Delicate superconductivity in nodal-line NaAlGe single crystal.

Nodal-line superconductor NaAlSi with a transition temperature (Tc) of 7 K has attracted considerable attention in recent years, whereas its Ge counterpart, NaAlGe, does not superconduct down to the lowest temperature regardless of their similar atomic and electrical structures. To tackle this enigma, we resort to the growth of NaAlGe single crystal and characterize its ground state. Interestingly, when hole doped by oxidation or extracting Na, single-crystalline NaAlGe transforms from a semimetal/semiconductor to a superconductor (Tc=1.8∼3.3 K) with zero resistivity and a diamagnetic shielding fraction over 100%, but without a thermodynamic response in heat capacity. Continuous x-ray diffraction reveals a transient new structure with a largeraxis, which is suggested to have arisen from the minor loss of Na and to be responsible for the emergence of the delicate superconductivity. Our findings place NaAlGe on an equal footing with NaAlSi and provide an alternative for studying the intriguing relationship between superconductivity and nodal-line topology.