Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Y26: Glassy and Amorphous Systems |
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Sponsoring Units: DCMP Room: 328 |
Friday, March 20, 2009 8:00AM - 8:12AM |
Y26.00001: Long Term Aging of As$_{x}$Se$_{1-x }$glasses and the Intermediate Phase Ping Chen, Jacob Wachtman, P. Boolchand The reversibility window in As$_{x}$Se$_{1-x}$ glasses was reported\footnote{D.G. Georgiev et al. Phys. Rev. B \underline {62}, R9228(2000).} 8 years ago to reside in the 28{\%} $<$ x $<$ 37{\%} range. We have re-examined those samples in m-DSC and Raman scattering. Both the 8 year old hermetically sealed samples in Al holders (set A) and samples from the same batch preparation but stored in plastic vials (set B) at laboratory ambient environment were studied. The reversibility window in samples of set A, after 8 years of aging, is found to be intact. In set B, analysis of the T$_{g}$ endotherm becomes difficult because of a precursor exotherm that appears in the 32{\%} $<$ x $<$ 60{\%} range, and steadily increases with x. In addition, in both set of samples, one observes a sub T$_{g}$ endotherm upon aging in the 90 $^{\circ}$C $<$ T $<$ 120 $^{\circ}$C range. These m-DSC results supported by Raman scattering suggest that the exotherm is due to light induced nanocrystallization (nc) of As$_{4}$Se$_{4}$ fragments (an extrinsic effect), while the sub-T$_{g}$ feature is due to nc fragments of trigonal Se formed upon long term aging(an intrinsic effect). These findings will be compared to a recent report.\footnote{R. Golovchak et al. Phys. Rev. B \underline {78}, 014202(2008).} [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y26.00002: Molecular structure of virgin and T$_{g}$ cycled (Ag$_{2}$Se)$_{x}$ (AsSe)$_{1-x}$ bulk glasses Jacob Wachtman, Ping Chen, P. Boochand AsSe, the base glass (x = 0) in the titled ternary, is an interesting example of a chalcogenide that is partially de-mixed into As$_{4}$Se$_{4}$ molecules segregated from a connected AsSe network, with the latter determining glass network properties. Raman scattering reveals sharp modes of the Realgar molecules that are superimposed on broad modes coming from of the backbone. Upon T$_{g}$ cycling virgin samples (as quenched melts), the concentration of de-mixed As$_{4}$Se$_{4}$ molecules decreases, suggesting that thermally induced polymerization occurs; molecules break up to form part of the connective tissue. Modulated DSC experiments reveal a broad exotherm near 140 $^{\circ}$C in virgin samples, which becomes nearly extinct in T$_{g}$ cycled samples. The exotherm may represent Realgar molecules nano-crystallizing as the temperature approaches T$_{g}$. Compositional trends in thermal parameters such as T$_{g}$(x), $\Delta $C$_{p}$(x), and the $\Delta $H$_{nr}$(x) as a function of Ag$_{2}$Se content `x' of the glasses will be reported. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y26.00003: Atomistic Picture of the Intermediate Phase in GexSe1-x Glasses: A Joint Theoretical and Experimental Study Fakhar ul Inam, Gang Chen, Denyago Tafen, David Drabold Raman and calorimetric studies on~~GexSe1-x glasses have provided evidence for the existence of the intermediate phase (IP) in chalcogenide and other glasses. Here, Ab-initio molecular dynamics (MD) based models of these glasses are discussed, and an atomistic picture of the IP, based upon the models and available experiments, is presented. A thorough analysis of our models reveals that the IP in GexSe1-x glasses appears due to the competition between the percolating~GeSe2 phase~and the a-Se phase, which gives rise to the ``flattening'' of the observables in the IP window. We present X-Ray Absorption Near Edge Structure (XANES) measurements on germanium selenide glasses in the IP composition range, and detect an electronic signature of the IP in terms of the shift of the XANES white line (WL) and non-linear behavior o their intensities in the IP window. We show that these models appear to properly represent the XANES results. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y26.00004: Boson modes and Floppy modes in network glasses D. Novita, Ping Chen, P. Boolchand Network glasses differ from their crystalline counterparts in a significant way- the presence of an excess of low-frequency vibrations. Here we show that in a covalent\footnote{P. Chen et al. ArXiv 0810.3637} ( As$_{x}$S$_{1-x})$ and in a super-ionic\footnote{D. Novita et al. ArXiv 0808.1154} (AgI$_{(x)}$AgPO$_{3(1-x)})$ glass system, Raman Bose peak scattering strength (I$_{B})$ displays commonalities; in the flexible phase of these systems, I$_{B}$ is found to increase almost linearly as networks become more flexible or their connectivity decreases. Moreover, the rate at which dI$_{B}$/d\textbf{\textit{r}} changes is reminiscent of the variation df/d\textbf{\textit{r}} of the floppy mode count (f) with \textbf{\textit{r }}in rigidity theory. These results suggest that at least in the flexible phase, contributions to the boson peak must result in large part from floppy modes. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y26.00005: Characterization of dry B$_{2}$O$_{3}$ glass K. Vignarooban, D. Novita, Ping Chen, P. Boolchand A sample of Puratronic B$_{2}$O$_{3}$ (Aesar) was vacuum ( 10$^{-6 }$Torr) melted in a Pt crucible at 520\r{ }C for 3 days and slow cooled to room temperature to obtain a glass. All sample manipulations were performed in a N$_{2}$ gas purged glove box. T$_{g }$ of the sample from inflexion point of the reversing heat flow in an m-DSC experiment ,using a scan rate of 3\r{ }C/min, gave a value of T$_{g}$ (mDSC) = 308(1)\r{ }C. A traditional DSC experiment, using a scan rate of 10\r{ }C/min, gave a value of T$_{g}$(DSC) = 309(2)\r{ }C. Our T$_{g}$ (DSC) value is 12\r{ }C to 20\r{ }C higher than previous reports$^{2}$ using the same scan rate. Vibrational features in IR reflectance in the 1200-1600 cm$^{-1}$ range (LO and TO modes) , and in the 3200-3600 cm$^{-1}$ range ( free and bonded water) evolve as transparent platelets are exposed to laboratory environment, providing evidence for water reactivity of dry samples. Raman scattering$^{3}$ results complement IR reflectance ones. We confirm$^{2}$ density of dry samples (1.805(4)gms/cm$^{3})$ to be somewhat less than wet ones (1.815(4) gms/cm$^{3})$. $^{2}$ Ramos et al. JNCS \underline {221}, 170 (1997). $^{3~}$F. Galeener et al, PRB \underline {22}, 3983 (1980). [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y26.00006: Silicon under mechanical shear: molecular dynamics study Ali Kerrache, Normand Mousseau, Laurent J. Lewis Relaxation processes and defect behavior in amorphous silicon (a-Si) under shear are investigated by molecular dynamics simulations using the empirical Environment Dependent Inter-atomic Potential. Shear deformations allow us to reproduce a vast range of interesting dynamics in disordered materials. For example, it has been shown recently that high-energy ion irradiation deforms plastically a-Si samples, following a pattern similar to the application of a shear. While large shear reproduce high-irradiation effects, moderate one can accelerate defect diffusion. A controlled application of shear can therefore help us to understand better the nature of defect diffusion in a-Si, in addition to generating new defects that could be placed with the appropriate external field at critical positions or even lead, in some cases, to crystallization of the a-Si. The properties of a-Si under shear are analyzed as a function of imposed shear velocity and as a function of the distance from the wall. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y26.00007: Liquid-liquid transition in supercooled silicon determined by first-principles simulation P. Ganesh, M. Widom First principles molecular dynamics simulations reveal a liquid-liquid phase transition in supercooled elemental silicon. Two phases coexist below T$_{c} \quad \sim $ 1232K. The low density phase is nearly tetra-coordinated, with a pseudogap at the Fermi surface, while the high density phase is more highly coordinated and metallic in nature. The transition is observed through the formation of van der Waals loops in pressure-volume isotherms below T$_{c}$. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y26.00008: High thermal conductivity of a hydrogenated amorphous silicon film J.L. Feldman, Xiao Liu, D.G. Cahill, R.S. Crandall, Noam Bernstein, D.M. Photiadis, M.J. Mehl, D.A. Papaconstantopoulos, Ho-Soon Yang We measured the thermal conductivity $\kappa$ of an 80~$\mu$m thick hydrogenated amorphous silicon ({\em a}-Si:H) film from 80\,K to room temperature with the 3$\omega$ method and at room temperature with the time-domain thermoreflectance (TDTR) method. The {\em a}-Si:H sample with 1~at.\% hydrogen was prepared by hot-wire chemical-vapor deposition (HWCVD), a procedure which was found previously to produce superior material properties with a near absent atomic tunneling states that are ubiquitous in glasses. We find that $\kappa$ is higher than any of the previous temperature dependent measurements, and shows a strong phonon mean free path dependence. We also performed numerical calculations on three 1000 atom models using Kubo theory and a tight binding electronic structure method. Due to the restraints of the TDTR results on low frequency extrapolations of calculated phonon diffusivities, the Kubo thermal conductivityis seen to be too small to explain our experiments. We conclude that the HWCVD a-Si:H sample has superior structural ordering relative to any amorphous silicon previously studied. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y26.00009: Specific heat of amorphous materials outside of the universal regime: $a$-Si and $a$-Si:H Daniel Queen, Qi Wang, Richard Crandall, Frances Hellman We present specific heat measurements for $a$-Si and $a$-Si:H films that are known to lack the broad distribution of tunneling level systems (TLS) as measured by internal friction. Below 1K the TLS model describes the universal behavior seen in specific heat, thermal conductivity, and other measurements. However, this model does not address the universal high temperature features, namely, the plateau in k and peak in C/T$^{3}$ that occur around 10K. Tetrahedrally bonded systems, such as $a$-Si and $a$-Ge, have long been thought to lack TLS due to the over constrained nature of their bonding. These materials prove difficult to quench from a bulk melt but are routinely grown by vapor deposition. We use our MEMS nanocalorimeter for heat capacity measurements between 2-300K on a range of $a$-Si and $a-$Si:H films prepared by e-beam evaporation and Hot-wire CVD. Changes in the high temperature specific heat will be discussed for films that are known to have orders of magnitude lower densities of TLS as measured by internal friction. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y26.00010: Hydrogen microstructure and voids in amorphous silicon hydride: A first principles study Rajendra Timilsina , Parthapratim Biswas We study distribution of hydrogen and hydride configurations in several models of a-Si:H using first-principles density functional calculations. Motivated by recent experimental result via small angle x-ray scattering, which reveals the presence of large voids (of linear dimension up to 4 nm) in a-Si:H, we develop models for a range of concentration, and study the effect of voids on hydrogen distribution. In particular, we investigate the presence of voids in two different concentration regime: high (14 at. \% and above) and low (below 14 at. \%) following a recent experimental observation from infrared absorption spectroscopy . The bonding environment of H atoms, and the local electronic structure near the voids are also presented. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y26.00011: Atomistic modeling of amorphous silicon carbide: A first-principles study Parthapratim Biswas, Raymond Atta-Fynn Localized basis {\it ab initio} molecular dynamics techniques within density functional theoretic framework have been used to model a realistic atomistic configuration of amorphous silicon carbide ({\it a}-Si$_{0.5}$C$_{0.5}$) containing 1000 atoms. The structural, electronic and vibrational properties have been studied and compared to existing theoretical models and available experimental data. Our study clearly reveals that the short-range chemical order in this material is predominant due to presence of heteronuclear Si-C bonds with coordination defect concentration less than 5\% and the chemical disorder parameter was $\chi$ = 0.083. Our 1000-atom model shows the presence of a clean gap in the spectrum and we also study the nature of the localization of the electronic band tail states as well as the vibrational eigenmodes. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y26.00012: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y26.00013: Glass Formability of Aqueous Solutions, and the Critical Nucleation Radius of Cubic Ice Ryan Dunn, Matthew Warkentin, Robert Thorne We have determined critical cooling rates and critical warming rates for a range of concentrations of different solutes in aqueous solutions using high-speed video microscopy. Our results show that the glass formability of aqueous solutions is exponential in the concentration for all solutes tested, with a different characteristic concentration for each solute. The characteristic concentration correlates with molecular radius. A simple modification of critical droplet theory relates the characteristic concentration to the critical nucleation radius in pure water, and explains the relationship between molecular radius and the characteristic. This simple, general theory of glass formability in aqueous solutions is important at a fundamental level, and will also have broad consequences for the field of cryobiology. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y26.00014: Nanocalorimetry as a means to explore thin films of vapor-deposited organic glasses Kenneth L. Kearns, Heiko Huth, Mathias Ahrenberg, Christoph Schick, M. D. Ediger Vapor deposition was used to prepare nanometer thick films of small molecule organic glasses. Films of indomethacin (IMC) and 1,3,5-(tris)naphthylbenzene (TNB) with a range of stabilities and thicknesses were created and characterized using differential nanocalorimetry. The heat capacity-like calorimetric signal was lower for the stable vapor-deposited glass films at temperatures below the glass transition T$_{g}$ than for an ordinary glass prepared by cooling the liquid. A gradual increase in the calorimetric signal was observed during the isothermal transformation above T$_{g}$ from stable to ordinary glass with the fastest transformation taking place in about 200 $\tau _{\alpha }$. The time for this transformation was dependent on film thickness with 350 nm thick films transforming approximately 5 times faster than 40 $\mu $m thick films. Aging experiments on the ordinary glass also showed a thickness dependence with thinner films aging more rapidly. This thickness dependent behavior is consistent with a mechanism where the dynamics at the film interfaces are faster than those in the bulk. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y26.00015: Evidence for a new molecular packing at low temperatures in vapor-deposited indomethacin Kevin Dawson, Ken Kearns, Werner Steffen, Lian Yu, Mark Ediger Thin films of the low molecular weight organic glassformer indomethacin were prepared at different substrate temperatures using physical vapor deposition. When analyzed by wide angle x-ray scattering, samples prepared at T$_{g}$-50 K showed a broad high intensity peak that is not present in samples prepared at T$_{g}$. When such samples were annealed at T$_{g}$+4 K the extra peak eventually vanished but only after 24,000 s (more then 500 $\tau _{\alpha })$. At low deposition rates the WAXS signature of this new molecular packing is only observed for substrate temperatures below T$_{g}$-20 K. Based on WAXS data on supercooled indomethacin, the new WAXS peak is unexpected. These results suggest that ordinary indomethacin glasses aged to equilibrium 20 K below the conventional T$_{g}$ would undergo a first order transition to a new amorphous phase. [Preview Abstract] |
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