Delicate Superconductivity in NaAlGe Single Crystal

TL;DR

This study demonstrates that hole doping NaAlGe induces superconductivity with a transition temperature around 2.3 to 3.5 K, revealing a delicate superconducting state linked to structural changes, and clarifies the differences with NaAlSi.

Contribution

It provides the first evidence of superconductivity in NaAlGe upon hole doping and links structural modifications to the emergence of superconductivity.

Findings

NaAlGe becomes superconducting after hole doping.

Structural analysis shows a transient phase with a larger c axis.

Superconductivity occurs without a clear heat capacity anomaly.

Abstract

Nodal-line superconductor NaAlSi with a transition temperature ($T_c$) 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 semimetallic ground state. Interestingly, when hole doped by oxidation, single-crystalline NaAlGe transforms from a semimetal to a superconductor ($T_c = 2.3 \sim$ 3.5 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 larger $c$ axis, 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 resolve the controversies in NaAlSi and NaAlGe, while their similarities and differences provide an ideal opportunity to investigate a variety of exciting topological quantum states.