  The nodal-line semimetals NaAlSi and NaAlGe have significantly different
ground states despite having similar electronic structures: NaAlSi exhibits
superconductivity below 7 K, while NaAlGe exhibits semiconductive electrical
conductivity at low temperatures, indicating the formation of a pseudogap at
approximately 100 K. The origin of the pseudogap in NaAlGe is unknown but may
be associated with excitonic instability. We investigated hole-doping effects
on the ground state in the solid solution Na(Al1-xZnx)Ge and discovered that
the pseudogap is suppressed continuously with increasing Zn content, followed
by the appearance of a superconducting dome with the highest transition
temperature of 2.8 K. This superconductivity most likely results from excitonic
fluctuations.
