Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session S18: Disordered and Glassy Systems II |
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Sponsoring Units: DCMP GSNP Chair: Sindee Simon, Texas Tech University Room: 403 |
Thursday, March 6, 2014 8:00AM - 8:12AM |
S18.00001: Density of states of Frenkel excitons in strongly disordered two-dimensional systems Robert Siemann, Abdelkrim Boukahil We present the calculation of the density of states of Frenkel excitons in strongly disordered two-dimensional systems. A random distribution of transition frequencies with variance $\sigma ^{\mathrm{2}}$ characterizes the disorder. The Coherent Potential Approximation (CPA) calculations show a strong dependence of the density of states (DOS) on the disorder parameter $\sigma $. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S18.00002: Effects of disorder on the optical properties of Frenkel excitons Abdelkrim Boukahil, Robert Siemann The Coherent Potential Approximation (CPA) is used to study the effects of disorder on the absorption line shapes of Frenkel excitons in one-, two-, and three-dimensional systems. A Gaussian distribution of transition frequencies with rms width $\sigma $ was used. Several values of the disorder parameter $\sigma $. The CPA results show that short tails on the high-energy side of the peaks are $\sigma $ dependent, and long tails on the low-energy side of the peaks are independent of the disorder parameter $\sigma $. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S18.00003: Depolarized Photon Correlation Spectroscopic Study of the Glass-Forming Liquid Cumene at Very High Pressures Kevin Lyon, Tim Ransom, William Oliver In recent years full-spectrum analysis of light-scattering data has been utilized to explore the liquid-glass transition at variable temperatures and ambient pressure. We have developed methods for doing depolarized photon correlation spectroscopy (PCS) in the diamond anvil cell in order to probe directly the structural relaxation time of glass-forming liquids at very high pressures. Here we present results for liquid cumene at 25~C between 1 bar and pressures approaching the room-temperature glass transition at 2.1~GPa. Data along higher-temperature isotherms will also be presented. Methods for minimizing any undesired heterodyne component in the collected light as well as the use of the longitudinal modes of the Brillouin spectrum to aid in the acquisition and spatial filtering of the scattered light will be discussed. Intensity-intensity correlation data were found to be well represented by the KWW equation with a nearly constant stretching parameter of ${\rm g}=$ 0.66 for 25~C. Furthermore, the relaxation time as a function of pressure is described will using a modified VTF expression: (P)$=_{\mathrm{0}}$exp\textbraceleft DP/(P$_{\mathrm{0}}$-P)\textbraceright , with values of $_{\mathrm{0}} =$ 11.9 ps, D $=$ 18.6, and P$_{\mathrm{0}} =$ 3.4 GPa at T $=$ 25 $^{\circ}$C. Thus, (P) has been obtained at 25 $^{\circ}$C for Cumene over seven decades from about a microsecond to several seconds and is found to be in excellent agreement with previously determined values for the alpha relaxation at lower pressures obtained from Brillouin data [G. Li, \textit{et al.}, Phys. Rev. Lett. \textbf{74}, 2280 (1995)]. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S18.00004: Glass-Forming Liquids under Extreme Conditions William Oliver, Kevin Lyon, Tim Ransom The nature of glass-forming liquids and the glass transition remain incompletely understood despite intense effort over many years. Though important contributions to our understanding of viscous liquids and glasses at high pressure have been made during this time, the overwhelming majority of studies have consisted of temperature-dependent studies at 1 bar. Recent experimental advances have begun to change this situation in important new ways [see, e.g., A.A. Pronin \textit{et al.}, JETP Letters \textbf{92}, 479 (2010)]. Glass-forming liquids can be exposed to record high pressures of several GPa with the diamond anvil cell (DAC); however, sample volumes are tiny (nanoliters) and the DAC is most amenable to optical techniques. Recent methods for probing glass-forming systems in the DAC will be highlighted in this presentation including direct measurement of Tg(P), the combination of depolarized Brillouin and photon correlation spectroscopies to measure the alpha relaxation time as a function of pressure from picoseconds to many seconds, and lastly, in the spirit of recent temperature dependent studies at one bar [see, e.g., Zhang \textit{et al}., Phys. Rev. E \textbf{70}, 011502 (2004)], we can now carry out full spectrum analyses in which depolarized backscattering with forward scattering spectra are combined in a self-consistent way to determine the significance of things such as rotation-translation coupling. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S18.00005: High Pressure Light Scattering Study of Relaxation in the Glass Former Cumene Tim Ransom, Kevin Lyon, William Oliver To understand relaxation dynamics in glassy systems, a light scattering study on Cumene has been carried out in a diamond anvil cell (DAC) at pressures from 0.2 GPa to 2.5 GPa isothermally at 75 $^{\circ}$C. Polarized and depolarized spectra were taken in both near-backscattering and equal-angle 60$^{\circ}$ forward-scattering geometries at several free spectral ranges from 0.5 GHz to 300 GHz. Depolarized backscattering spectra are converted into susceptibility featuring the evolution of the $\alpha $-relaxation peak, yielding structural relaxation times $\tau _{\alpha}$ from 10 ps to 1 ns. We have also developed photon correlation spectroscopy (PCS) in a DAC, giving $\tau_{\alpha}$ from $\sim$ 1 $\mu$s to 1 s. We fit $\tau_{\alpha}$ over these many decades with a modified VFTH equation $\tau _{\alpha} = \tau_{0}$exp[DP/(P$_{0}$-P)] giving parameters $\tau _{0}=$ 9.2 ps, D$=$ 17.5, and P$_{\mathrm{0}}=$ 4.5 GPa at 75$^{\circ}$C. After the $\alpha $-relaxation peak moves into lower frequencies (P $\sim$ 1 GPa), we observe the emergence of the $\beta $-relaxation minimum region. We fit the $\beta $-minimum to a power law scaling form $\chi $''($\omega )=$b($\omega $/$\omega _{\mathrm{min}}$)$^a$ $+$ a($\omega_{\mathrm{min}}$/$\omega$)$^b$. Polarized backscattering and forward scattering gives frequency shift $\omega_{\mathrm{B}}$ and linewidth $\Gamma _{\mathrm{B}}$ values of the longitudinal acoustic modes at two different q. We observe that the usual peak in linewidths does not coincide with $\omega_{\mathrm{B}}\tau_{alpha} \approx $ 1, indicating that the longitudinal acoustic modes do not couple with structural relaxation. Tansverse acoustic modes also appear in the depolarized forward scattering spectra. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S18.00006: Two-State Dynamics in Liquids and Glass on a Picosecond Timescale Marcus Cicerone, Miaochan Zhi, Juan dePablo We present results from neutron scattering, atomistic MD simulations, and optical Kerr effect spectroscopy (OKE) to demonstrate that liquids and glasses exhibit two dynamic states at short times. We provide evidence that the two dynamic states arise from molecules that are either tightly caged or loosely caged on a ps timescale. This heterogeneous motion is associated with hopping at low temperature, but the two-state scenario persists well above the melting point, and also contributes significantly to transport at the higher temperatures. Using concepts derived from this model we are able to quantitatively predict self-diffusion of small molecule glassformers well into the supercooled regime. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S18.00007: Fast Scanning Calorimetry Studies of Glassy Films of Toluene Deepanjan Bhattacharya, Vladislav Sadtchenko Fast scanning calorimetry was used to prepare and characterize micron thick vapor-deposited and liquid-cooled films of toluene on a thin filament. At temperatures above and below standard glass transition temperature (Tg) of toluene, the vapor-deposited films were prepared by physical vapor deposition at deposition rates of approximately 15 nm/s and the liquid-cooled films were prepared by quenching of liquid, 10 K above Tg, at a rate of approximately 5 K/s. It was found that vapor-deposited films have lower enthalpy and higher kinetic stability than liquid-cooled films even at temperatures approaching slightly above Tg. The most stable vapor-deposited films were prepared at temperatures 5 K below Tg. The kinetic stability of this film increased by about 4 K when the deposition rate was lowered from 40 nm/s to about 0.5 nm/s. A negligible change in the kinetic stability of this film was observed as long as the thickness was above 200- 300 nm range. The nature of the substrate had negligible impact on the phase's kinetic stability. We will report the results of our FSC studies and compare them with those of other contemporary findings. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S18.00008: Thermophysical and Rheological Properties of Imidazolium-Based Ionic Liquids: The Effect of Aliphatic versus Aromatic Functionality Ran Tao, Lianjie Xue, George Tamas, Edward Quitevis, Sindee Simon As a material class, ionic liquids possess attractive properties and have a wide range of potential uses. In this work, a series of imidazolium-based ionic liquids with the same carbon number varying from aliphatic to aromatic functionalities are investigated. The effects of cation symmetry and larger aromatic polycyclic functionality are studied. The thermal properties, including the glass transition temperature, melting temperature, and decomposition temperature, are characterized, and the density and the ionic conductivity are measured as a function of temperature. Rheological studies are performed using both steady-state and dynamic shear modes. The Cox-Merz relationship between the steady shear viscosity and the dynamic viscosity is examined. The temperature dependence of viscosity is described by the Vogel-Fulcher-Tammann equation and the dynamic fragility is calculated for each ionic liquid and compared to the fragility obtained from calorimetry. Master curves of dynamic shear responses are also constructed and will be discussed. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S18.00009: Atomistic study of rejuvenation of amorphous metals via thermal loading Masato Wakeda, Junji Saida, Ju Li, Shigenobu Ogata Rejuvenation is the structural excitation of amorphous system accompanied by enthalpy and free volume rise, and it is one of the promising approaches for improving the deformability of amorphous metals, which usually exhibit macroscopic brittle fracture. However, methods for controlling the rejuvenation and feasibility conditions of the rejuvenation remain unclear because of few experimental evidences and lack of clear knowledge of nonequilibrium glass properties. In this study, we investigate a method to control the rejuvenation through thermal loading and the feasibility conditions of the thermal rejuvenation. Using molecular dynamics techniques, we constructed an amorphous alloy model via melt-quenching process, and then conducted annealing and quenching processes. It is observed that thermal rejuvenation occurs via a thermal loading process of annealing at temperatures above a critical value and subsequent quenching at a cooling rate that is higher than that of the initial melt-quenching process. The level of rejuvenation increases with increasing annealing temperature and quenching rate. We discuss the background nature of rejuvenation and potential application of thermal rejuvenation to control the mechanical properties of amorphous metals. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S18.00010: Cooperativity in glassy dynamics investigated by higher-harmonic dielectric spectroscopy Thomas Bauer, Peter Lunkenheimer, Alois Loidl In recent years, due to experimental advances initiated by hole burning experiments, nonlinear dielectric spectroscopy has gained increasing interest in the field of glass-forming matter. For example, refining the technique of high-field permittivity measurements, we found a surprising lack of nonlinearity in the so-called excess wing region, that could not be accessed by this method before [1]. In the present contribution, we report new, detailed measurements of the third-order nonlinear dielectric susceptibility $\chi_{3}$ of four glass-forming liquids for a broad temperature range [2]. We find a significant hump in $\chi_{3}(\nu )$, from which we deduce the number of correlated molecules N$_{corr}$. We detect a continuous increase of N$_{corr}$ on approaching the glass-transition temperature. Comparing these results with the temperature-dependent apparent energy barriers in these systems, our experiments finally prove the old notion that intermolecular correlations of glassy systems are responsible for the non-canonical temperature development of glassy dynamics. [1] Th. Bauer, P. Lunkenheimer, S. Kastner, A. Loidl, Phys. Rev. Lett. \textbf{110}, 107603 (2013) [2] Th. Bauer, P. Lunkenheimer, A. Loidl, Phys. Rev. Lett., in press (arXiv:1306.4630) [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S18.00011: Plasticity of amorphous carbon Julian von Lautz, Michael Moseler, Lars Pastewka We use molecular dynamics simulations to probe the plastic response of representative bulk volumes of amorphous carbon at densities from 2.0 g cm$^{\mathrm{-3}}$ to 3.3 g cm$^{\mathrm{-3}}$ in simple and triaxial shear. After an initial elastic response the samples yield with only little strain hardening or softening. Individual plastic events in this network forming glass are strikingly similar to those observed for bulk metallic glasses: We find that plasticity is carried by fundamental rearrangements of regions of around 100 atoms, the shear transformation zone. In the simple shear geometry, those events coalesce to form a shear-band on longer time scales. During plastic deformation, the material changes its hybridization by transforming sp$^{\mathrm{3}}$ carbon atoms to sp$^{\mathrm{2}}$. We provide evidence that this transformation of the structural state occurs before the material yields, hence weakening the material. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S18.00012: Anomalous Temperature Dependence and Isotope Effect in the Structural Dynamics of Deeply Supercooled Water Alexander L. Agapov, Alexander I. Kolesnikov, Vladimir N. Novikov, Ranko Richert, Alexei P. Sokolov Despite simple chemical structure and its importance in our life, water remains one of the most puzzling liquids.\footnote{Angell, C. A. \textit{Science} 319, 582-587 (2008).} Combining neutron scattering and dielectric spectroscopy we show that quantum fluctuations have a pronounced effect on dynamics in deeply supercooled water. Dielectric measurements revealed that water has an anomalously weak temperature dependence of structural dynamics close to $T_{g} \approx $ 136K with unphysical low fragility index $m \approx $ 14. Additionally, we observed an anomalously large isotope shift of $T_{g}$ between H$_{2}$O and D$_{2}$O, $\Delta T_{g}$ $\sim$ 8-10K, in a strong contrast to the isotope effect on $T_{g}$ observed in other hydrogen bonding liquids. The observed anomalous behavior is consistent with the recently suggested idea of quantum zero-point vibrations affecting dynamics of supercooled water.\footnote{Novikov, V. N. {\&} Sokolov, A. P. \textit{Phys. Rev. Lett.} 110, 065701 (2013).} We speculate that the apparent fragile-to-strong crossover in dynamics of water can be ascribed to quantum effects dominating structural relaxation at low temperatures. These results have significant implications for our understanding of water dynamics and its peculiar behavior at low temperatures. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S18.00013: Liquid-liquid coexistence and crystallization in supercooled ST2 water Fausto Martelli, Jeremy Palmer, Pablo Debenedetti, Roberto Car We have computed the free energy landscape of ST2 water in the supercooled regime (228.6 K and 2.4 kbar) using several state-of-the-art computational techniques, including umbrella sampling and metadynamics. Such results conclusively demonstrate coexistence between two liquid phases, a high-density liquid (HDL) and a low-density liquid (HDL), which are metastable with respect to cubic ice. We show that the three phases have distinct structural features characterized by the local structure index and ring statistics. We also find that ice nucleation, should it occur, does so from the low-density liquid. Interestingly, we find that the number of 6-member rings increases monotonically along the path from HDL to LDL, while non-monotonic behavior is observed near the saddle point along the LDL-ice Ic path. This behavior indicates a complex re-arrangement of the H-bond network, followed by progressive crystallization. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S18.00014: Coherent neutron scattering and collective dynamics on mesoscale Vladimir Novikov, Kenneth Schweizer, Alexei Sokolov By combining and extending, a variety of theoretical concepts for the dynamics of liquids in the supercooled regime, we formulate a simple analytic model for the temperature and wavevector dependent collective density fluctuation relaxation time that is measurable using coherent dynamic neutron scattering. Comparison with experiments on the ionic glass-forming liquid CKN in the lightly supercooled regime suggests the model captures the key physics in both the local cage and mesoscopic regimes, including the unusual wavevector dependence of the collective structural relaxation time. The model is consistent with the idea that the decoupling between diffusion and viscosity is reflected in a different temperature dependence of the collective relaxation time at intermediate wavevectors and near the main (cage) peak of the static structure factor. More generally, our analysis provides support for the ideas that decoupling information and growing dynamic length scales can be at least qualitatively deduced by analyzing the collective relaxation time as a function of temperature and wavevector, and that there is a strong link between dynamic heterogeneity phenomena at the single and many particle level. Though very simple, the model can be applied to other systems, such as molecular liquids. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S18.00015: A genetic algorithm to determine metastable MS1 phase of the Al-Sm system Zhuo Ye, Feng Zhang, Yang Sun, Manh Cuong Nguyen, Mikhail Mendelev, Matthew Kramer, Cai-Zhuang Wang, Kai-Ming Ho An efficient genetic algorithm (GA) was used to determine a metastable Al$_{60}$Sm$_{11}$ phase [termed MS1 in Mater. Sci. Eng. A179--A180, 600 (1994)] that evolves during rapid solidification of an amorphous melt-spun Al-10{\%}Sm alloy. The MS1 phase is of particular interest as it is the 1st observed phase during devitrification and is believed to possess a strong connection to the undercooled liquids. It also presents a severe challenge to theoretical crystal structure prediction methods since it 1) has a big unit cell with a $\sim$1.4 nm, 2) is metastable and not necessarily the ground state, and 3) contains site-occupancy and anti-site defects. A GA combined with experimental characterization of phase transitions and Rietveld refinements provides the necessary identification of the MS1 crystal structure. Calculated X-ray diffraction patterns of the MS1 phase match perfectly with experiments. Interestingly, the MS1 phase shares the same motif as undercooled Al-10{\%}Sm liquids. The topological connection between undercooled liquid and crystal structures is worth further investigation, to understand how the topological order in the starting amorphous phase correlates with phase selection during devitrification. [Preview Abstract] |
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