Line Broadening and Decoherence of Electron Spins in Phosphorus-Doped   Silicon Due to Environmental 29^Si Nuclear Spins
  Phosphorus-doped silicon single crystals with 0.19 % <= f <= 99.2 %, where f is the concentration of 29^Si isotopes, are measured at 8 K using a pulsed electron spin resonance technique, thereby the effect of environmental 29^Si nuclear spins on the donor electron spin is systematically studied. The linewidth as a function of f shows a good agreement with theoretical analysis. We also report the phase memory time T_M of the donor electron spin dependent on both f and the crystal axis relative to the external magnetic field. 
Eisuke Abe, Akira Fujimoto, Junichi Isoya, Satoshi Yamasaki, and Kohei   M. Itoh