Publications

Warm dense matter is the region in phase space of density and temperature where the thermal, Fermi, and Coulomb energies are approximately equal. The lack of a dominating scale and physical behavior makes it challenging to model the physics to high fidelity. For Sandia, a fundamental understanding of the region is of importance because of the needs of our experimental HEDP programs for high fidelity descriptive and predictive modeling. We show that multi-scale simulations of macroscopic physical phenomena now have predictive capability also for difficult but ubiquitous materials such as stainless steel, a transition metal alloy.

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Mechanisms for enhanced low-dose-rate sensitivity are described. In these mechanisms, bimolecular reactions dominate the kinetics at high dose rates thereby causing a sub-linear dependence on total dose, and this leads to a dose-rate dependence. These bimolecular mechanisms include electron-hole recombination, hydrogen recapture at hydrogen source sites, and hydrogen dimerization to form hydrogen molecules. The essence of each of these mechanisms is the dominance of the bimolecular reactions over the radiolysis reaction at high dose rates. However, at low dose rates, the radiolysis reaction dominates leading to a maximum effect of the radiation.

Density functional calculations show that the electric field effect on Si ad-dimer diffusion on Si(0 0 1) is largely a reflection of the position dependence of the ad-dimer's dipole moment. Surface diffusion barriers' dependence on perpendicular electric fields can be used to discriminate between diffusion mechanisms. Since the previously accepted mechanism for ad-dimer diffusion on Si(0 0 1) has the opposite field dependence to what is observed, it cannot be the one that dominates mass-transport. We identify an alternate process, with a similar barrier at zero electric field and field dependence in agreement with measurements. For rotation, calculations to date show linear field dependence, in contrast to experiments. © 2003 Elsevier Science B.V. All rights reserved.

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We change the diffusion mechanism of adsorbed Ge-Si dimers on Si(001) using the electric field of a scanning tunneling microscope tip. By comparing the measured field dependence with first-principles calculations we conclude that, in negative field, i.e., when electrons are attracted towards the vacuum, the dimer diffuses as a unit, rotating as it translates, whereas, in positive field the dimer bond is substantially stretched at the transition state as it slides along the substrate. Furthermore, the active mechanism in positive fields facilitates intermixing of Ge in the Si lattice, whereas intermixing is suppressed in negative fields. © 2003 The American Physical Society.