Oxide interfaces: Instrumental insights.

S ir Arthur Stanley Eddington said, " We used to think that if we knew one, we knew two, because one and one are two. We are finding that we must learn a great deal more about 'and'. " Combining multiple materials with desirable properties is an oft-pursued route to achieve artificial composite materials with functionalities superior to any of their parts. When the dimensions at which the constituents are interleaved in the composite reach that of the wavelength of the electrons travelling within the material, unanticipated behaviour can occur 1. Considerable efforts over the past decade have focused on how to stabilize those new states by forming superlattices with nanometre-to subnanometre-scale periodicities in a class of materials known as complex transition metal oxides 2. These oxides contain correlated electrons, which means that every electron simultaneously is influenced by all other electrons in the system, owing to the open electronic shell configuration of the transition metal cations. This aspect makes predicting their properties a challenge. The uncertain meaning of 'and' in two-component atomic-scale oxide superlattices further compounds the problem of rational control of interface-derived phases, because simple sum rules rarely apply. To surmount such challenges requires advanced instruments — atomic-level structure and electronic property interrelationships are essential in navigating the vast phase space of chemistries, strains and periodicities available to artificial oxide superlattices. Writing in Nature Materials, Eric Monkman and colleagues 3 demonstrate the powerful insights that can be gained from an integrated materials discovery instrument (Fig. 1) that combines atomic-level control over oxide superlattice growth with the capability to explore the interfacial electronic structure without ever breaching vacuum. The unprecedented capabilities of the consolidated tool enable Monkman and colleagues to reveal how the metal–insulator transition 4 in a systematic series of manganite superlattices with different interleaving sequences arises from strong quantum many-body interactions induced by the dimensionality of the superlattice. The unique instrument used by Monkman et al. for the growth and study of correlated oxide superlattices consists of molecular beam epitaxy (MBE) and angle-resolved photoemission spectroscopy (ARPES), respectively. The former technique provides the ability to create oxide superlattices with subnanometre periodicities and atomically sharp interfaces. The latter enables the electronic band structure and interactions of a system to be pieced together. ARPES relies on a momentum conservation book-keeping exercise, which exploits Einstein's photoelectric effect underpinning the quantum nature of all matter: electrons emitted by a material that are …