Surprising properties

What are the results of Philippe Ghosez and his team’s work? The materials on which they are mainly working are ABO₃-type complex oxides with an identical cubic structure, known as perovskite.

Element A (often an alkaline, alkaline earth or rare earth metal) occupies the corners of the structure while B (often a transition metal) is in the middle, enclosed in an octahedral cage of oxygen. Identified in the 19^th century, this structure is common to numerous compounds, which present a wide variety of interesting properties according to the A and B metals chosen. These compounds can have an insulating, semi-conductor, metallic or even superconductor behaviour. They are used in numerous devices (memories, transistors, sensors, actuators) since they include amongst of the best ferroelectric compounds (BaTiO₃, PbTiO₃), piezoelectric compounds (PbZr_xTi_1-xTiO₃), multiferroic magnetoelectric compounds (BiFeO₃), dielectric compounds (Ba_xSr₁-xTiO₃), magneto-resistive compounds (AMnO₃) or non-linear optically active compounds (BaTiO₃, LiNbO₃ with a related trigonal structure).

In 2008 (read the article A New Nanomaterial), the teams of Philippe Ghosez and Jean-Marc Triscone published an article in Nature (1) in which the physicists described the development of an artificial nanomaterial with special ferroelectric properties. The scientists discovered that when different oxides are stacked in superlattices, the properties of the whole system can be different from those of the oxides on an individual level. In this particular instance, they revealed a new mechanism that can produce spontaneous polarisation, a phenomenon that has since raised a growing interest and which researchers now refer to as hybrid improper ferroeletricity. This is the result of an unusual coupling between different structural distortions allowed by the material’s layered structure: the researchers have indeed shown that this resulted from interactions on an atomic scale at the interfaces between the layers. “This opened a completely new field of investigation”, Professor Ghosez remembers. “It was opening the door to the design of new materials by engineering properties at interfaces on an atomic scale.” Why such enthusiasm? Let’s draw an analogy.  Current electronics is based on a very simple material: silicon. Using this unique compound, we have been able to create an incredible variety of devices: diodes, transistors, memories! How? By creating interfaces between differently doped areas or with other materials. Functionality is created by the interface! The challenge was therefore to control these interfaces as precisely as possible. “This is what we would like to achieve with our materials, the ABO₃ oxides, which are more complex, naturally possess functional properties much more exciting than silicon, and can be combined and stacked like Lego blocks thanks to their identical perovskite structure. Another spectacular example is that of the interface between two insulators, SrTiO₃ and LaAlO₃ (2), in which a two-dimensional electron gas can be induced reversibly, opening the way for numerous possibilities in the field of electronics (read the article The interface between two non-conductive materials can be conductive). Controlling (3) and acting on the interfaces in the ABO3 nanostructures should allow us to generate a vast number of new devices with unique properties, and perhaps replace silicon one day… A dream sometimes known as ‘oxitronics’! "

(1) Improper ferroelectricity in perovskite oxide artificial superlattices, E. Bousquet, M. Dawber, N. Stucki, C. Lichtensteiger, P. Hermet, S. Gariglio, J.-M. Triscone and Ph. Ghosez, Nature (London) 452, 732 (2008).
(2) Tunable conductivity threshold at polar oxide interfaces. M.L. Reinle-schmitt, C. Cancellieri, D. Li, D. Fontaine, M. medarde, E. Pomjakushina, C.W. Schneider, S. Gariglio, Ph. Ghosez, J.-M. Triscone & P.R. Willmott, Nature Communications 3, 932 (2012).
(3) Atomically precise interfaces from non-stoichiometric deposition. Y.F. Nie, Y. Zhu, C.-H. Lee, L.F. Kourkoutis, J.A. Mundy, J. Junquera, Ph. Ghosez, D.J. Baek, S. Sung, X.X. Xi, K.M. Shen, D.A. Muller & D.G. Schlom, Nature Communications 5, 4530 (2014).