LOW-ENERGY QUANTUM DYNAMICS OF ATOMS AT DEFECTS - INTERSTITIAL OXYGENIN SILICON

The problem of the low-energy highly anharmonic quantum dynamics of is olated impurities in solids is addressed by using path-integral Monte Carlo simulations. Interstitial oxygen in silicon is studied as a prot otypical example showing such a behaviour. The assignment of a 'geomet ry' to the defect is discussed. Depending on the potential (or on the impurity mass), there is a 'classical' regime, where the maximum proba bility density for the oxygen nucleus is at the potential minimum. The re is another regime, associated with highly anharmonic potentials, wh ere this is not the case. The two regimes are separated by a sharp tra nsition. Also, the decoupling of the many-nuclei problem into a one-bo dy Hamiltonian to describe the low-energy dynamics is studied. The adi abatic potential obtained from the relaxation of all of the other degr ees of freedom at each value of the coordinate associated with the low -energy motion gives the best approximation to the full many-nuclei pr oblem.