Physics Department Faculty:
Efthimios Kaxiras
Gordon McKay Professor of Applied Physics & Professor of PhysicsPhD 1987, MIT
The behavior of a material – whether the propagation of a crack in a turbine blade or the speed of an electronic switch – is determined by the interaction between valence electrons and ions in a solid. Real solids cannot be described as ideal infinite crystals because they contain many defects (such as surfaces, interfaces, grain boundaries, stacking faults, dislocations, vacancies, interstitials, and so on) whose presence can dramatically affect the properties of a material. Since the 1980s, physicists have developed a theoretical framework for studying the nature and properties of solids, using a quantum mechanical description that does not rely on adjustable parameters. Advances in computational power have made it possible to use this approach for describing realistic solids, including their defects, with remarkable accuracy - even to predict properties of novel materials not yet synthesized in the laboratory.
Efthimios Kaxiras and his coworkers use these theoretical methods to study the properties of solids (including, for example, materials assembled from exotic carbon fullerenes), surfaces, and interfaces. They also employ them to explain the microscopic reasons for ductile or brittle behavior. In recent work, they have applied them, in combination with other techniques, to semiconductor crystal growth and other nonequilibrium phenomena. First, they calculate a few key quantities, such as diffusion and exchange barriers, using the quantum mechanical approach. They then use these quantities in more elaborate, stochastic (Monte Carlo) simulations that make direct contact with experiment.
Kaxiras has used this approach to unravel the features of complex dynamical phenomena such as surfactant-mediated hetero-epitaxial growth of semiconductors. Their analysis correctly reproduces growth instabilities, including the transition from layer-by-layer to island growth, and more detailed microscopic features, such as the fractal nature of islands and their size distribution during growth.