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Low-energy quantum dynamics of atoms at defects; interstitial oxygen in silicon

The problem of the low-energy highly anharmonic quantum dynamics of isolated impurities in solids is addressed by using path-integral Monte Carlo simulations. Interstitial oxygen in silicon is studied as a prototypical example showing such a behaviour. The assignment of a `geometry' to the defect is discussed. Depending on the potential (or on the impurity mass), there is a `classical' regime, where the maximum probability density for the oxygen nucleus is at the potential minimum. There is another regime, associated with highly anharmonic potentials, where this is not the case. The two regimes are separated by a sharp transition. Also, the decoupling of the many-nuclei problem into a one-body Hamiltonian to describe the low-energy dynamics is studied. The adiabatic potential obtained from the relaxation of all of the other degrees of freedom at each value of the coordinate associated with the low-energy motion gives the best approximation to the full many-nuclei problem.