https://phys.org/news/2018-07-expressway-electrons-oxide-heterostructures.html

The research team co-led by Prof Andrivo RUSYDI and Prof ARIANDO, both from the Department of Physics and Nanoscience and Nanotechnology Institute (NUSNNI) NanoCore, NUS has developed a new methodology involving a combination of advanced measurement techniques (spectroscopic ellipsometry, synchrotron-based soft X-ray absorption spectroscopy and charge transport measurements) to determine the influence of localised charges on the mobility of electrons at the oxide interface. These localised charges can shield (or "screen") electrons in such a way that they do not "see" each other, significantly reducing the coulomb repulsion between them. Screening the coulomb repulsion helps to reduce correlation effects between electrons. This is known as the "screening effect" and it allows the electrons at the interface to travel with higher mobility. The new method developed by the NUS research team allowed them to detect both screened and unscreened electrons, thereby shedding light on how they dictate the electronic properties of a complex oxide heterostructure, particularly at a buried interface.

The researchers involved in this team have applied this method to an oxide heterostructure made up of lanthanum strontium aluminium tantalate ((La0.3Sr0.7)(Al0.65Ta0.35)O3 (LSAT) and SrTiO3. They discovered the presence of a new midgap state populated by localised charges (which are transferred from the surface of LSAT) at the interface. A midgap state is a state occurring within the optical band gap. Interestingly, they found that such a midgap state is responsible for determining the transport properties of the interface. When there are more localised charges at the interface, the mobile electrons are further shielded from those in the surrounding bulk material. This significantly increases the interface electron mobility.