Undercompressive Shock Formation during Ion Bombardment of Solid Surfaces
Mark Bradley
Department of Physics, Colorado State University
Date and time:
Tuesday, February 3, 2015 - 4:30pm
Location:
ECOT 226
Abstract:
Bombarding a solid surface with a broad ion beam can produce a remarkable variety of nanoscale patterns. Oblique-incidence bombardment, for example, can lead to the formation of surface ripples with wavelengths as short as 10 nanometers. The anisotropic Kuramoto-Sivashinsky (AKS) equation has traditionally been used to model the formation and propagation of these ripples.
We have introduced an equation of motion for the surface of an ion-bombarded material that differs from the AKS equation by the inclusion of a cubic nonlinearity. Our results establish that this term has a crucial influence on the dynamics --- it can lead to the formation of a terraced topography that coarsens in time, in accord with experimental observations. The rapid variation in the slope at the edge of a terrace is due to the formation of an undercompressive shock. The cubic nonlinearity can also induce a reversal of the ripple propagation direction as the ripple amplitude grows. This could explain why in most experiments the ripples are observed to move in the direction opposite to that predicted by the linear theory.