Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session T30: Disordered and Glassy Systems (non-polymeric) |
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Sponsoring Units: DCMP Chair: Nicolas Giovambattista, Brooklyn College Room: 338 |
Thursday, March 21, 2013 8:00AM - 8:12AM |
T30.00001: Two-State ``Hopping'' Dynamics in Molecular Liquids and Glasses Marcus Cicerone, Qin Zhong, Madhusudan Tyagi, Joseph Curtis, Devin Averett, Juan de Pablo Hopping has long been suspected as an important mode of transport in supercooled liquids at temperatures below $T_c$. It has been observed in model systems, but until now, has not been directly observed in molecular liquids. We show that incoherent quasi-elastic neutron scattering (QENS) reveals a two-state scenario where, on a 1 ps timescale, molecules are either confined to motion on a lengthscale of 0.05 $r_H$, or free to undergo motion on a much larger lengthscale of roughly 0.3 $r_H$, where $r_H$ is the hydrodynamic radius. The motion executed by the less-constrained molecules fits the description of hopping motion observed in model simulations and colloid experiments. The population free to he latter giving rise to hopping at low temperature where the mobile states are long-lived. We show also that this two-state scenario holds well above $T_c$, where the mobile state lifetime exhibits apparently universal behavior, and transport appears to proceed by both small-step diffusion and larger-step ``hopping'' processes. Our interpretation of the neutron scattering data is confirmed by atomistic MD simulations, which reveal additional richness, and suggest that this very short-time two-state behavior may be the precursor to dynamic heterogeneity as observed on longer timescales. [Preview Abstract] |
Thursday, March 21, 2013 8:12AM - 8:24AM |
T30.00002: Models of two level systems for anisotropic glassy materials Dragos-Victor Anghel, Irina Mihaela Dumitru, Alexandru George Nemnes, Dmitrii Churochkin We use an extended version of the standard tunneling model to explain the sound absorption in anisotropic glassy materials and heat transport in mesoscopic slabs and bridges. The glassy properties are determined by an ensemble of two level systems (TLS). In our model a TLS is characterized by a $3\times3$ symmetric tensor, $[T]$, which couples to the strain field, $[S]$, through a $3\times3\times3\times3$ tensor of coupling constants, $[[R]]$. The structure of $[[R]]$ reflects the symmetry of the host lattice. We also propose microscopic theoretical methods and models of TLS by which we test some of the most well known models of glassy materials, together with our own model. [Preview Abstract] |
Thursday, March 21, 2013 8:24AM - 8:36AM |
T30.00003: ABSTRACT WITHDRAWN |
Thursday, March 21, 2013 8:36AM - 8:48AM |
T30.00004: Configurational excitations of simple liquids Takuya Iwashita, Takeshi Egami The dynamics of glass-forming liquids has not been fully understood at the atomic-scale level, even for normal liquids because the basic mechanism regarding to liquid dynamics remain unknown. An elementary process of liquids, in which an atom loses or gains one of its nearest neighbors, was studied using MD simulations of various metallic liquids at high temperatures. The result was presented in terms of Maxwell relaxation time, represented by viscosity/G, and the lifetime of local topology of atomic connectivity. Above crossover temperature, TA, the Maxwell relaxation time is almost equal to the lifetime of local topology, suggesting the topological excitation as the elementary excitation in high temperature liquid metal. We also showed that the TA may be associated with the propagation of transverse sound wave beyond an atomic shell. Below TA the Maxwell relaxation time becomes larger than the lifetime of local topology. This result implies an importance of the interaction between local configulational excitations in the supercooled state. [Preview Abstract] |
Thursday, March 21, 2013 8:48AM - 9:00AM |
T30.00005: Supercooled Liquids with Enhanced Orientational Order Michael W\"{u}bbenhorst, Simona Capponi, Simone Napolitano The nature of the glass transition, the transformation of a liquid into a disordered solid, still remains one of the most intriguing unsolved problems in materials science. Recent models rationalize crucial features of vitrification with the presence of medium-range ordered regions coexisting with the isotropic liquid. In lines with this prediction, here we report an extraordinary enhancement in bond orientational order (BOO) in ultrathin films of supercooled polyols, grown by physical vapour deposition. By varying the deposition conditions and the molecular size, we could tune the kinetic stability of the liquid phase enriched in BOO towards conversion into the ordinary liquid phase. We observed a strong increase in the dielectric strength with respect to the ordinary supercooled liquid and slower structural dynamics, suggesting the existence of a metastable liquid phase with improved orientational correlations[1]. [1] 3. S. Capponi, S. Napolitano, and M. W\"{u}bbenhorst, Nat. Commun. doi: 10.1038/ncomms2228 (2012). [Preview Abstract] |
Thursday, March 21, 2013 9:00AM - 9:12AM |
T30.00006: The Kinetics of the Glass Transition and Physical Aging in Germanium Selenide Glasses Haoyu Zhao, Yung Koh, Sindee Simon, Sabyasachi Sen The kinetics associated with the glass transition is investigated using differential scanning calorimetry (DSC) for germanium selenide glasses with Ge content ranging from 0 to 30 atom{\%} Ge and mean coordination numbers ranging from 2.0 to 2.6. As Ge content increases, the glass transition region broadens and the step change in heat capacity at Tg decreases. As a result of physical aging, enthalpy overshoots are observed in DSC heating scans and the corresponding change in enthalpy can be calculated as a function of aging time. The enthalpy loss on aging linearly increases with the logarithm of aging time and then levels off at an equilibrium value that increases with decreasing aging temperature. The time required to reach equilibrium increases with decreasing aging temperature and, at a given temperature, with decreasing germanium content. The results indicate that all samples show expected physical aging behavior, and no evidence for a Boolchand intermediate phase characterized by high stability and absence of physical aging is found. [Preview Abstract] |
Thursday, March 21, 2013 9:12AM - 9:24AM |
T30.00007: Dynamical and structural heterogeneities close to liquid-liquid phase transitions: The case of gallium Alex Antonelli, Samuel Cajahuaringa, Maurice de Koning Liquid-liquid phase transitions (LLPT) have been proposed in order to explain the thermodynamic anomalies exhibited by some liquids. Recently, it was found, through molecular dynamics simulations, that liquid elemental gallium, described by a modified embedded-atom model, exhibits a LLPT between a high-density liquid (HDL) and a low-density liquid (LDL), about 60 K below the melting temperature. In this work [1], we studied the dynamics of supercooled liquid gallium close to the LLPT. Our results show a large increase in the plateau of the self-intermediate scattering function ($\beta$-relaxation process) and in the non-Gaussian parameter, indicating a pronounced dynamical heterogeneity upon the onset of the LLPT. The dynamical heterogeneity of the LDL is closely correlated to its structural heterogeneity, since the fast diffusing atoms belong to high-density domains of predominantly 9-fold coordinated atoms, whereas the slow diffusing ones are mostly in low-density domains of 8-fold coordinated atoms. The energetics suggests that the reason for the sluggish dynamics of LDL is due to its larger cohesive energy as compared to that of the HDL. [1] S. Cajahuaringa, M. de Koning, and A. Antonelli, J. Chem. Phys. $\textbf{136}$, 064513 (2012). [Preview Abstract] |
Thursday, March 21, 2013 9:24AM - 9:36AM |
T30.00008: Pressure Dependence of the Glass Transition Temperature in the Fragile Glass Former Cumene Tim Ransom, William Oliver The glass transition temperature, T$_{g}$, is one of the most important characteristics of glassy systems. While T$_{g}$ has been measured for many systems at atmospheric pressure, direct measurement of the glass transition is difficult at high pressures due to small sample sizes and long time scales. T$_{g}$(P) measurements to date mostly involve extrapolations of high-pressure viscosity or relaxation data to $\eta $~$=$~10$^{13}$~P~or t~$=$~100~s, respectively. In this study we present direct measurement of T$_{g}$ at pressures up to several GPa through a combination of pressure gradient tracking and observation of increases in the thermal expansion coefficient upon heating from the glass to the viscous liquid state. High pressures are attained through the use of a diamond anvil cell and precise temperatures are maintained via custom heating and cryogenic systems. By directly mapping this phase boundary, we can compare models for T$_{g}$(P). In addition, high-pressure analysis requiring knowledge of T$_{g}$ at pressure will be greatly aided. [Preview Abstract] |
Thursday, March 21, 2013 9:36AM - 9:48AM |
T30.00009: Average Oscillator Strength Per State of a one-dimensional disordered Frenkel exciton system in the Coherent Potential Approximation Abdelkrim Boukahil, Robert Siemann, David Huber We report the results of studies of the low energy side of the Average Oscillator Strength Per State $f(E) = F(E)/\rho(E)$, where $F(E)$ is the line shape function and $\rho(E)$ is the density of states function of one dimensional Frenkel excitons in the Coherent Potential Approximation (CPA). A Gaussian distribution of the transition frequencies with rms width $\sigma $ ($0.07\le \sigma \le 0.4)$ is used. Our CPA theory predicts that on the low energy side of the peak the tails are short and independent of the disorder parameter $\sigma $; implying a behavior consistent with the Urbach rule. Our CPA results are in excellent agreement with previous investigations. [Preview Abstract] |
Thursday, March 21, 2013 9:48AM - 10:00AM |
T30.00010: Atomistic Modeling of Mechanical Loss in Amorphous Oxides Rashid Hamdan, Jonathan Trinastic, Hai-Ping Cheng The mechanical and optical loss in amorphous solids, described by the internal friction and light scattering susceptibility are investigated using classical, atomistic molecular dynamics simulation. We implemented the trajectory bisection method and the non-local ridge method in DL-POLY molecular dynamics simulation software. These methods were used to locate the different local potential energy minima that a system visits through an MD trajectory and the transition state between any two consecutive minima. From the distributions of the barrier height and asymmetry, and the relaxation time of the different transition states we calculated the internal friction of pure amorphous silica and mixed oxides. [Preview Abstract] |
Thursday, March 21, 2013 10:00AM - 10:12AM |
T30.00011: The nature of the $\beta$-peak in the loss modulus of amorphous solids Yossi Cohen, Smarajit Karmakar, Itamar Procaccia, Konrad Samwer Glass formers exhibit, upon an oscillatory excitation, a response function whose imaginary and real parts are known as the loss and storage moduli respectively. The loss modulus typically peaks at a frequency known as the $\alpha$ frequency which is associated with the main relaxation mechanism of the super-cooled liquid. In addition, the loss modulus is decorated by a smaller peak, shoulder or wing which is referred to as the $\beta$-peak. The physical origin of this secondary peak had been debated for decades, with proposed mechanisms ranging from highly localized relaxations to entirely cooperative ones. Using numerical simulations, we expose a clear and unique cooperative mechanism for the said $\beta$-peak which is distinct from that of the $\alpha$-peak. [Preview Abstract] |
Thursday, March 21, 2013 10:12AM - 10:24AM |
T30.00012: Density of Surface States in a-Si/Ge Using a Two Parameter Hamiltonian Eliezer Richmond To rigorously investigate the contribution of surfaces to the density of electronic states of a-Si/Ge and the effect of the topology on the density of surface states (DOS), a surface for amorphous homopolar tetrahedral solids is defined. The density of unsaturated bonds is 0.106 bonds/{\AA}$^{2}$. Reconstruction enables a 88{\%} reduction in the density of unsaturated bonds. The effects on the DOS in the valence band and energy gap is investigated using a two parameter Hamiltonian. The local and configuration DOS are computed for the unsaturated bond and the four back bond hybrids. The ring structure effects the DOS in the valence band, but not the more localized energy gap states. The spectral feature due to surface atoms with only one unsaturated bond is affected by the topology. The antibonding spectral feature in the energy gap deriving from surface atoms with 2 or 3 unsaturated bonds is independent of all topological effects while the bonding spectral feature from these same surface atoms is not. Comparison with empirical results verifies the contribution of the unsaturated bonds to ESR signals and elucidates the origin of the subtle valence band features in UPS spectra. [Preview Abstract] |
Thursday, March 21, 2013 10:24AM - 10:36AM |
T30.00013: Fluctuating Mobility Generation and Transport in Glasses Apiwat Wisitsorasak, Peter Wolynes Complex spatiotemporal structures developing in glasses during aging and heating processes involve the interplay between fluctuating mobility generation and transport. To understand these structures, we extend mode-coupling theory to inhomogeneous system and combine the theory with activated events within the framework of Random-First Order Transition theory of glasses. We explore using numerical methods the process of fluctuating mobility generation and transport in glasses as the glasses age after cooling and as they rejuvenate after heating. This scheme allows us to investigate the dynamical heterogeneity in glasses below the glass transition temperature. We found a growing length scale and an increasing relaxation time upon the aging process. On the contrary, in the rejuvenating process, the mobility propagates from the high mobility at free surfaces into the bulks which resembles flame front propagation in combustion theory. [Preview Abstract] |
Thursday, March 21, 2013 10:36AM - 10:48AM |
T30.00014: Many-body localization in one dimension as a dynamical renormalization group fixed poin Ronen Vosk, Ehud Altman We formulate a dynamical real space renormalization group approach to describe the time evolution of a random spin-1/2 chain, or interacting fermions, initialized in a state with fixed particle positions. Within this approach we identify a many-body localized state of the chain as a dynamical infinite randomness fixed point. Near this fixed point our method becomes asymptotically exact, allowing analytic calculation of time dependent quantities. In particular we explain the striking universal features in the growth of the entanglement seen in recent numerical simulations: unbounded logarithmic growth delayed by a time inversely proportional to the interaction strength. Lack of true thermalization in the long time limit is attributed to an infinite set of approximate integrals of motion revealed in the course of the RG flow, which become asymptotically exact conservation laws at the fixed point. Hence we identify the many-body localized state with an emergent generalized Gibbs ensemble. Within the RG framework we show that long range resonances are irrelevant at strong randomness, and formulate a criterion for when they do become relevant and may cause a delocalization transition. [Preview Abstract] |
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