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Abstract Angle‐resolved photoemission spectroscopy (ARPES) has been a widely adopted technique in the studies of quantum materials. The surface sensitivity of photoelectric effect also makes it a powerful tool to investigate surface and shallow interface phenomena. While an overwhelming majority of its use focuses on extracting the eigenenergy of the electron Bloch states in momentum space, attempts to extract information of the wave function via ARPES has been limited to molecular systems. In this perspective, it is proposed and advocated use ARPES to investigate and unravel wave function properties, as opposed to only the electron energy‐momentum dispersion relation, in crystalline solids and their interfaces. This can help enhance the rapidly growing development of material properties based on the spatial and geometric properties of the electronic wave functions. 

Dimensionality and electronic correlations are crucial elements of many quantum material properties. An example is the change of the electronic structure accompanied by the loss of quasiparticles when a metal is reduced from three dimensions to a lower dimension, where the Coulomb interaction between carriers becomes poorly screened. Here, using angle-resolved photoemission spectroscopy, we report an orbital-selective decoherence of spectral density in the perovskite nickelate LaNi⁢O3 toward the monolayer limit. The spectral weight of the 𝑑𝑧⁢2 band vanishes much faster than that of the 𝑑𝑥⁢2−𝑦⁢2 band as the thickness of the LaNi⁢O3 layer is decreased to a single unit cell, indicating a stronger correlation effect for the former upon dimensional confinement. Dynamical mean-field theory calculations show an orbital-selective Mott transition largely due to the localization of 𝑑𝑧⁢2 electrons along the 𝑐 axis in the monolayer limit. This orbital-selective correlation effect underpins many macroscopic properties of nickelates, such as metal-to-insulator transition and superconductivity, where most theories are built upon a 𝑑𝑥⁢2−𝑦⁢2–𝑑𝑧⁢2 two-band model. 

Two-dimensional electron gas (2DEG) states at oxide interfaces between two ferroic materials have been fertile ground to realize controllable multiferroicity. Here, we investigate the 2DEG states at the interface of ferroelectric BaTi⁢O3 and a magnetic layer of iron using angle-resolved photoemission spectroscopy. Orbital-selective charge transfer occurs on the surprisingly robust 2DEG. Based on first-principles calculations, we show how the interfacial hybridization can give rise to the unexpected charge transfer in the magnetic 2DEG. Our study reveals a close interplay on a 2DEG between magnetic and ferroelectric interfaces, which sheds light on future design principles of multiferroic 2DEG states.