$ on-site excitation at 3.1 eV revealed different dynamics. Notably, coupling between the on-site and charge transfer excitations was strongly suppressed above the antiferromagnetic ordering temperature, demonstrating the strong influence of charge and spin order on polaron dynamics. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L33.00002: Amplitude and phase gratings based on spatially modulated densities of optically generated polarons in thermally reduced LiNbO$_3$ Hauke Bruening, Mirco Imlau In thermally reduced, nominally pure LiNbO$_3$ a variety of small polarons can be observed, being responsible for the distinct photochromic properties of this material. In this contribution we use a spatially modulated excitation of polarons for the recording of holographic gratings. These gratings inherit some of the pronounced features of the polarons like a stretched-exponential relaxation behavior with a lifetime in the ms-range. Beside amplitude gratings we also find phase gratings leading to a high diffraction efficiency in some recording and readout geometries. The origin of these phase gratings can't be explained by the classic photorefractive effect due to Fe$_{\mathrm{Li}}$ or other photorefractive dopants. In contrast, our findings are discussed in the frame of a model taking into account a local change of the refractive index by the polaronic distortion of the crystal lattice. Measurements of activation energies also indicate that these gratings can be attributed to the small bound ($\mathrm{Nb}^{4+}_{\mathrm{Li}}$)-polaron. [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L33.00003: Hamiltonian approach to Feynman's path-integral polaron treatment J.T. Devreese, S.N. Klimin The Feynman path-integral based all-coupling variational approach for the Fr\"{o}hlich-polaron is re-formulated and extended using the Hamiltonian formalism with time-ordered operator calculus. Special attention is devoted to the excited polaron states. The energy levels and the inverse lifetimes of the excited polaron states are, for the first time, explicitly calculated within this all-coupling approach. The resulting transition energies are compared with the peak positions of the polaron optical conductivity, as recently calculated numerically using diagrammatic quantum Monte Carlo. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L33.00004: Comparison of the Raman low frequency spectra of NBT and KLT Daniel Jackson, Radha Pattnaik, Haosu Luo, Dwight Viehland, Jean Toulouse We present the results of a detailed comparative study of the low frequency central peak in sodium bismuth titanate (Na$_{0.5}$Bi$_{0.5}$TiO$_{3}$ or NBT) and potassium lithium tantalate (K$_{1-x}$ Li$_{x}$TaO$_{3}$ or KLT) from 90 degree angle Raman scattering with a resolution of 1 cm$^{-1}$. The Raman spectra of NBT were obtained over a wide temperature range from 78 to 950 K, spanning the two transitions, from cubic to tetragonal at $\sim $820 K and tetragonal to rhombohedral in the range 480-600 K. In an effort to better understand the nature of these phase transitions in NBT, we performed a detailed analysis of the central peak and soft mode combined, using different models. In particular, we compare the model in which these two features are uncoupled with the model in which they are coupled with a strength parameter, $\delta ^{2}$. These models are also discussed in the more general context of A-site substituted ABO$_{3}$ perovskites. The effects of an external electric field and mechanical pressure on the transitions will also be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L33.00005: Phonon structures and IR intensities in strained BaTiO3 Aldo Raeliarijaona, Huxiang Fu While soft modes and structural instability in cubic ferroelectrics have been well studied, the vibration properties in structurally stable phases are relatively less understood, however. Here we have carried out first-principle calculations, using density-functional perturbation theory, to determine the phonon structures and IR intensities at gamma point for tetragonal Barium titanate under different in-plane strains, in which lattice parameter ranges from 3.93 {\AA} to 3.80 {\AA}. We find that some modes shift strongly with the inplane strain, while other modes show surprisingly little change. The response of IR intensity is also revealed to be mode-dependent. The microscopic insight for these behaviors is examined. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L33.00006: Neutron Diffuse Scattering Measurements of PZT D. Phelan, P.M. Gehring, J. Rodriguez, C. Stock, X. Long, Y. Xie, Z.-G. Ye Neutron diffuse scattering measurements were performed on a single crystal of PbZr$_{0.675}$Ti$_{0.325}$O$_3$. The data is compared and contrasted with other Pb-based piezoelectrics, such as the relaxor PbMg$_{1/3}$Nb$_{2/3}$O$_3$, and is discussed within the context of random fields. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 4:18PM |
L33.00007: A Neutron Study of the Structure and Lattice Dynamics of Single Crystal PZT Invited Speaker: The outstanding piezoelectric properties of PbZr$_{1-x}$Ti$_{x}$O$_{3}$ (PZT) perovskite ceramics have long been exploited in numerous device applications, making PZT arguably the most technologically important ferroelectric material in use today. Efforts to understand the piezoelectric mechanism have inspired a plethora of structural studies spanning decades, but solving the PZT phase diagram has proven to be famously problematic because single crystals have not been available save for Zr- and Ti-rich compositions that lie very near the end members PbZrO$_{3}$ and PbTiO$_{3}$, where the piezoelectricity is weakest. Thus, whereas PZT has been the subject of thousands of powder and ceramic investigations, no consensus regarding the crystal structures of PZT exists. We report the first neutron diffraction study of single-crystal PZT with compositions x = 0.325 and 0.460 [1]. Our data refute the thesis that the ferroelectric phases of PZT within this composition range, all of which are highly piezoelectric, are purely monoclinic (Cc or Cm). The broadening of certain Bragg peaks can be interpreted in terms of coexisting rhombohedral and monoclinic domains, whereby monoclinic order is enhanced by Ti-doping. This is consistent with the theoretical proposal that the tendency to form macroscopic monoclinic phases facilitates the mechanism of polarization rotation by reducing the energy required to reorient the electric polarization. Dispersions of the lowest energy TO and TA phonon modes were measured on a single crystal of PZT with x = 0.325 in the paraelectric phase at 650 K [2]. The TO mode energy drops at small wave-vectors suggesting that it is a soft mode associated with the ferroelectric phase transition at 590 K. Evidence of a second soft-mode, corresponding to a phase transition at 370 K at the R-point, is provided based on the redistribution of spectral weight as a function of temperature. \\[4pt] [1] D. Phelan \textit{et al}., Phys. Rev. Lett. \textbf{105}, 207601 (2010). \\[0pt] [2] D. Phelan \textit{et al}., submitted. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L33.00008: Correlations between tetragonality, polarization, and ionic displacement in lead titanate-derived ferroelectric perovskite solid solutions Andrew Rappe, Tingting Qi, Ilya Grinberg We use first-principles density functional theory calculations to investigate the dependence of tetragonality on local structure in a variety of ferroelectric solid solutions. We demonstrate that tetragonality is strongly coupled to the $B$-cation displacement and weakly coupled to the $A$-cation displacement. Examination of various Bi$M^{3+}$O$_3$ additives to PbTiO$_3$ for different $M^{3+}$ ionic sizes reveals that substitution of either small $B$-cations or low doping of large $B$-cations gives rise to large spontaneous polarization and tetragonality. Understanding how the phase transition temperature ($T_c$) and tetragonality are affected by Pb- and Bi-based perovskite additives provides a rational path for designing new high-temperature piezoelectric materials. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L33.00009: First principles prediction of a morphotropic phase boundary in the Bi(Zn$_{1/2}$Ti$_{1/2}$)O$_3$-(Bi$_{1/2}$Sr$_{1/2}$)(Zn$_{1/2}$Nb$_{1/2}$)O$_3$ alloy Valentino R. Cooper, Asegun S. Henry, Shigeyuki Takagi, David J. Singh We present a density functional theory study on alloys of the tetragonally distorted Bi(Zn$_{1/2}$Ti$_{1/2}$)O$_3$ (BZT) and the rhombohedrally oriented (Bi$_{1/2}$Sr$_{1/2})($Zn$_{1/2} $Nb$_{1/2})$O$_3$ (BSZN). We find that compositions with $\ge$ 50\% BZT are tetragonally distorted with the polarization pointing mainly along the [001] direction. Conversely, for low concentrations of BZT the polarization is rhombohedrally oriented. Based on these results we propose a phase diagram with a possible monoclinc phase between 25\% and 50\% BZT where this material may have a useful piezoelectric response. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L33.00010: Probing Ferroelectricity in Thin-film Perovskite SnTiO$_3$ with First-principles Structural Instability Analysis William Parker, James Rondinelli, Serge Nakhmanson Perovskite SnTiO$_3$ has been recently identified in a number of computational studies as an attractive, environmentally-friendly material with potential to replace ferroelectric PbTiO$_3$. However, additional computational evidence suggests that bulk perovskite SnTiO$_3$ may be metastable. Using density-functional theory and phonon-band instability analysis, we investigate avenues for epitaxial stabilization of ferroelectric perovskite SnTiO$_3$ thin films with applied strains of up to $\pm$ 2\%. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L33.00011: Room temperature metastable monoclinic phase in BaTiO$_{3}$ crystals Tom Lummen, Jianjun Wang, Martin Holt, Amit Kumar, Eftihia Vlahos, Sava Denev, Long-Qing Chen, Venkatraman Gopalan Low-symmetry monoclinic phases in ferroelectric materials are of considerable interest, due to their associated enhanced electromechanical coupling. Such phases have been found in Pb-based perovskite solid solutions such as lead zirconate titanate (PZT), where they form structural bridges between the rhombohedral and tetragonal ground states in compositional space. In this work, we directly image such a monoclinic phase in BaTiO$_{3}$ crystals at room-temperature, using optical second harmonic generation, Raman, and X-ray microscopic imaging techniques. Phase-field modeling indicates that ferroelectric domain microstructures in BaTiO$_{3}$ induce local inhomogeneous stresses in the crystals, which can effectively trap the transient intermediate monoclinic structure that occurs across the thermal orthorhombic-tetragonal phase boundary. The induced metastable monoclinic domains are ferroelectrically soft, being easily moved by electric fields as low as 0.5 kV cm$^{-1}$. Stabilizing such intermediate low-symmetry phases could very well lead to Pb-free materials with enhanced piezoelectric properties. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L33.00012: Infrared spectroscopy of KDP under high pressure Ana Akrap, Christopher C. Homes, Ricardo P.S.M. Lobo, Patrick Simon We have determined infrared reflectivity of potassium dihydrogen
phosphate (KDP) in
the paraelectric ($T>T\rm_c=$135 K) and ferroelectric phase
($T |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L33.00013: Elastic Anizotropy and Domain Stability in Ferroelectric Thin Films and Problem of Critical Thickness for Memory Alexander Bratkovsky, A.P. Levanyuk The most important effect of the depolarizing field in thin ferroelectric (FE) capacitors is the emergence of domains in place of monodomain state desirable for memory applications, and it depends on the parameters of system Ferroelectric- Electrode. We have studied FE films of BaTiO3, PbTiO3, and Pb (Zr)0.5(Ti)0.5O3 with SrRuO3 electrodes on SrTiO3 (100) substrate. Due to lattice misfit, the FE film becomes tetragonal with the polar axis perpendicular to the film. We have studied rarely addressed topic of relation between the equilibrium domain structure and limits of absolute stability of the monodomain state. We have found that in films with thickness close to the minimal one compatible with FE the stripe domains form with domain walls along the cubic axes in BTO and PTO films, while in PZT it is at 45 degrees to the axes. The orientational dependence of their energy is actually very weak, less than 1\% is all above systems. The threshold of absolute instability of the monodomain state is shifted by electrostriction most significantly in BTO and PTO, where it gets close to the (formal) ``critical thickness'' for FE in monodomain films. In PZT, on the other hand, electrostriction hardly affects it [1]. \\[0pt] [1] A.M. Bratkovsky and A.P. Levanyuk, Phil. Mag. 90, 113 (2009); arXiv: 0801.1669; Phys. Rev. Lett. 100, 149701 (2008). [Preview Abstract] |
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