Extreme Reduction of the Spin-Orbit Splitting of the Deep Acceptor Ground State of ${\mathrm{Zn}}_{\mathit{S}}^{-}$ in Si

Electric-dipole spin resonance of the deep acceptor ${\mathrm{Zn}}_S^{-}$ in Si reveals close ${\Gamma }_8$ and ${\Gamma }_7$ ground states with zero-field separation of only 0.31 meV as compared to the 43 meV of the two valence bands. With Landé's formula for the $g$ factors of a ${}_{}{}^2{}_{}{}^{}T_2^{}{}_{}{}^{}$ state split by spin-orbit interaction into ${\Gamma }_8$ and ${\Gamma }_7$ this nearness can be interpreted as strong quenching of the orbital moment. The observed dependence on the Zn isotopic mass indicates a dynamic contribution of the acceptor atom to the electronic state as is expected for a Jahn-Teller effect.