While most devices thus far have utilized electrical charge modulation, the introduction of other physical degrees of freedom can provide numerous opportunities to design novel interfacial properties. In particular, strongly correlated electron systems of transition metal oxides provide optimal interfaces to integrate charge, spin, orbital, and lattice degrees of freedom. [6,7] So, correlated oxide heterostructures have a high potential to implement multi-state memories or multifunctional devices. [6][7][8] Most correlated oxides have high carrier densities and therefore their interactions at the heterointerfaces are significantly short. Previous studies have shown that, in the correlated oxide interfaces, charge modulations, [9][10][11] as well as magnetic, [12] orbital, [13] and structural [14] reconstructions occur within several atomic layers from the interface. There have been a few systematic studies of highelectron-mobility oxide heterostructures whose carrier densities are comparable to that of conventional semiconductors. [15,16] The short length scale of the interfacial reconstruction indicates Interfaces between dissimilar correlated oxides can offer devices with versatile functionalities, and great efforts have been made to manipulate interfacial electronic phases. However, realizing such phases is often hampered by the inability to directly access the electronic structure information; most correlated interfacial phenomena appear within a few atomic layers from the interface. Here, atomic-scale epitaxy and photoemission spectroscopy are utilized to realize the interface control of correlated electronic phases in atomic-scale ruthenate-titanate heterostructures. While bulk SrRuO 3 is a ferromagnetic metal, the heterointerfaces exclusively generate three distinct correlated phases in the single-atomic-layer limit. The theoretical analysis reveals that atomic-scale structural proximity effects yield Fermi liquid, Hund metal, and Mott insulator phases in the quantum-confined SrRuO 3 . These results highlight the extensive interfacial tunability of electronic phases, hitherto hidden in the atomically thin correlated heterostructure. Moreover, this experimental platform suggests a way to control interfacial electronic phases of various correlated materials.
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