A publication in Advanced Science

Unique interface between two insulators!

In Research

Sébastien Lemal and Philippe Ghosez from the Theoretical Physics of Materials Laboratory (CESAM Research Unit), in collaboration with researchers from the University of Geneva, have succeeded in rationalizing the appearance of electrons at the interface between two insulators, based on their superconducting properties. These results were published in Advanced Science(1).


echnological innovation is mainly based on exploiting the physical properties of certain materials, whether in construction, aeronautics, energy or information technology. Materials science research is therefore essential to meet new technological challenges. Often the properties of individual materials are not sufficient to create a functional device. Indeed, modern electronics are based on properties emerging at interfaces created by the association of several materials, such as in diodes, transistors, etc..

Most materials are described as either conductors or insulators, depending on their ability to conduct electricity. Perovskite structural oxides SrTiO3 and LaAlO3, for example, are both insulating. However, they present a particularity : when one carries out a junction between these two materials, their interface becomes conductive ! If we take a closer look, we see that a thin layer of electrons forms next to this interface. Where do these electrons come from? How are they distributed near the interface? Sébastien Lemal and Philippe Ghosez from the Theoretical Physics of Materials Department (PhyTheMa - Research Unit CESAM), in collaboration with experimenters from Professor Jean-Marc Triscone's group at the University of Geneva, have succeeded in rationalising the appearance of electrons at this interface on the basis of their superconducting properties. Their results are discussed in a recent article published in the journal Advanced Science(1).

Gaz electron isoalnts

An electron gas appears between two insulators! When connecting the insulators LaAlO3 (red) and SrTiO3 (blue), a confined electron gas appears near the interface (yellow). The density of this gas depends on the composition of LaAlO3: by replacing it with an alloy mixing 50% LaAlO3 and 50% SrTiO3, the surface density of the gas is halved. Moreover, at low temperatures, this difference in density modifies the expansion of the gas, which moves away from the interface when the density decreases. This results in a clear difference in the expansion of the superconducting phase. [Figure: Sébastien Lemal, copyright: PhyTheMa@ULiège]

One of the major discoveries of recent years in the domain of functional oxide doamine is the conductive nature of the interface between SrTiO3 and LaAlO3 insulating oxides. Their junction is characterized by the existence of a "gas" of electrons confined near the interface that makes it metallic, without having to rely on external doping as performed in conventional semiconductor devices. The origin of this gas, which exhibits many properties including superconductivity and very high mobility in the junction plane, remains subject to debate in the scientific community. Moreover, this type of interface can be realized at the scale of a few atomic layers only!

Research conducted by researchers from ULiège and Geneva has confirmed the controversial mechanism behind this electronic "gas". "Since LaAlO3 has an electrical polarity different from that of SrTiO3, the "gas" appears at the interface to avoid the presence of an electric field in the layer of LaAlO3, unfavourable energetically, explains Sébastien Lemal, PhD student in the laboratory of Theoretical Physics of Materials (PhyTheMa). This mechanism imposes a surface electronic density of 3.3 1014 cm-2 at the interface. In addition, by replacing the LaAlO3 layer with a polar alloy layer consisting of 50% LaAlO3 and 50% SrTiO3, this density increases to 1.7 1014 cm-2. "The numerical models developed in Liege show that only these densities can explain the differences in the confinement of this "gas" at low temperature, and thus validate the polar mechanism. "They are determined from measurements of the transverse expansion of the superconducting phase measured from the interface, which evolves from ~10 nm to ~30 nm, a factor of 3," says Philippe Ghosez, director of the laboratory. »

These results therefore provide an experimental protocol for indirectly evaluating the electronic density of the gas from superconductivity measurements, and confirm the possibility of controlling the containment of this gas on the basis of the composition of the polar alloy. Such discoveries complete an already well-provided array of properties for this extraordinary interface, and bode well for the exploitation of these materials for technological applications in the field of electronics.

Scientific reference

(1) Probing Quantum Confinement and Electronic Structure at Polar Oxide Interfaces , D. Li, S. Lemal, S. Gariglio, P. Wu, A. Fête, M. Boselli, Ph. Ghosez and J.-M. Triscone, Adv. Sci. 1800242 (2018). https://doi.org/10.1002/advs.201800242


Sébastien LEMAL | UR CESAM | Q-MAT I slemal@uliege.be | +32(0)4 366 36 12

Pr. Philippe GHOSEZ | UR CESAM | PhyTheMa I Philippe.Ghosez@uliege.be | +32(0)4 366 36 11

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