Ashivni Shekhawat

Imaging Atomic Rearrangements in Two-Dimensional Silica Glass: Watching Silica’s Dance

Glassy Eyed In crystalline materials, the collective motion of atoms in one- and two-dimensional defects—like dislocations and stacking faults—controls the response to an applied strain, but how glassy materials change their structure in response to strain is much less clear. Huang et al. (p. 224; see the Perspective by Heyde) used advanced-transmission electron microscopy to investigate the structural rearrangements in a two-dimensional glass, including the basis for shear deformations and the atomic behavior at the glass/liquid interface. Dynamics of individual atoms in a two-dimensional silicate glass have been observed using transmission electron microscopy. [Also see Perspective by Heyde] Structural rearrangements control a wide range of behavior in amorphous materials, and visualizing these atomic-scale rearrangements is critical for developing and refining models for how glasses bend, break, and melt. It is difficult, however, to directly image atomic motion in disordered solids. We demonstrate that using aberration-corrected transmission electron microscopy, we can excite and image atomic rearrangements in a two-dimensional silica glass—revealing a complex dance of elastic and plastic deformations, phase transitions, and their interplay. We identified the strain associated with individual ring rearrangements, observed the role of vacancies in shear deformation, and quantified fluctuations at a glass/liquid interface. These examples illustrate the wide-ranging and fundamental materials physics that can now be studied at atomic-resolution via transmission electron microscopy of two-dimensional glasses.

From Damage Percolation to Crack Nucleation Through Finite Size Criticality

We present a unified theory of fracture in disordered brittle media that reconciles apparently conflicting results reported in the literature. Our renormalization group based approach yields a phase diagram in which the percolation fixed point, expected for infinite disorder, is unstable for finite disorder and flows to a zero-disorder nucleation-type fixed point, thus showing that fracture has a mixed first order and continuous character. In a region of intermediate disorder and finite system sizes, we predict a crossover with mean-field avalanche scaling. We discuss intriguing connections to other phenomena where critical scaling is only observed in finite size systems and disappears in the thermodynamic limit.

Deformation of Crystals: Connections with Statistical Physics

We give a birds-eye view of the plastic deformation of crystals aimed at the statistical physics community, as well as a broad introduction to the statistical theories of forced rigid systems aimed at the plasticity community. Memory effects in magnets, spin glasses, charge density waves, and dilute colloidal suspensions are discussed in relation to the onset of plastic yielding in crystals. Dislocation avalanches and complex dislocation tangles are discussed via a brief introduction to the renormalization group and scaling. Analogies to emergent scale invariance in fracture, jamming, coarsening, and a variety of depinning transitions are explored. Dislocation dynamics in crystals challenge nonequilibrium statistical physics. Statistical physics provides both cautionary tales of subtle memory effects in nonequilibrium systems and systematic tools designed to address complex scale-invariant behavior on multiple length scales and timescales.

Large-Scale Molecular Dynamics and High-Resolution Transmission Electron Microscopy Study of Graphene Grain Boundaries

Graphene is a promising material for various technological applications, due to its excellent electrical and [1] mechanical properties [2]. New graphene deposition methods are continually increasing the maximum fabricated sheet size of a single crystallographic orientation, but most deposition methods still produce polycrystalline sheets, containing grain boundaries (GBs) [3]. Graphene GBs are very interesting scientifically due to the two-dimensional nature of graphene. Bulk three-dimensional materials require 5 angles to characterize the macroscopic degrees of freedom, while two-dimensional materials require only 2 angles: the misorientation angle θM, defined as the angle between the unit cell vectors of each grain, and the boundary line direction θL, defined as the angle between the boundary vector and the symmetric tilt boundary vector. Thus the parameter space for 2D GBs is far smaller than that of 3D grain boundaries. With modern computer simulation methods, it is now possible to enumerate this entire parameter space in a very fine-grained manner.