STO and LAO are wide-gap insulators with a perovskite structure that are routinely used as substrates for growing high-temperature-superconducting cuprates, colossal magnetoresistive manganites and multiferroics, among other materials. Recently, researchers have also discovered a number of interesting properties in various oxide STO heterostructures, including quasi-two-dimensional electron gas (2DEG) behaviour, magnetism, resistance switching, the giant thermoelectric effect and colossal ionic conductivity. Such characteristics mean that these materials might be ideal for all-oxide electronics, thermoelectrics and solid-oxide fuel cells.

Four years ago, Yunzhong Chen of the Technical University of Denmark and colleagues reported on the discovery of a new type of heterostructure based on STO that has a conducting interface, despite the fact that it contains a non-crystalline overlayer. The same team is now saying that a single-unit cell of insulating La1-xSrxMnO (LMO) inserted at the interface between disordered LAO and crystalline STO increases the speed at which electrons move in these 2DEGs. Enhancing carrier mobilities at such complex oxide interfaces has proved difficult until now.

Extremely clean metallic oxide interface

Thanks to resonant X-ray spectroscopy and transmission electron microscopy, the researchers observed that the metallic oxide interface was extremely clean and that electron mobilities there were around 100 times higher than at an unbuffered interface. They say that the buffer layer, or spacer as it is also called, acts as an electron sink at the interface, which has an empty or partially filled sub-band lying at a lower energy than the Fermi level of the system. "In this way, the spacer can be used to tune the conduction at the interface at will, something that leads to the boost in electron mobility we observed," explains Chen.

"The buffer layer suppresses the scattering of electrons in the 2DEG by separating the electrons from their charged donors – as is done in conventional semiconductor heterostructures," he adds. "It also prevents redox reactions at the oxide interface and removes intrinsic defects, such as oxygen vacancies on the STO side. In my opinion, this latter effect plays a non-negligible role in enhancing the mobilities of electrons in the system.

Unearthing fundamental new physics and designing novel quantum devices

"The extremely clean oxide interface we made could be used not only to unearth fundamental new physics but also to design novel quantum devices and structures that work thanks to strongly correlated electrons," he tells Such structures include ferromagnetic 2D electron liquids and systems in which the unconventional quantum Hall effect is at play.

The team says that it now busy looking into the quantum transport in its oxide interfaces when they are made up into conventional devices. "Patterning oxide interfaces is generally more challenging than patterning conventional semiconductor heterointerfaces," says Chen, "but, happily, we can use routine lithography technology to create these structures, and this at room temperature."

The researchers, reporting their results in Nature Materials doi:10.1038/nmat4303, are also trying to determine and understand how magnetism in the oxide interfaces emerges when buffered by the spacer. This work is being done with collaborators at the University of British Columbia and the Canadian Light Source, adds Chen.