Discovering the secrets of oxide electronics

A great deal of research is being carried out in the field of oxide electronics, investigating this virtually unlimited source of materials with exotic properties, motivated partly by the search for a replacement for silicon as we approach the limits of what we can do with the current generation of electronics. Although we can easily measure the properties of these materials, their high-quality production is a challenge for materials science, and developing models to explain their properties is challenging for theoreticians. Observing electron-phonon interactions, thought to be the underlying cause for superconductivity in most materials, is a very important area of research for condensed matter physics.

Professor Felix Baumberger from the University of Geneva’s Department of Quantum Matter Physics, and the Spectorscopy of Novel Materials Group at the Paul Scherrer Institute.

SrTiO₃ is widely used as a substrate for growing oxide films, but is a highly fascinating material in its own right. A band insulator, it becomes a superconductor when doped with less than 1 excess electron per 10⁴ Ti ions, implying an exceptionally strong attractive interaction among diluted carriers. In combination with other materials, it plays a crucial role in creating novel physical properties, for example superconductivity above the temperature of liquid nitrogen in a single monolayer of FeSe grown on a SrTiO₃ base. This critical temperature is higher than in any iron-based bulk material, suggesting a key role for the SrTiO₃ substrate. 

The Fröhlich polaron

Visualisation of the complex electron phonon interaction in SrTiO₃ by ARPES. The series of replica bands at low carrier density is an unambiguous spectroscopic fingerprint of Fröhlich polarons. With increasing carrier density, the replica bands evolve into a subtle kink structure in much wider electronic bands. This shows directly that the polarons dissociate as the doping is increased.