Considering the results of previous studies 4, 5, 6, 7, 8, 12, 19, 20, 21, 22, this is indicative of Kondo-hybridisation (KH) between the localised γ- and itinerant β-bands. We also observe unexpected spectral weight suppression in the β-band (4 d xz/yz) (but not in the α-band (4 d xz/yz)). With variation in x and T, a gradual orbital-selective opening of a soft gap 17, 18 is observed in the γ (4 d xy)-band with spectral weight transfer from low- to high-binding energy (BE), suggesting the emergence of the OSMP. In this article, we report our systematic x- (0.2 ≤ x ≤ 0.5) and T-dependent ARPES results on CSRO. Part of the reason for these problems may arise from the fact that previous studies were performed with limited points in the parameter space, such as substitution ( x) and temperature ( T). However, controversy still remains even regarding the very existence of the OSMP, and the origin of HF behaviour is still unclear. This led to several angle-resolved photo-emission spectroscopy (ARPES) studies that aimed to obtain direct evidence for the OSMP in CSRO 14, 15, 16. Recently, unexpected HF states have been discovered in moderately localised 3 d-electron systems such as CaCu 3Ir 4O 12, AFe 2As 2 (A = K, Rb, Cs), and Fe 3GeTe 2 3, 4, 5, providing a strong impetus to search for possible Kondo-pairing even in less localised 4 d/5 d-electron systems.Ĭa 2-xSr xRuO 4 (CSRO), a 4 d transition metal oxide (TMO), has been reported to exhibit strong HF behaviour near x = 0.5 6, 7, 8, which triggered intensive theoretical/experimental investigations 9, 10, 11, 12, 13, 14, 15, 16 on the existence of an orbital-selective Mott phase (OSMP) for decades. In early studies, HF behaviour was mostly found in f-electron systems in which strongly localised f- and itinerant spd- hybridised orbitals coexist and pair to form Kondo-singlets 2. The heavy fermion (HF) state is one of the most important subjects in strongly correlated systems research and is often accompanied by exotic states such as superconductivity, quantum criticality and magnetism 1.
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