It's all at the interface. Again.

Over the last decade, there has been a great deal of exciting work in making electronically interesting systems at atomically sharp interfaces between different oxide materials (oxide heterostructures). Analogous efforts at semiconductor-dielectric interfaces have given us the conventional field-effect transistor, something like 10⁹ of which are being used to render this page for you. Likewise, heterointerfaces in compound semiconductor systems (especially the technologically relevant III-V materials like GaAs) have given us two Nobel Prizes and a great deal of quantum electronic fun. Oxides are much trickier beasts from the materials science side, making growth and interfacial control a major challenge. Moreover, with respect to basic science, transition metal oxides can be incredibly rich systems, because in many of them electron-electron interactions lead to competing electronic and magnetic phases, with consequences like the emergence of high temperature superconductivity.

A few years ago, this paper demonstrated that it was possible to get superconductivity at the interface between SrTiO₃ and LaAlO₃, two oxides that are both insulating if perfectly stoichiometric. Still, SrTiO₃ is known to superconduct if highly doped, and therefore this observation, while a great experiment, wasn't hugely shocking, given the existence of a high density electron gas at the STO/LAO interface. More recently, this paper showed that high temperature superconductivity could happen at the interface between a nominally insulating oxide and a metallic (but not superconducting) cuprate related to the high-Tc materials. This past week on the arxiv, a logical successor to these works appeared here. The authors use two nominally insulating oxides (STO again, and CaCuO₂. Because of imperfect stoichiometry at the interface (excess oxygen, apparently), there is a conducting layer at the interface, with a superconducting transition around 50 K (in one sample, though others all show transitions exceeding 25 K). Bearing in mind that this is a preprint (and therefore has not been refereed), it is still very exciting. We are finally approaching the ability to engineer complex materials (not just semiconductors) on the atomic layer level, and this should be an incredible playground for basic science and materials engineering. It'd be great to get plugged into a collaboration working in this area.