"Aftereffects'' Phenomenon in $^{111}$In($\rightarrow$$^{111}$Cd)-Implanted $\alpha$-Al$_2$O$_3$ Single Crystals: Novel Approach Integrating Experimental Double-Model Analysis with Density-Functional Theory

Abstract

We develop an experimental double-model analysis, combined with density-functional theory (DFT), to explore the origins of dynamic hyperfine interactions (HFIs) linked to the electron-capture decay ''aftereffects ''(ECAE) phenomenon. This electronic effect, reversible with temperature, has been observed in time-differential perturbed $\gamma$-$\gamma$ angular correlations (TDPAC) experiments on oxides doped with ($^{111}$In (EC)$\rightarrow$)$^{111}$Cd probe atoms. Besides identifying the electronic configuration that yields the stable final electric-field gradient (EFG) after the dynamic process ends, we determine the initial configurations around the probe nucleus and their corresponding EFGs whose fluctuations produce these dynamic HFIs. We demonstrate the equivalence between parameters of the two most widely used methods for analyzing this type of dynamic HFI, enabling us to obtain these initial electronic configurations at each temperature. In this framework, to unravel controversial TDPAC results reported for $^{111}$In-implanted $\alpha$-Al$_2$O$_3$ single crystals, we perform a DFT study of Cd-doped $\alpha$-Al$_2$O$_3$, examining their defect-formation energies, as functions of the Cd impurity level's charge state. We show that the stable final EFG for the expected interaction HFI$_u$ originates from $^{111}$Cd probes located at defect-free substitutional Al sites (without trapped electron holes) across all measured temperatures. Those of the unexpected HFI$_d$ originate from probes at Al sites, but with different degrees of occupation of the Cd impurity level. We show that one trapped hole for HFI$_u$ and at least five for HFI$_d$ are responsible for the dynamic regime when the ''aftereffects'' are more pronounced. The proposed scenario accounts for the observation of well-defined EFGs when the dynamic regime does not end.