Hyperfine Clock Transitions of Bismuth Donors in Silicon Detected by   Spin Dependent Recombination
  Bismuth donors ion-implanted in $^{28}$Si and $^\text{nat}$Si are studied using magnetic resonance spectroscopy based on spin dependent recombination. The hyperfine clock transition, at which the linewidth is significantly narrowed, is observed for the bismuth donors. The experimental results are modeled quantitatively by molecular orbital theory for a coupled pair consisting of a bismuth donor and a spin dependent recombination readout center, including the effect of hyperfine and Zeeman interactions. 
Pierre-Andr\'e Mortemousque, Simon Berger, Takeharu Sekiguchi,   Christophe Culan, Robert G. Elliman, Kohei M. Itoh
cond-mat.mtrl-sci