Reactive Path Ensembles within Nonequilibrium Steady States
Rare reactive events encapsulate a range of phenomena relevant to chemical and statistical physics. While their study has been simplified through decades of computational and theoretical work, the field is predominantly informed by systems in thermodynamic equilibrium. In this talk, I’ll discuss two specific problems in the intersection of kinetics and nonequilibrium statistical mechanics to illuminate reactions and reactive path ensembles in nonequilibrium steady states. In the first part, I’ll characterize why the addition of nonequilibrium activity tends to accelerate kinetics using insights from our recent work on stochastic optimal control theory and spectral theory. In the second part, I’ll consider the shear-induced folding and unfolding transition of a polymer to illustrate how the quintessential notion of reactive processes as barrier-crossing events is insufficient out of equilibrium, due to the emergence of currents that can couple to the reactive mode and modulate the reaction.
Aditya Singh is a 4th-year Chemistry Ph.D. student in the David Limmer group. Aditya received a B.S. in Chemistry and Computer Science from the University of Wisconsin-Madison, where he researched on investigating the complexation and coacervation of polyelectrolytes using theoretical and computational methods. Aditya’s work focuses on developing theoretical and computational methods to investigate rare reactive events with a specific focus geared toward systems in nonequilibrium steady states.
First Principle Studies of Cation Disorder and Point Defect in Semiconductors
Defects in semiconductors can act as a knob for tuning properties or as an undesirable feature affecting semiconductor performance. The understanding of defects often requires combinatorial studies of experiments and theory. In this talk, I will discuss two computational studies of defects, the role of cation disorder in noval photoactive ZnTiN2, and the accurate point defect level prediction using non-empirical hybrid functional.
Cation disorder is often found in divalent ternary nitride semiconductors, with significant consequences for optoelectronic properties. For example, recently synthesized wurtzite-derived ZnTiN2 exhibits cation disorder and has a measured band gap of about 2 eV, 1.5 eV smaller than that predicted for its cation-ordered structure by density functional theory (DFT) calculations with hybrid functionals. Using state-of-the-art first principles DFT calculations, we demonstrate that cation disorder in ZnTiN2 creates locally charge-unbalanced chemical environments that further introduce states near band edges, leading to band gap reduction. We study how the cation disorder affects the local chemical environment, energetics, electronic structure, and optoelectronic properties in ZnTiN2.
Despite Kohn-Sham DFT is widely used to study defects, semilocal functionals usually fail to predict deep mid-gap defects due to its tendency of delocalization. Here, we apply a recently developed nonempirical Wannier-localized optimally-tuned screened range-separated hybrid (WOT-SRSH) functional to properties of the oxygen vacancy in ZnO. We show that in addition to quantitatively capturing the wurtzite ZnO band gap, our WOT-SRSH calculations predict an accurate defect level for oxygen vacancy. Moreover, we show the bulk WOT-SRSH
Sijia Ke is a 4th year Materials Science and Engineering PhD student in Jeffrey Neaton’s group in Physics department. Her research focus is to use ab-initio calculations to study electronic structure and optical properties of complex solid materials. She obtained her bachelor’s degree in Physics at the University of Science and Technology of China.