Integrated Calendar

Grain boundaries (GBs) are the 2D interfaces between crystals of the same material with different orientations. The dynamics of GBs is central to both microstructure evolution and the mechanics of polycrystals. GB dynamics are largely controlled by the motion of line defects that are constrained to lie in the GB. These line defects, known as disconnections, have both dislocation character (Burgers vector) and step character (step height). Possible Burgers vectors and step heights are completely determined by crystallography (i.e., crystal structure and the relative orientations of the two grains). In this talk, I will discuss disconnections, their crystallography, their nucleation and motion, and present a statistical mechanics-based description of a wide range of GB properties based on disconnection dynamics. In particular, I will discuss the thermal roughening of GBs, the migration of GBs, GB shear coupling, and how GBs interact with with applied stresses and compare these predictions with both molecular dynamics and experimental results. I will end by describing the remaining challenges in developing a quantitative approach to the microstructure evolution of polycrystalline materials.

Mesoscopic physics, pioneered by Joe Imry nearly 4 decades ago, explores the behavior of matter on length scales where dimensionality, coherence, and interactions compete to produce material properties that are fundamentally different from their bulk counterparts. For example, the conventional wisdom of superconductivity, developed in 1957 by Bardeen, Cooper and Schrieffer (BCS) describes this state in terms of a condensate of electron pairs arranged in a spatially isotropic wave function with no net momentum or angular momentum (a spin-singlet configuration). However, on mesoscopic length scales entirely different types of superconductivity may be realized such as unconventional pairing where electrons are arranged in triplet rather than singlet configurations. Such superconductors
may enable dissipationless transport of spin and may also give rise to elementary excitations that do not obey the conventional Fermi or Bose statistics but rather have non-Abelian statistics where the exchange of two particles transforms the state of the system into a new quantum mechanical state.
In this talk I will describe some of our recent work that explores the proximity effect between a conventional superconductor and a semiconductor with strong spin-orbit interaction. Using supercurrent interference, we show that we can tune the induced superconductivity
continuously from conventional to unconventional, that is from singlet to triplet. Our results open up new possibilities for exploring unconventional superconductivity as well as provide an exciting new pathway for exploring non-Abelian excitation.