Signature of Kondo hybridisation with an orbital-selective Mott phase in 4 d Ca 2−x Sr x RuO 4

The heavy fermion state with Kondo-hybridisation (KH), usually manifested in f -electron systems with lanthanide or actinide elements, was recently discovered in several 3 d transition metal compounds without f -electrons. However, KH has not yet been observed in 4 d /5 d transition metal compounds, since more extended 4 d /5 d orbitals do not usually form flat bands that supply localised electrons appropriate for Kondo pairing. Here, we report a substitution- and temperature-dependent angle-resolved photoemission study on 4 d Ca 2−x Sr x RuO 4 , which shows the signature of KH. We observed a spectral weight transfer in the γ-band, reminiscent of an orbital-selective Mott phase (OSMP). The Mott localised γ-band induces the KH with an itinerant β-band, resulting in spectral weight suppression around the Fermi level. Our work demonstrates the evolution of the OSMP with possible KH among 4 d electrons, and thereby expands the material boundary of Kondo physics to 4 d multi-orbital systems.