Shunfeng Chen

This year marks the 20th anniversary of the discovery of LaAlO₃/SrTiO₃ (LAO/STO) oxide heterointerfaces. Since their discovery, transition metal oxide (TMO) interfaces have emerged as a fascinating and fast‐growing area of research, offering a variety of unique and exotic physical properties which has provided a strong impetus for the rapid advances and actualization of oxide electronics. This review revisits the fundamental mechanisms accounting for the two‐dimensional (2D) conducting interfaces, and how new models proposed to better account for the unique interfacial effects. Recent breakthroughs in the theoretical and experimental domains of oxide interfaces are also discussed including the detection and investigation of 2D quasiparticle. Moving beyond the well‐known LAO/STO interface, this review delves into other systems where unconventional interfacial superconductivity, interfacial magnetism, and spin polarization are dealt with in greater detail. In terms of device applications, this review proceeds with a treatment on the recent developments in domains including field effect transistors and freestanding heterostructure membranes. By emphasizing the opportunities and challenges of integrating oxide interfaces with existing technologies, the review will end off with an outlook projecting the progress and the trajectory of this research domain in the years to come.

Two-dimensional (2D) perovskite oxide interfaces are ideal systems to uncover diverse emergent properties, such as the arising polaronic properties from short-range charge–lattice interactions. Thus, a technique to detect this quasiparticle phenomenon at the buried interface is highly coveted. Here, we report the observation of 2D small-polarons at the LaAlO3/SrTiO3 conducting interface using high-resolution spectroscopic ellipsometry. First-principles investigations show that interfacial electron–lattice coupling mediated by the longitudinal phonon mode facilitates the formation of these polarons. This study resolves the long-standing question by attributing the formation of interfacial 2D small polarons to the significant mismatch between experimentally measured interfacial carrier density and theoretical values. Our study sheds light on the complexity of broken periodic lattice-induced quasi-particle effects and its relationship with exotic phenomena at complex oxide interfaces. Meanwhile, this work establishes spectroscopic ellipsometry as a useful technique to detect and locate optical evidence of polaronic states and other emerging quantum properties at the buried interface.

Two-dimensional (2D) perovskite oxide interfaces are ideal systems where diverse emergent properties can be uncovered.The formation and modification of polaronic properties due to short-range strong charge-lattice interactions of 2D interfaces remains hugely intriguing.Here, we report the direct observation of small-polarons at the LaAlO3/SrTiO3 (LAO/STO) conducting interface using high-resolution spectroscopic ellipsometry.First-principles investigations further reveals that strong coupling between the interfacial electrons and the Ti-lattice result in the formation of localized 2D small polarons.These findings resolve the longstanding issue where the excess experimentally measured interfacial carrier density is significantly lower than theoretically predicted values.The charge-phonon induced lattice distortion further provides an analogue to the superconductive states in magic-angle twisted bilayer graphene attributed to the many-body correlations induced by broken periodic lattice symmetry.Our study sheds light on the multifaceted complexity of broken periodic lattice induced quasi-particle effects and its relationship with superconductivity.