Closing the gap between experiment and theory: crystal growth by temperature accelerated dynamics

We present atomistic simulations of crystal growth where realistic experimental deposition rates are reproduced, without needing any a priori information on the relevant diffusion processes. Using the temperature accelerated dynamics method, we simulate the deposition of 4 monolayers (ML) of Ag/Ag(100) at the rate of 0.075 ML/s, thus obtaining a boost of several orders of magnitude with respect to ordinary molecular dynamics. In the temperature range analyzed (0-70 K), steering and activated mechanisms compete in determining the surface roughness.     

Direct transformation of vacancy voids to stacking fault tetrahedra

Defect accumulation is the principal factor leading to the swelling and embrittlement of materials during irradiation. It is commonly assumed that, once defect clusters nucleate, their structure remains essentially constant while they grow in size. Here, we describe a new mechanism, discovered during accelerated molecular dynamics simulations of vacancy clusters in fcc metals, that involves the direct transformation of a vacancy void to a stacking fault tetrahedron (SFT) through a series of 3D structures. This mechanism is in contrast with the collapse to a 2D Frank loop which then transforms to an SFT. The kinetics of this mechanism are characterized by an extremely large rate prefactor, tens of orders of magnitude larger than is typical of atomic processes in fcc metals.     

Vacancy formation and strain in low-temperature Cu/Cu(100) growth

The development of compressive strain in metal thin films grown at low temperature has recently been revealed via x-ray diffraction and explained by the assumption that a large number of vacancies were incorporated into the growing films. The results of our molecular dynamics and parallel temperature-accelerated dynamics simulations suggest that the experimentally observed strain arises from an increased nanoscale surface roughness caused by the suppression of thermally activated events at low temperature combined with the effects of shadowing due to off-normal deposition.     

Mechanisms and rates of interstitial H2 diffusion in crystalline C60

Parallel replica dynamics and minimum energy path calculations have been used to study the diffusion mechanisms of H2 in fcc C60. Isolated interstitial H2 molecules bind preferentially in the lattice octahedral (O) sites and diffuse by hopping between O and tetrahedral sites. The simulations reveal an unexpected mechanism involving an H2 molecule diffusing through an already occupied O site, creating an H2 dimer, with a lower activation barrier than diffusion into an empty O site. Kinetic Monte Carlo simulations of a lattice model based on these mechanisms indicate that events involving dimers greatly enhance the self-diffusion rates of interstitial H2 in fcc C60.