Abstract:
With the assistance of modern thin-film growth techniques, perovskite oxides with a three-dimensional crystal structure can now be grown in a layer-by-layer manner at atomic-level precision on heterostructural substrates, opening up vast opportunities for unprecedented phenomena at the two-dimensional (2D) oxide interfaces. The emergence of a conductive interface between the two band insulators, LaAlO3 (LAO) and SrTiO3 (STO), represents the most celebrated exemplification in this context. Up to the date, a plethora of unexpected properties have been established at oxide heterointerfaces, ranging from 2D electron gas, 2D superconductivity, 2D orbital reconstruction to 2D magnetic ordering. However, why can the oxide interfaces be so surprising? This remains an outstanding problem to be addressed promptly. In this Colloquium, I will elucidate on how we disentangled the puzzle using simultaneous STEM-EELS tackling of charge, lattice, and electronic-structure degrees of freedom, atom-by-atom and unit-cell-by-unit-cell. The localization of a 2D electron density at an insulating (Nd,Sr)MnO3/STO interface [PRB, 2013], the condensation of the 2D interfacial charges in (Nd,Sr)MnO3/STO into one-dimensional electron chains [Nature Commun., 2014], and hidden lattice instabilities as the origin of the conductive LAO/STO interface [Nature Commun., 2016] were readily resolved. Perspectives on 2D oxide-interfacial phenomena and STEM-EELS instrumentations will also be discussed.
Brief Bio:
Dr. Ming-Wen Chu obtained his PhD degree in Materials Science in 2002 at University of Nantes, France. He then moved to Max-Planck Institute of Microstructure Physics, Halle (Germany), for his first postdoctoral research in 2003 and National Institute for Materials Science, Tsukuba (Japan), for the second postdoctoral work in 2004. In January 2005, he joined Center for Condensed Matter Sciences (CCMS) at National Taiwan University as assistant research fellow. He was promoted to associate research fellow in 2009 and to research fellow in 2014. Dr. Chu’s graduate and post-graduate trainings have been on x-ray crystallography and electron scattering of oxides. After having joined CCMS, he changed his research focus to atomically-resolved electron spectroscopy using scanning transmission electron microscopy (STEM) in conjunction with electron energy-loss spectroscopy (EELS) and readily demonstrated the first unveiling of plasmonic dark modes in 2009 (highly cited paper in Physics, 2015), the first fluorescent-X-rays elemental map at atomic resolution in 2010, and the first unit-cell-by-unit-cell charge counting at oxide interface in 2013. He is now one of the most active experts in STEM and EELS and his recent research interests are centered on atomic-scale unraveling of emergent phenomena at oxide interfaces.