Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session S34: Small Molecule Glasses |
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Sponsoring Units: GSOFT Chair: Kate Jensen, Yale University Room: 337 |
Thursday, March 17, 2016 11:15AM - 11:27AM |
S34.00001: Point-to-set correlations and rugged landscapes Sho Yaida, Ludovic Berthier, Patrick Charbonneau, Gilles Tarjus Upon approaching the glass transition a liquid gets sluggish without obvious structural changes. The glassy slowdown is instead attributed to an increasing roughness in the underlying free-energy landscape. Cavity point-to-set (PTS) correlations are real-space tools for characterizing the evolution of this rugged landscape, but their measurement is a serious computational challenge. Here, we first describe how advanced Monte Carlo techniques can be used to dramatically enhance sampling in cavities, extending the range over which PTS correlations can be obtained. By suitably generalizing the notion of PTS correlations to capture any type of growing order in liquids, be it local or amorphous, we then establish a criterion for distinguishing a dynamical slowdown due to critical ordering from one due to glassiness. These methodological advances shed a new light on the interplay between structure and dynamics in model glass formers, and tie in with recent field-theoretic results about the nature of jumps between metastable minima in rough landscapes. [Preview Abstract] |
Thursday, March 17, 2016 11:27AM - 11:39AM |
S34.00002: Evidence for a second-order phase transition to a low-entropy glass C. Patrick Royall, Francesco Turci, Thomas Speck The physics underlying the glass transition is a major outstanding. Central its solution is whether there is some kind of thermodynamic transition to a “ideal glass”, a disordered state with extremely low entropy, or whether in principle a liquid may be supercooled to arbitrary low temperature. Among the challenges that lie in tackling the glass transition are the immense timescales involved. Computer simulation, which might otherwise be able to pick up hints of a thermodynamic transition is limited by the small time-window over which a liquid can be equilibrated. Here we address this challenge using trajectory sampling in a system undergoing a first order nonequilibrium phase transition to a glassy state rich in low-energy geometric motifs. Extrapolation to equilibrium indicates that the transition would occur at a similar temperature at which the ideal glass transition is expected from extrapolation of dynamic and thermodynamic measurements. We further reweight nonequilibrium data to equilibrium leading to configurations representative of extremely low temperature, which indicate a transition to a low energy state at the ideal glass transition temperature. We thus interpret the ideal glass transition as the lower critical endpoint of this nonequilibrium transition. [Preview Abstract] |
Thursday, March 17, 2016 11:39AM - 11:51AM |
S34.00003: Softness Correlations Across Length Scales Robert Ivancic, Amit Shavit, Jennifer Rieser, Samuel Schoenholz, Ekin Cubuk, Douglas Durian, Andrea Liu, Robert Riggleman In disordered systems, it is believed that mechanical failure begins with localized particle rearrangements. Recently, a machine learning method has been introduced to identify how likely a particle is to rearrange given its local structural environment, quantified by \textit{softness}. We calculate the softness of particles in simulations of atomic Lennard-Jones mixtures, molecular Lennard-Jones oligomers, colloidal systems and granular systems. In each case, we find that the length scale characterizing spatial correlations of softness is approximately a particle diameter. These results provide a rationale for why localized rearrangements---whose size is presumably set by the scale of softness correlations---might occur in disordered systems across many length scales. [Preview Abstract] |
Thursday, March 17, 2016 11:51AM - 12:03PM |
S34.00004: Onset of cooperative dynamics in equilibrium glass-forming metallic liquids Abhishek Jaiswal, Yang Zhang Onset of cooperative dynamics has been observed in the metastable regime of many molecular liquids, colloids, and granular materials approaching their respective glass or jamming transition points. It is also considered to play a significant role in the emergence of slow dynamics. However, the nature of such dynamical cooperativity remains elusive in multicomponent metallic liquids characterized by complex many-body interactions and high mixing entropy. Herein, we report indications of the onset of cooperative dynamics in an equilibrium glass-forming metallic liquid (ZrCuNiAl). This is revealed by deviation of the experimentally measured mean diffusion coefficient from its high temperature Arrhenius behavior below $T_{o}$~$\approx $ 1300~K, i.e., a crossover from uncorrelated dynamics above $T_{o}$~to landscape-influenced correlated dynamics below $T_{o}$. The onset/crossover in this system is observed at approximately twice of its calorimetric glass transition temperature ($T_{g}$~$\approx $ 697~K) and in the stable liquid phase, unlike many molecular liquids. Furthermore, we show the presence of such a dynamical onset phenomenon in ten other glass-forming metallic liquids, universally occurring at approximately twice of their $T_{g}$ and in their liquid phases. [Preview Abstract] |
Thursday, March 17, 2016 12:03PM - 12:15PM |
S34.00005: Differences in dynamic heterogeneity in strong and fragile glass formers Hannah Staley, Elijah Flenner, Grzegorz Szamel We study dynamic heterogeneity in a model strong glass former. We examine the spatial extent $\xi_4^a(t)$ and the strength $\chi_4^a(t)$ of the heterogeneity of the dynamics at two length scales $a$. One length scale corresponds to the nearest neighbor separation and the other length scale corresponds to the length scale of the tetrahedral network. We find that the dynamic correlation length $\xi_4^a$ grows much slower with increasing relaxation time at both length scales than for model fragile glass formers. We also find that the dynamically correlated regions are more ramified for the strong glass former than for model fragile glass formers. However, we do find that Stokes-Einstein violation indicates a change in the character of the dynamic heterogeneities for the strong glass former and the fragile glass formers. [Preview Abstract] |
Thursday, March 17, 2016 12:15PM - 12:27PM |
S34.00006: Correlating structural and dynamic fragility in glass-forming liquids Dmitry Voylov, Philip Griffin, Brandon Mercado, Jong Keum, Vladimir Novikov, Alexei Sokolov The glass transition was attracting wide interest over the last several decades, but still remains the topic of intensive research and discussions. One of the most intriguing and well-known observations is a drastic change of dynamic properties with only slight variations of structure upon cooling down to the glass transition temperature Tg. This has led many to believe that the changes of dynamics during approach to Tg have no structural signatures which would be significant and common to different types of glass-forming liquids. Here we demonstrate analysis of temperature dependence of the main diffraction peak in a static structure factor of various glass-formers. We show that the relative changes of its width with temperature correlates with fragility of these materials. This observation was analyzed using Adam-Gibbs approach establishing a connection between the structural and dynamical properties of glass-forming materials. [Preview Abstract] |
Thursday, March 17, 2016 12:27PM - 12:39PM |
S34.00007: Aging and random-field magnetism in ferromagnet/antiferromagnet bilayers. Tianyu Ma, Ryan Freeman, Xiang Cheng, Stefan Boettcher, Sergei Urazhdin Exchange interaction at the interface between a ferromagnet (F) and an antiferromagnet (AF) results in a random effective exchange field acting on both F and AF [1], which can produce complex equilibrium and dynamical states. We utilized anisotropic magnetoresistance to look for signatures of such states in epitaxial Py$=$Permalloy/Fe50Mn50 and polycrystalline CoO/Py bilayers. For thin AF layers, both systems exhibit slow cooperative aging indicative of a complex glassy state [2]. Aging follows the same small power-law or logarithmic dependence and is observed over a wide range of temperatures and fields, suggesting a universal aging mechanism. Glassy relaxation is not observed at any temperature for AF thickness above 3.5nm. We argue that these observations are inconsistent with the usual ``granular'' and ``domain-state'' models of F/AF systems. We discuss the implications of our results for the random field magnetism, and the relationship between the dimensionality and the topological properties of magnetic systems. \begin{enumerate} \item A.P. Malozemoff, Phys. Rev. B 35, 3679(R) (1987). \item T.C. Proctor, D.A. Garanin, and E.M. Chudnovsky, Phys. Rev. Lett. 112, 097201 (2014). \end{enumerate} [Preview Abstract] |
Thursday, March 17, 2016 12:39PM - 12:51PM |
S34.00008: Aging in the two-dimensional random-field systems Xiang Cheng, Tianyu Ma, Sergei Urazhdin, Stefan Boettcher Random fields introduced into the classical Ising and Heisenberg spin models can roughen the energy landscape, leading to complex nonequilibrium dynamics. The effects of random fields on magnetism have been previously studied in the context of dilute antiferromagnets (AF), impure substrates, and magnetic alloys $[1]$. We utilized random-field spin models to simulate the observed magnetic aging in thin-film ferromagnet/antiferromagnet (F/AF) bilayers. Our experiments show extremely slow cooperative relaxation over a wide range of temperatures and magnetic fields $[2]$. In our simulations, the experimental system is coarse-grained into a random field Ising model on a 2D square lattice. Monte Carlo simulations indicate that aging processes may be associated with the glassy evolution of the magnetic domain walls, due to the pinning by the random fields. The scaling of the simulated aging agrees well with experiments. Both are consistent with either a small power-law or logarithmic dependence on time. We further discuss the topological effects on aging due to the dimensional crossover from the Ising to the Heisenberg regime.\\ $[1]$T. Nattermann, Spin glasses and random fields, 12 (1997):277\\ $[2]$ S. Urazhdin, arXiv:1503.08380 (2015)(arxiv.org/pdf/1503.08380.pdf) [Preview Abstract] |
Thursday, March 17, 2016 12:51PM - 1:03PM |
S34.00009: Qualitative change in structural dynamics of some glass-forming systems Vladimir Novikov, Alexei Sokolov Analysis of temperature dependence of structural relaxation time $\tau (T)$ in supercooled liquids revealed a qualitatively distinct feature - a sharp, cusp-like maximum in the second derivative of log $\tau _{\mathrm{\alpha }}(T)$ at some $T_{max}$. It suggests that the super-Arrhenius temperature dependence of $\tau_{\mathrm{\alpha }}(T)$ in glass-forming liquids eventually crosses over to an Arrhenius behavior at \textit{T\textless T}$_{max}$, and there is no divergence of $\tau_{\mathrm{\alpha }}(T)$ at non-zero $T$. $T_{max}$ can be above or below $T_{g}$, depending on sensitivity of $\tau (T)$ to change in liquid's density quantified by the exponent $\gamma $ in the scaling $\tau_{\mathrm{\alpha }}(T)$ \textasciitilde exp($A/T\rho^{\mathrm{-\gamma }})$. These results might turn the discussion of the glass transition to the new avenue -- the origin of the limiting activation energy for structural relaxation at low $T$. [Preview Abstract] |
Thursday, March 17, 2016 1:03PM - 1:15PM |
S34.00010: Percolation Thresholds in Angular Grain media: Drude Directed Infiltration Donald Priour Pores in many realistic systems are not well delineated channels, but are void spaces among grains impermeable to charge or fluid flow which comprise the medium. Sparse grain concentrations lead to permeable systems, while concentrations in excess of a critical density block bulk fluid flow. We calculate percolation thresholds in porous materials made up of randomly placed (and oriented) disks, tetrahedrons, and cubes. To determine if randomly generated finite system samples are permeable, we deploy virtual tracer particles which are scattered (e.g. specularly) by collisions with impenetrable angular grains. We hasten the rate of exploration (which would otherwise scale as $n_{\mathrm{coll}}^{1/2}$ where $n_{\mathrm{coll}}$ is the number of collisions with grains if the tracers followed linear trajectories) by considering the tracer particles to be charged in conjunction with a randomly directed uniform electric field. As in the Drude treatment, where a succession of many scattering events leads to a constant drift velocity, tracer displacements on average grow linearly in $n_{\mathrm{coll}}$. By averaging over many disorder realizations for a variety of systems sizes, we calculate the percolation threshold and critical exponent which characterize the phase transition. [Preview Abstract] |
Thursday, March 17, 2016 1:15PM - 1:27PM |
S34.00011: Influence of Hydrogen Bonding on the Kinetic Stability of Vapor Deposited Triazine Glasses Audrey Laventure, Ankit Gujral, Olivier Lebel, Christian Pellerin, Mark D. Ediger Physical vapor deposition (PVD) can produce glasses with enhanced kinetic stability, high density and anisotropy. However, the influence of hydrogen bonding on these properties has not been fully explored. We vapor deposit a series of triazine derivatives containing functional groups with different H-bonding capability, i.e. NHMe (H-bond donor), OMe (H-bond acceptor) and Et (none) using a wide range of substrate temperatures, from 0.60 to 1.05Tg. PVD glasses of the NHMe derivative have inferior kinetic stability compared to its OMe and Et analogues. This behavior can be rationalized by the higher average number of bonded NH per molecule found in PVD glasses of the NHMe derivative, as quantified by infrared spectroscopy (IR). Despite this difference in H-bonding, IR and wide angle X-ray scattering reveal that all three compounds show a tendency to orient parallel to the substrate at low substrate temperatures. Our results support the hypothesis that strong intermolecular interactions, such as H-bonds, can hinder mobility of the molecules at the interface and thus limit their possibility to sample the potential energy landscape to produce stable glasses. [Preview Abstract] |
Thursday, March 17, 2016 1:27PM - 1:39PM |
S34.00012: Thermal properties of composite materials: a complex systems approximation J. L. Carrillo, Beatriz Bonilla, J. J. Reyes, Victor Dossetti We propose an effective media approximation to describe the thermal diffusivity of composite samples made of polyester resin and magnetite inclusions. By means of photoacoustic spectroscopy, the thermal diffusivity of the samples were experimentally measured. The volume fraction of the inclusions was systematically varied in order to study the changes in the effective thermal diffusivity of the composites. For some samples, a static magnetic field was applied during the polymerization process, resulting in anisotropic inclusion distributions. Our results show a significant difference in the thermal properties of the anisotropic samples, compared to the isotropic randomly distributed. We correlate some measures of the complexity of the inclusion structure with the observed thermal response through a multifractal analysis. In this way, we are able to describe, and at some extent predict, the behavior of the thermal diffusivity in terms of the lacunarity and other measures of the complexity of these samples [1]. [1] F. Cervantes-Alvarez, J J Reyes-Salgado, V Dossetti, and J L Carrillo, J. Phys. D: Appl. Phys. 47 (2014) 235303; J. J. Reyes-Salgado, B. Bonilla, V. Dossetti, and J L Carrillo, J. Phys. D: Appl. Phys. 48, (2015) [Preview Abstract] |
Thursday, March 17, 2016 1:39PM - 1:51PM |
S34.00013: Preparing anisotropic glasses from structural analogs of liquid crystal formers by physical vapor deposition Jaritza Gomez, Mark Ediger Physical vapor deposition (PVD) can be used to tune molecular orientation in glasses by depositing at substrate temperatures (T$_{\mathrm{substrates}})$ just below the glass transition temperature (T$_{\mathrm{g}})$. Glasses of a smectic A liquid crystal (LC) former, itraconazole, deposited at a T$_{\mathrm{substrate\thinspace }}=$ T$_{\mathrm{g}}$ have been shown to inherit the structure of the equilibrium smectic liquid and orient nearly perpendicular to the substrate. Here we report the deposition of glasses prepared from molecules that are structural analogs to known LC formers: posaconazole and a functionalized perylenemonoimide (PMI), analogs to itraconazole and a previously reported columnar LC, respectively. Spectroscopic ellipsometry and infrared spectroscopy are used to characterize average molecular orientation in the as-deposited glasses. Surprisingly, we find that molecular orientation in glasses of posaconazole deposited at different T$_{\mathrm{substrates}}$ does not follow the previously observed trends for linear molecules without LC states, but more closely follows itraconazole. In addition, we find that glasses deposited at T$_{\mathrm{g}}$ are not isotropic, even though liquid-cooled glasses do not show preferential molecular orientation. Similarly, glasses from a functionalized PMI, structural analog to a known columnar LC, show molecular orientation at T$_{\mathrm{substrate\thinspace }}=$ T$_{\mathrm{g}}$. These results may provide insights into the mechanism by which physical vapor deposition can produce glasses with tunable molecular orientation. [Preview Abstract] |
Thursday, March 17, 2016 1:51PM - 2:03PM |
S34.00014: Fragility of Ionic Liquids Measured by Flash Differential Scanning Calorimetry Ran Tao, Eshan Gurung, Edward L. Quitevis, Sindee L. Simon Ionic liquids are a class of materials that possess attractive properties. They generally have low rates of crystallization due to their bulky and asymmetrical ion structure, and are often considered as good glass-forming materials. In this work, a series of imidazolium-based ionic liquids with varying functionalities from aliphatic to aromatic groups and a fixed anion are characterized using fast scanning differential scanning calorimetry. The limiting fictive temperature Tf', which is equivalent to the glass transition temperature Tg, is measured on heating as a function of cooling rate using Flash differential scanning calorimetry. Different calculation methods are employed and compared for the determination of Tf'. The dynamic fragility is obtained for the series of ionic liquids, and using this data along with a compilation of data from the literature reveals the relationship between molecular structure and fragility for ionic liquids. [Preview Abstract] |
Thursday, March 17, 2016 2:03PM - 2:15PM |
S34.00015: Generating tunable structures in glassy materials: Smectic-like layering in glasses of a liquid crystal system prepared by vapor deposition Ankit Gujral, Jaritza Gomez, Jing Jiang, Chengbin Huang, Kathryn O'Hara, Michael Toney, Michael Chabinyc, Lian Yu, Mark Ediger Anisotropic packing, particularly in highly ordered liquid crystalline configurations, has been shown to be useful in organic electronic and optoelectronic applications. In this work, vapor deposited glasses of a model smectic liquid crystal-forming molecule, itraconazole, are investigated. The films are characterized using x-ray scattering, FTIR and spectroscopic ellipsometry, and are found to exhibit unprecedented structural and optical anisotropy for a macroscopically homogeneous solid. A smectic-like layered structure is observed in the glasses that are prepared by depositing the glass at a substrate temperature during deposition (T$_{sub}$) maintained below the glass transition temperature, T$_{g}$, of the molecule. The layer spacing, and the associated average tilt angle of the molecules, is found to be tunable as a function of T$_{sub}$. The layer spacing reduces by 16\% as T$_{sub}$ is lowered. These features are retained in the films when heated to at least T$_{g}$ of the molecule. [Preview Abstract] |
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