The magnetic and superconducting properties of thin films of oxide compounds are useful in many practical applications, particularly in ‘heterostructures’ comprised of different oxide layers. Researchers from the University of Tokyo1 have realized a major advance in the study of highly conductive thin-film oxide systems with the observation of a two-dimensional layer of mobile superconducting electrons in an oxide heterostructure.

High-mobility, two-dimensional electron systems are readily produced using semiconductor heterostructures, and have led to fundamental breakthroughs in the study of electronic quantum effects. Oxide systems are also interesting in that they hold the promise of novel phenomena due to the unique nature of the electronic states involved.

Fig. 1: Two-dimensional superconducting electron sheet (superconducting electron pairs are indicated by arrows) in a thin niobium-doped SrTiO3 film (yellow) sandwiched between insulating undoped SrTiO3 layers.

The team of researchers produced their thin conductive oxide films using strontium titanate, the oxide compound for which thin film growth is most advanced. When doped with niobium, excess electrons are introduced into SrTiO3, causing the film to become electrically conductive. To achieve a two-dimensional confined electron system, the team grew the thin niobium-doped film between undoped, and therefore insulating, SrTiO3 layers (Fig. 1).

The device was superconducting at low temperatures, in accordance with the known superconductivity of niobium-doped SrTiO3. Notably, however, the sample ceased to be superconducting when exposed to a high magnetic field, and the electrical resistivity of the structure showed characteristic oscillations with increasing magnetic field strength. These quantum oscillations are a signature of electron confinement within the niobium-doped layer, and proof that the scientists had produced a two-dimensional electronic quantum system.

This achievement has important implications, as it is a testament to the versatility that has now been achieved in the growth of oxide heterostructures, says Harold Hwang from the research team. “To date, you had to live with whatever systems could be grown. Here we have demonstrated a key step in getting oxide heterostructures to the level of conventional semiconductors, in the sense of band-structure engineering.”

Hwang says that in the future, electrical fields could be used to control the distribution of electrons in the niobium-doped layers. Furthermore, devices containing multiple conducting layers could reveal unique physics arising from the interaction between the electron sheets.