Session H40: Soft Excitations in Glasses and Jammed Solids

Suppressed beta relaxations and reduced heat capacity in ultrastable organic glasses prepared by physical vapor deposition

Glasses play an important role in technology as a result of their macroscopic homogeneity (e.g., the clarity of window glass) and our ability to tune properties through composition changes. A problem with liquid-cooled glasses is that they exhibit marginal kinetic stability and slowly evolve towards lower energy glasses and crystalline states. In contrast, we have shown that physical vapor deposition can prepare glasses with very high kinetic stability. These materials have properties expected for ``million-year-old'' glasses, including high density, low enthalpy, and high mechanical moduli. We have used nanocalorimetry to show that these high stability glasses have lower heat capacities than liquid-cooled glasses for a number of molecular systems. Dielectric relaxation has been used to show that the beta relaxation can be suppressed by nearly a factor of four in vapor-deposited toluene glasses, indicating a very tight packing environment. Consistent with this view, computer simulations of high stability glasses indicate reduced Debye-Waller factors. These high stability materials raise interesting questions about the limiting properties of amorphous packing arrangements.   

Ideality and Tunneling Level Systems (TLS) in amorphous silicon films.

Heat capacity, sound velocity, and internal friction of covalently bonded amorphous silicon (a-Si) films with and without hydrogen show that low energy excitations commonly called tunneling or two level systems (TLS) can be tuned over nearly 3 decades, from below detectable limits to the range commonly seen in glassy systems. This tuning is accomplished by growth temperature, thickness, growth rate, light soaking or annealing. We see a strong correlation with atomic density in a-Si and in literature analysis of other glasses, as well as with dangling bond density, sound velocity, and bond angle distribution as measured by Raman spectroscopy, but TLS density varies by orders of magnitude while these other measures of disorder vary by less than a factor of two. The lowest TLS films are grown at temperatures near 0.8 of the theoretical glass transition temperature of Si, similar to work on polymer films and suggestive that the high surface mobility at relatively low temperature of vapor deposition can produce materials close to an ideal glass, with higher density, lower energy, and low TLS due to fewer nearby configurations with similarly low energy. The TLS measured by heat capacity and internal friction are strongly correlated for pure a-Si, but not for hydrogenated a-Si, suggesting that the standard TLS model works for a-Si, but that a-Si:H possess TLS that are decoupled from the acoustic waves measured by internal friction. Internal friction measures those TLS that introduce mechanical damping; we are in the process of measuring low T dielectric loss which yield TLS with dipole moments in order to explore the correlation between different types of TLS. Additionally, a strong correlation is found between an excess T$^{\mathrm{3\thinspace }}$term (well above the sound velocity-derived Debye contribution) and the linear term in heat capacity, suggesting a common origin.   

Breakdown of elasticity in low temperature amorphous solids

Understanding how a system responds to external perturbations is crucial to investigate the statistical correlations between the microscopic degrees of freedom. Close to a second order phase transition, long range correlations show up in the divergence of properly defined susceptibilities. In this talk I will explore this paradigm to investigate the nature of correlations in amorphous solids. The solution of structural glass models in high dimension predicts that deep in the glass phase a second order glass-to-glass transition arises at low enough temperature. I will thus investigate the behavior of elastic responses on approaching this phase transition showing that standard elasticity breaks down at the critical point. In the low temperature phase the elastic response is history and time dependent. This may clarify the very jerky nature of stress-strain curves of low temperature amorphous solids.\\ \\In collaboration with Giulio Biroli.   

Exploring relaxation pathways in rheology and aging of jammed soft solids

Stress heterogeneities frozen-in upon solidification, elasticity, structural disorder and thermal fluctuations conspire to select, in jammed soft solids, the microscopic relaxation pathways that ultimately determine their mechanical response as well as their progressive aging over time. We have used 3d numerical simulations of model solids to unravel such interplay and gain new insights into the nature of the relaxation pathways, their relevant time and lengthscales, and their dependence on the nature of the stress fluctuations in the material. I’ll review our recent findings and discuss their implications for soft excitations in different types of jammed soft solids.   

Soft modes in the perceptron model for jamming.

I will show how a well known neural network model —the perceptron— provides a simple solvable model of glassy behavior and jamming. The glassy minima of the energy function of this model can be studied in full analytic detail. This allows the identification of two kind of soft modes the first ones associated to the existence a marginal glass phase and a hierarchical structure of the energy landscape, the second ones associated to isostaticity and marginality of jamming. These results highlight the universality of the spectrum of normal modes in disordered systems, and open the way toward a detailed analytical understanding of the vibrational spectrum of low-temperature glasses.