Experimental evidence of the Kondo effect induced by oxygen vacancies in IrO2 crystals

Abstract

Study of the well-known electrocatalyst iridium dioxide (IrO₂) has been renewed recently because of its interesting topological properties and orbital two-channel Kondo effects. To solidly build up the orbital Kondo effect in IrO₂, we grew the IrO₂ single crystal that is most suitable to elucidate its intrinsic physical properties, and then studied its Kondo effect through electrical transport, magnetic characterizations, and Raman spectroscopy. The electrical transport and magnetoresistance of IrO₂ showed a low-temperature upturn in resistivity together (<25 K) with isotropic magnetoresistance, which are consistent with Kondo-like transport behaviors. Taken together with analyses of magnetic and X-ray photoelectron spectroscopy data, the point defects in IrO₂ appeared to be paramagnetic impurity-oxygen vacancies. These results suggest that IrO₂ has the Kondo effect resulting from oxygen vacancies, rather than the conventional Kondo effect induced by magnetic impurities.