2019 Simons Collaboration on Cracking the Glass Problem Annual Meeting

The third annual meeting of the Cracking the Glass Problem collaboration took place March 7–8, 2019, at the Simons Foundation in New York. It gathered together a group of 92 researchers from the United States, Europe, Japan, Brazil and India. It featured stimulating talks by collaboration scientists and distinguished external speakers, and a poster session by the other members of the collaboration. The interactive setting provided by the Simons Foundation enabled an environment that fostered vibrant discussions among the diverse crowd of attending scientists.

The collaboration’s goal is to develop a complete and quantitative description of the glass transition, connecting the explicit and quantitative mean-field (i.e., infinite dimension, d=∞) and zero-temperature theories that have been pioneered by the collaborators. The aim is to develop a similarly quantitative and predictive theory of glassy dynamics in finite dimensions at finite temperatures. The tools developed in creating this framework are having important ramifications in a wide range of fields outside of physics, including computer science (e.g., in machine learning and optimization problems) and biology (e.g., evolutionary population dynamics and glassy behavior of tissues). Over the last two and a half years, the collaboration has obtained major breakthroughs: It obtained and explored the exact infinite-dimensional solution of the dynamics of glassy hard-sphere systems, provided a nearly complete characterization of a new type of amorphous phase transition — namely the Gardner transition — and introduced new powerful algorithms, such as SWAP Monte Carlo, that has opened the door to simulations of supercooled liquids in regimes completely out of reach until now. This latter breakthrough has allowed, among other advances, for the first simulation of the brittle-to-ductile transition and provided convincing evidence of the existence of a finite-temperature ideal glass transition.

Read more of the report in the section below:

Talks

Sidney Nagel
University of Chicago

The State of the Collaboration

Scientists are customarily taught to understand solid materials by treating them as ideal crystals. This approach, however, becomes untenable in an intrinsically disordered material such as a glass: A crystal is an abysmal starting point for understanding the rigidity or excitations of a glass with no obvious long-range order.

In this lecture, Sidney Nagel will discuss how physicists are working to understand glassy matter. He will explain jamming, an alternative starting point for describing solids where order, rather than disorder, is treated as a perturbation. He will then show how physicists have learned to exploit disorder to create solids with unique, varied, textured and tunable behavior including long-range interactions inspired by the allosteric behavior of proteins. Applying these techniques, however, requires computing the response of each bond between particles. Can such detailed computations be avoided? Because a material has a memory of the conditions under which it was prepared and subsequently aged, scientists can now direct aging using Nature’s (as distinct from a computer’s) so-called greedy algorithms to achieve novel kinds of mechanical functionality.

This talk will review the state of the collaboration Cracking the Glass Problem. Nagel will first review the fundamental issues about glasses, jamming and the nature of the amorphous state that initially inspired the collaboration’s work. The collaboration has made great strides in finding solutions to some of these problems, and Nagel will describe what the team has accomplished so far. These include finding new algorithms for extending simulations of glassy dynamics into new regimes, understanding the Garner phase of matter, deriving the dynamics in the mean-field limit and the extension to the creation of metamaterials. These accomplishments have opened up new opportunities for further work. Nagel will give a vision of our plans for the future and where we are going.

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For further information on Professor Nagel’s Lecture, see its event page

Lisa Manning
Syracuse University

How Do Glasses Flow and Fail in Dimensions from Two to Infinity?

Solids resist shear forces, but under enough applied strain, they will yield and deform. We have recently established exact infinite-dimensional solutions for amorphous solids under strain, which predict where a solid will remain stable as a function of strain and density and provide a strong foundation for understanding rheology in glasses. However, infinite-dimensional theories cannot predict the temporal and spatial dynamics of deformation in amorphous solids, which vary dramatically from ductile and homogeneous flow in poorly annealed glasses to brittle fracture in deeply quenched glasses. We now understand that these processes are controlled by quasi-localized excitations that are common in 2-D and 3-D but disappear in higher dimensions. Analytically solvable elasto-plastic models include a mean-field coupling between localized excitations. We have demonstrated that the dynamics of such models are quite similar to those in 2-D and 3-D numerical simulations and indicate that the brittle-to-ductile transition is a random critical point. To make these theories quantitatively predictive, we are currently working to understand how material preparation and shear affect the population of low energy excitations, how the coupling between excitations differs from mean-field and how finite strain rates and temperatures impact these quantities.

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Vincenzo Vitelli
James Franck Institute

Odd Elasticity

Hooke’s law states that the deformations or strains experienced by an elastic object are proportional to the applied forces or stresses. The number of coefficients of proportionality between stress and strain (i.e., the elastic moduli) is constrained by energy conservation. In this talk, we generalize continuum elasticity to active media with nonconservative (or nonreciprocal) microscopic interactions. This generalization, which we dub ‘odd elasticity,’ reveals that two additional elastic moduli can exist in a 2-D isotropic solid with strain-dependent activity. Such an odd-elastic solid can be regarded as a distributed engine: work is locally extracted, or injected, during quasi-static cycles of deformation. By coarse graining illustrative microscopic models, we show how odd elasticity emerges in active metamaterials composed of springs that actuate internal torques in response to strain. Our predictions, corroborated by simulations, uncover phenomena ranging from activity-induced auxetic behavior and buckling, to wave propagation powered by self-sustained active elastic cycles.

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Andrea Liu
University of Pennsylvania

Mechanical Metamaterials and the Malleability of Disorder

Systems with complex landscapes have far more variation in their properties than those with simple ones. This natural variation can be pushed even further by design, allowing us to tune in unusual properties and novel functions into materials. For example, when most materials are stretched in one direction, they tend to shrink in the orthogonal directions. Materials that do the opposite and expand in the orthogonal directions when stretched are ‘auxetic’ and have attracted attention for applications such as high energy absorption. We have found that mechanical spring networks can be tuned easily to the extreme limit of auxetic behavior. Likewise, our collaboration has shown that properties common in living matter, such as the ability of proteins (e.g., hemoglobin) to change their conformations upon binding of an atom (oxygen) or molecule, the ability of the brain’s vascular network to send enhanced blood flow and oxygen to specific areas of the brain associated with a given task, or the ability to retain a memory, can be designed into disordered systems using similar principles. The ability to design properties and functions further gives new insight into the relation between microscopic structure and function that may help us both to understand living systems and to design new biologically inspired materials.

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Marc Mézard
École Normale Supérieure

Statistical Physics and Statistical Inference 

A major challenge of contemporary statistical inference is the large-scale limit, where one wants to discover the values of many hidden parameters, using large amounts of data. In recent years, ideas from statistical physics of disordered systems, notably the cavity method, have helped to develop new algorithms for important inference problems, ranging from community detection to compressed sensing, machine learning (notably neural networks) and generalized linear regression. The talk will review these developments and explain how they can be used, together with the replica method, to identify phase transitions in benchmark statistical ensembles of inference problems.

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Frances Hellman
University of California, Berkeley

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 three 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; Hellman is 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 T³ term (well above the sound velocity-derived Debye contribution) and the linear term in heat capacity, suggesting a common origin.

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Mark Ediger
University of Wisconsin, Madison

Exploring the Limits of Amorphous Packing with Vapor-Deposited Glasses

Glasses formed by cooling a liquid inherit both their structure and their limited stability from the liquid state. In contrast, glasses prepared by vapor deposition can avoid both of these limitations. By utilizing the high mobility present near the free surface of many organic glasses, vapor deposition can build glasses with low-enthalpy, high-density and high-thermal stability. Based upon their position on the potential energy landscape, these materials approach “ideal glass” packing that otherwise could only be achieved by annealing a liquid-cooled glass for thousands or millions of years. Vapor deposition of organic semiconductors produces glasses with improved properties for organic electronics, including the ability to produce anisotropic glasses with a wide range of structures. Remarkably, this “anti-epitaxy” process uses the free-surface structure as its template, rather than the substrate structure. Recent work has shown that optimizing vapor deposition can produce organic light emitting diodes (OLEDs) that are more efficient and have extended lifetimes.

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Jorg Kurchan
LPENS, Paris

Glassy Dynamics

Almost every meaningful scientific question we ask about supercooled liquids and glasses involves dynamics. If we want to control their behavior, we need to understand the way these systems flow, age or respond to external fields or violent stresses. However, there is another aspect about why the study of glassy dynamics is so basic: it has emergent and universal behavior — quantities and rules that transcend the immediate example being studied. These rules are independent of the microscopic structure of any particular material or example. Thus, what we learn by studying the dynamics in glasses and liquids teaches us lessons that are applicable far beyond the original manifestation.

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