A defining characteristic of complex transition-metal compounds(TMCs)is that they contain incomplete d sub-shells thus having a multiple of possible ground states.This triggers their vast variety of physical properties,emerging from a complicated competition between multiple energy scales and the close coupling with lattice and spin degrees of freedom.On one hand,both chemical and physical complexity presents a formidable challenge in condensed matter physics and materials science in general.On the other hand,the complexity is directly responsible for their tunability,offering a fabulous playground for making artificial structures of these materials with new functionalities [1].One can anticipate that new physics will emerge when ultrathin layers of TMCs are grown into artificial heterostructures with electronic and magnetic properties manipulated by broken symmetry and modified interface couplings--"the interface is a platform."It has become increasingly clear that surfaces [2],interfaces [3],thin films and heterostructures [4] of TMCs display a rich diversity of fascinating properties that are related,but not identical to,the bulk phenomena,while the origin is still fiercely contested.Specifically,many ultrathin films of transition-metal oxides exhibit nonmetallic behavior,in contrast to their metallic bulk counterpart,thus displaying a metal-insulator transition(MIT)as the film thickness is reduced.In this talk,I will focus on the understanding of the nature of MIT,a long-standing issue in the epitaxial oxide research community,by using the films of La2/3S1/3MnO3(LSMO)as a prototype example [5].Based on our recent studies by atomically controlled growth and characterization with Low Energy Electron Diffraction,Angle-resolved X-ray photoelectron spectroscopy,scanning tunneling and scanning transmission electron microscopy/spectroscopy,I will discuss the surface termination,layer-by-layer variation of composition and structure distortion,and their profound effect on the electronic and magnetic property,including the emergent insulating behavior in uitrathin films.