Critical metal-insulator transition due to nuclear quantum effects in Mn-doped GaAs

Mn-doped GaAs exhibits a critical metal-insulator transition at the Mn concentration of ${x}_{\mathrm{crit}}\ensuremath{\approx}1%$. Our self-interaction corrected first principles calculation shows that for Mn concentrations $x\ensuremath{\gtrsim}1%$, hole carriers are delocalized in host valence states, and for $x\ensuremath{\lesssim}1%$, holes tend to be trapped in impurity-band-like states. We further show that for a finite range of concentrations around ${x}_{\mathrm{crit}}$ the system exhibits a nonadiabatic superposition of these states, i.e., a mixing of electronic and nuclear wave functions. This means that the phase transition is continuous, and its criticality is caused by quantum effects of the atomic nuclei. In other words, the apparently electronic phase transition from the insulator to metal state cannot be described by electronic effects alone.