Impurity effects on the superconductivity in RuO₂ thin films

RuO₂ has attracted salient interest as a representative candidate for the altermagnetic ordered state. Although the magnetic order of this material is still under debate, some experiments have reported that the ground state of RuO₂ thin films grown on a TiO₂ substrate is superconducting. Therefore, RuO₂ thin films are an intriguing platform for investigating the relationship between altermagnetic and superconducting orders.

In this study, we try to elucidate the superconducting symmetry of the RuO₂ thin films through impurity effects. Impurity effects are a powerful probe to distinguish between isotropic conventional superconductivity and anisotropic unconventional superconductivity. While conventional superconductivity is immune to nonmagnetic impurities, unconventional superconductivity is strongly suppressed by disorders. For the systematic control of impurity concentrations, we apply high-energy electron irradiation, which can create Frenkel pairs in solids that act as point defects.

Performing the electrical resistivity measurements on the pristine and irradiated RuO₂ thin films, we confirmed that the superconducting transition temperature is systematically suppressed by the irradiation. Based on these results, we discuss the pairing symmetry and possible relationship between altermagnetic and superconducting states.