Dynamic hyperfine interactions in (111In→)111Cd-doped SnO2 and SnO semiconductors

Abstract

In the PAC literature, the so-called “after-effects” phenomenon refers to the electronic relaxations processes of a probe-atom following the electron-capture (EC) nuclear-decay of its radioactive parent. This effect has been observed in PAC experiments on semiconducting and insulating oxides doped with 111In(→111Cd), through the existence of dynamic hyperfine interactions (i.e., with time-dependent anisotropies), dependent and reversible with measuring temperature. We present here results of PAC experiments in (111In →)111Cd-doped SnO2 and SnO that have been successfully analyzed with a time-dependent on-off model for the perturbation factor. These results showed combined dynamic plus static hyperfine interactions whose electric-field gradients (EFG) were associated to different stable electronic configurations close to the 111Cd nucleus. The dynamic regime is originated in fast fluctuations between these different electronic configurations. Density functional theory-based electronic-structure calculations of Cd-doped SnO2 and SnO were performed as a function of the charge state of the impurity. In SnO2, the impurity introduces a double acceptor level in the top of the valence band together with isotropic outward relaxations of the nearest oxygen neighbors. The variation of the calculated EFG tensor as a function of the charge state of the Cd impurity level shows an interesting behavior that explains the experimental results, giving strong support from first principles to the electron-capture after-effects proposed scenario. A similar ab initio analysis in Cd-doped SnO, in which Cd is nominally an isovalent impurity, also gave account succesfully of the experimental results. The EC decay of 111In to 111Cd and the electric nature of the host are shown to contribute to the existence of these types of time-dependent hyperfine interactions, showing that the acceptor character of the impurity seems not to be a necessary condition to produce after effects at least in Sn oxides.