Evolution of Electronic States and Emergence of Superconductivity in the Polar Semiconductor GeTe by Doping Valence-Skipping Indium

GeTe is a chemically simple IV--VI semiconductor which bears a rich plethora of different physical properties induced by doping and external stimuli. Here, we report a superconductor-semiconductor-superconductor transition controlled by finely-tuned In doping. Our results reveal the existence of a critical doping concentration ${x}_{c}=0.12$ in ${\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{In}}_{x}\mathrm{Te}$, where various properties, including structure, resistivity, charge carrier type, and the density of states, take either an extremum or change their character. At the same time, we find indications of a change in the In-valence state from ${\mathrm{In}}^{3+}$ to ${\mathrm{In}}^{1+}$ with increasing $x$ by core-level photoemission spectroscopy, suggesting that this system is a new promising playground to probe valence fluctuations and their possible impact on structural, electronic, and thermodynamic properties of their host.