Octahedral tilt driven Kondo lattice behavior with orbital-selective Mott transition of 4d Ca_2-xSr_xRuO₄

The physics of heavy fermion (HF) state, which manifests in f-electron system [1], begins to germinate in d-orbital system, for example, in 3d single-orbital van der Waals ferromagnet, transition metal oxide(TMO) LiV₂O₄ and multi-orbital Fe based family [2, 3, 4]. However, the existence of the HF state and its underlying mechanism remain to be questionable in 4d multi orbital systems. Using angle-resolved photoemission spectroscopy (ARPES), we observe the HF state in orbital-selective Mott phase (OSMP) of 4d TMO, Ca_2-xSr_xRuO₄. Upon entering OSMP (x<0.5), we observe a massive spectral weight transfer, a 100meV-orbital selective gap for d_xy-originated γ and corresponding change in d_xz/yz-originated β band while d_xz/yz-originated α band remains hardly changed. With the temperature drop across a characteristic temperature T*, spectral weight transfer gradually occurs from lower to higher binding energy in β and γ bands. This possible Kondo lattice behavior can be well explained by the key role of the octahedral tilt driven d-band hybridization. Our work firstly, demonstrates the possible origin of the formation of the HF state from 4d TMOs with structural distortions, which expands the boundary of the HF-candidates to the 4d multi orbital systems.