Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session G37: Focus Session: Nanoscale Materials Physics of Phase Transitions I |
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Sponsoring Units: DMP Chair: Richard Haglund, Vanderbilt University Room: Baltimore Convention Center 340 |
Tuesday, March 14, 2006 8:00AM - 8:12AM |
G37.00001: Adsorbate-Induced Ferroelectricity in Nanostructures Jonathan Spanier, Alexie Kolpak, Ilya Grinberg, Jeffrey Urban, Lian Ouyang, Wansoo Yun, Andrew Rappe, Hongkun Park We discuss recent progress in understanding the size-dependent evolution of ferroelectricity in perovskite nanostructures and ultrathin films obtained via variable-temperature scanning probe microscopy investigations of individual nanorods and other ferroelectric nanostructures. We propose a mechanism in which the surface polarization charge is passivated by molecular adsorbates. A combination of density functional theory simulations, modified phenomenological Landau theory, and thermodynamic analysis demonstrates that the experimental behavior is well described by this mechanism. [Preview Abstract] |
Tuesday, March 14, 2006 8:12AM - 8:24AM |
G37.00002: Nucleation process on the 180${^\circ}$ domain wall of PbTiO$_{3}$ by the external electric field Young-Han Shin, Ilya Grinberg, I-Wei Chen, Andrew Rappe Ferroelectric oxides are extremely useful as nonvolatile memory storage materials, and the speed at which polar domains can be reversed is a critical characteristic for future development of these materials. However, the size of the critical nucleus during the polarization reversal is still unknown experimentally. If we assume that the magnitudes of local polarizations are the same and their directions are along the external field, it will be triangular and the height of the nucleus along the external field should be much larger than its width following the Miller and Weinreich's study in 1960s. We made an atomic potential for perovskite ferroelectrics based on the first-principles calculation, and performed molecular-dynamics simulations to understand the nucleation and growth process of ferroelectric domains. We find that its shape is close to a square not a triangle and its size much smaller than Miller and Weinreich's. It stems from the small polarizations and the voltex-like flow around the nucleus. To increase the system size we used the stochastic study using the nucleation and growth rates which were obtained from the molecular dynamics simulations. The overall speed of the domain wall motion can be estimated from this stochastic calculation. [Preview Abstract] |
Tuesday, March 14, 2006 8:24AM - 8:36AM |
G37.00003: Critical Behaviors in Pb(Zr,Ti)O$_{3}$ Ultrathin films Emad Almahmoud, Igor Kornev, Laurent Bellaiche The first-principles-derived approach of Ref.[1] is used to determine the thickness dependency of Curie temperature and of the spontaneous polarization in Pb(Zr,Ti)O$_{3}$ (PZT) thin films that are under stress-free and open-circuit boundary conditions. It is found that, above a thickness of 7 (B-)monolayers (ML), the Curie temperature follows the finite-size relation of Ref.[2] with a critical exponent $\lambda $ = 1.04. On the other hand, the Curie temperature deviates from this ``usual'' relationship for thickness of 6 monolayers and below, characterizing a crossover from a three- to a two-dimensional behavior. This striking crossover is also reflected in the critical exponent \textit{$\beta $}, that is associated with the power law describing the behavior of the polarization with thickness. As a matter of fact, such exponent decreases drastically from 0.14 to 0.1 between 7 and 4 ML. This work is supported by ONR grants N 00014-01-1-0365, N00014-04-1-0413 and 00014-01-1-0600, by NSF grants DMR-0404335 and DMR-9983678, and by DOE grant DE-FG02-05ER46188. \newline [1] E. Almahmoud, Y. Navtsenya, I. Kornev, H. X. Fu, and L. Bellaiche, \textit{Phys. Rev. B 70, 220102{\textregistered} (2004) } \newline [2] G. A. T. Allan, Phys. Rev. B1, 352 (1970). [Preview Abstract] |
Tuesday, March 14, 2006 8:36AM - 9:12AM |
G37.00004: Phase transitions in ferroelectric superlattices Invited Speaker: The construction of artificial ferroelectric superlattices with fine periodicity presents exciting possibilities for the development of new materials with extraordinary properties and furthermore a probe for understanding the fundamental physics of ferroelectric materials. Our superlattices of PbTiO$_{3}$/SrTiO$_{3}$ are prepared on conducting 0.5{\%} Nb doped (001) SrTiO$_{3}$ substrates using off-axis RF magnetron sputtering. Cross-sectional TEM investigations were performed on several samples and reveal the coherent growth and artificial layering of the samples. Further structural characterization using standard $\theta $-2$\theta $ x-ray diffraction was performed on a series of 20 bilayer superlattices in which the PbTiO$_{3}$ thickness was varied from 54 to 1 unit cells while the SrTiO$_{3 }$layer thickness was maintained at 3 unit cells. Intuitively one expects, as the thickness of the PbTiO$_{3}$ layers relative to the SrTiO$_{3}$ layers is reduced, a decrease of the ferroelectric polarization which should result in a concomitant decrease of the average lattice parameter. This is indeed the behaviour we observe for superlattices PbTiO$_{3}$/SrTiO$_{3}$ n/3 where n is greater than 3. However, surprisingly, the 2/3 and 1/3 superlattices display larger average lattice parameters which indicate a recovery of ferroelectricity at very small PbTiO$_{3}$ layer thicknesses, a finding we confirmed using atomic force microscopy. The experimental finding thus stands in stark contrast to the intuitive expectation of a ferroelectric-paraelectric phase transition in this system as the ferroelectric component is reduced and we find further that the temperature of the ferroelectric-paraelectric phase transition is also greatly modified. Due to the excellent quality of the samples we are able to present the results of a number of detailed structural and electrical characterizations, along with the development of first principles based theoretical models, which cast further light on the fascinating phase transition behaviour of this system. Through this we can gain an insight into how we can understand and control the behaviour of ferroelectricity as the physical dimensions are reduced and the relevant boundary conditions are modified. [Preview Abstract] |
Tuesday, March 14, 2006 9:12AM - 9:24AM |
G37.00005: Generalized thermodynamics of solid surfaces applied to size effects on equilibrium Robert Cammarata In his thermodynamics of liquid interfaces, Gibbs introduced the ``dividing surface'' and rigorously derived the general conditions for equilibrium, and showed how the size of phases can affect the equilibrium state. However, the case of systems with solid surfaces, he imposed certain restrictions, such as considering only single component solids, which limited the applicability of his approach. In the case of finite size and multicomponent solids, difficulties arise when defining the chemical potentials for the interface, which leads to difficulties in defining the surface energy. It will be shown how these difficulties can be overcome by using the concept of ``surface availability,'' analogous to the availability used in bulk thermodynamics. A generalized surface thermodynamics can be formulated consistent with Gibbs' analysis for liquid surfaces but that can be used for general multicomponent solid systems. Also, employing the surface availability allows one to dispense with the dividing surface construction and treat the interfacial region as a separate inhomogeneous phase. Applications to nanoscale materials will be discussed. [Preview Abstract] |
Tuesday, March 14, 2006 9:24AM - 9:36AM |
G37.00006: Theory of the Melting of Confined Nanocrystals Daryl Chrzan, Q. Xu, I.D. Sharp, D.O. Yi, C.W. Yuan, C.Y. Liao, A.M. Glaeser, J.W. Ager III, E.E. Haller Recent measurements of the melting point of Ge nanocrystals embedded within a silica matrix reveal that the nanocrystals can be heated approximately 200 K above the bulk melting point before melting, and cooled approximately 200 K below the bulk melting point before solidification. This behavior is in marked contrast to that observed for free standing nanocrystals. We have developed a classical, continuum thermodynamic model which demonstrates that the phenomenon is caused by kinetic barriers to the nucleation of the liquid phase. The kinetic barriers are associated with the density increase upon melting of Ge and the fact that liquid Ge does not wet silica. The model provides a quantitative description of the experimental observations. This research is supported by the U. S. Department of Energy under contract No. DE-AC02-05CH11231. [Preview Abstract] |
Tuesday, March 14, 2006 9:36AM - 9:48AM |
G37.00007: Theory of the Jahn-Teller Soliton Dennis Clougherty It is demonstrated that under common conditions a molecular solid subject to Jahn-Teller interactions supports stable Q- ball-like non-topological solitons. Such solitons represent a localized lump of excess electric charge in periodic motion accompanied by a time-dependent shape distortion of a set of adjacent molecules. The motion of the distortion can correspond to a true rotation or to a pseudo-rotation about the symmetric shape configuration. These solitons are stable for Jahn-Teller coupling strengths below a critical value; however, as the Jahn- Teller coupling approaches this critical value, the size of the soliton diverges signaling an incipient structural phase transition. The soliton phase mimics features commonly attributed to phase separation in complex solids. [Preview Abstract] |
Tuesday, March 14, 2006 9:48AM - 10:00AM |
G37.00008: Finite-size scaling of the critical temperatures of magnetic thin films with variable range of interactions. Timothy Bramfeld, Roy F. Willis Finite-size scaling in magnetic (spin) systems with an arbitrary range of spin interactions was first discussed by Domb and Dalton [1]. These authors explored the effect on the various critical exponents of the thermodynamic quantities of a generalized Ising model in which each spin interacts equally strongly with neighbors within some finite interaction distance beyond which the interaction goes to zero. Such a model was used by Zhang {\&} Willis [2] to explain the thickness dependence of the Curie temperatures of ferromagnetic nickel films. Specifically, they showed that Tc followed a power law, reduced temperature t $\sim $ L$^{-\lambda }$down to a critical thickness Lo = Ro, at which point the critical temperature reduced linearly with further decreasing thickness L. In this talk, we show that the demarcation point Lo = Ro scales with the range of spin interactions in alloy films. This parameter Ro is a function of the changing dimensions of the Fermi surface i.e. related to the period of RKKY oscillations in these itinerant ferromagnets. We examine the ramifications of an increasing range of spin interactions Ro on the finite-size critical behavior of a magnetic system. [1] C. Domb {\&} N.W. Dalton, Proc. Phys. Soc. \underline {89}, 859 (1966). [2] R. Zhang {\&} R.F. Willis, Phys. Rev. Lett. \underline {86}, 2665 (2001). [Preview Abstract] |
Tuesday, March 14, 2006 10:00AM - 10:12AM |
G37.00009: Phase Diagrams, Crossovers and Charge-Spin Separation in Small Clusters Armen Kocharian, Gayanath Fernando, Kalum Palandage, James Davenport The charge-spin separation effect and response thermodynamics in the presence of a magnetic field are calculated using exact analytical diagonalization and grand canonical ensemble method for small clusters. Rigorous criteria for the Mott-Hubbard, N{\'e}el and zero-magnetic field (spin) weak singularities ({\it saddle points}) in the thermodynamic charge and spin densities of states {\it versus} chemical potential and magnetic field are found in the exactly solvable two and four site Hubbard ($U\mp 0$) clusters. Large quantum spin fluctuations are signaled by the presence of spin and charge density peaks, pseudogaps and corresponding temperature driven crossovers. The numerically evaluated, exact expressions for the charge, spin susceptibilities and specific heat demonstrate separation of charge and spin degrees and presence of pseudogaps that disappear at the distinct critical temperatures of crossovers. The phase diagram in the ground state, when monitored as a function of doping, displays the presence of clearly identifiable quantum phase transitions and critical points (QCP). In overall, the obtained spin pseudogap, magnetic correlations with antiferromagnetic (spin) pseudogap structure and crossovers in small clusters at $U>0$ closely resemble the pseudogap phenomena and the normal-state phase diagram in high T$_c$ superconductors. [Preview Abstract] |
Tuesday, March 14, 2006 10:12AM - 10:24AM |
G37.00010: Electron-Spin Resonance of Chromium-Doped Vanadium Dioxide Patrick Bunton, D. Blane Baker, Kenneth Maynes, Kenneth Hartman, Andrej Halabica, Rene Lopez, Richard Haglund We have investigated the metal insulator transition (MIT) in vanadium dioxide using the electron-spin resonance (ESR) response of a chromium probe. Our goal is to elucidate information regarding potential trigger mechanisms for the MIT such as extrinsic defects or thermal excursion associated with soft modes. A chromium-doped vanadium dioxide film was prepared on fused-silica by pulsed laser deposition in oxygen followed by annealing in an oxygen atmosphere. The chromium dopant proved to be sensitive to the MIT, exhibiting a hysteretic dependence of chromium intensity. A 140~nm-thick film exhibits a factor of four decrease in chromium line intensity as the temperature increases across the MIT. ESR can yield information regarding thermal excursions of lattice ions as well as symmetry information -- typically through either linewidth or axial spin Hamiltonian parameter. In addition, decreases in ESR signal intensities are discussed in relation to changes in electron spin-lattice relaxation times. Extensions to two-dimensional films of contiguous nanoparticles and to nanoparticle arrays will be discussed. [Preview Abstract] |
Tuesday, March 14, 2006 10:24AM - 10:36AM |
G37.00011: Percolation effects in networks of vanadium dioxide nanocrystals John Rozen, Ren\'e Lopez, Richard Haglund, Leonard Feldman Bulk vanadium dioxide undergoes a phase transition from a semiconductor to a metal near 68 $^o$C upon heating. A unique aspect of VO$_2$ nanocrystals is that their small size leads to meta-stable states and to the widening of the hysteresis loop traced out by the semiconductor-to-metal phase transition. We report simultaneous measurements of the transmittance and of the resistance made on thin VO$_2$ nanocrystalline films. An offset between the effective electrical and optical switching temperatures is observed. Indeed, there is an intermediate state where the films are optically metallic but electrically semiconducting (opening the possibility of designing tri-state devices). This is explained in terms of current percolation through a network of nanometer-scale grains of different sizes undergoing the transition at distinct temperatures. An effective-medium approximation can model this behavior and proves to be an indirect method to calculate the surface coverage of the films. Also, the understanding of the collective properties of the nanocrystals leads to the reconstruction of the electrical hysteresis of the phase transition. [Preview Abstract] |
Tuesday, March 14, 2006 10:36AM - 10:48AM |
G37.00012: Transmission modulation through sub-wavelength hole arrays in metal-VO$_{2}$ double-layer nanostructures J. Y. Suh, E. U. Donev, R. Lopez, L. C. Feldman, R. F. Haglund Jr. Nanoscale optical systems require active devices able to control light in sub-wavelength structures. We report the use of a unique double layer structure which provides this function. Enhanced transmission of near-infrared light through a subwavelength hole arrays patterned in Ag-VO$_{2 }$and Au-VO$_{2}$ double-layer films shows that it is possible to modulate transmitted intensity by means of a semiconductor-to-metal phase transition in VO$_{2}$. The transmitted intensity in the near-infrared for the metallic phase of VO$_{2}$ is a factor of $\sim $10 greater than the intensity for the semiconducting phase. We explain this modulation in terms of the switchable permittivity difference of the VO$_{2}$ phases. [Preview Abstract] |
Tuesday, March 14, 2006 10:48AM - 11:00AM |
G37.00013: Optical diffraction in ordered VO$_{2}$ nanoparticle arrays Rene Lopez, Leonard Feldman, Richard Haglund The potential of oxide electronic materials as multifunctional building blocks is one of the driving concepts of the field. In this presentation, we show how nanostructured particle arrays with long-range order can be used to modulate an optical response through exploiting the metal-insulator transition of vanadium dioxide. Arrays of VO$_{2}$ nanoparticles with long-range order were fabricated by pulsed laser deposition in an arbitrary pattern defined by focused ion-beam lithography. The interaction of light with the nanoparticles is controlled by the nanoparticle size, spacing and geometrical arrangement and by switching between the metallic and semiconducting phases of VO$_{2}$. In addition to the near-infrared surface plasmon response observed in previous VO$_{2}$ studies, the VO$_{2}$ nanoparticle arrays exhibit size-dependent optical resonances in the visible region that likewise show an enhanced optical contrast between the semiconducting and metallic phases. The collective optical response as a function of temperature gives rise to an enhanced scattering state during the evolving phase transition, while the incoherent coupling between the nanoparticles produces an order-disorder-order transition. [Preview Abstract] |
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