A new artificial material shows the pathway to improved electronics

In the 10 April issue of Nature, a new artificial material is revealed that marks the beginning of a revolution in the development of materials for electronic applications. The discovery results from a collaboration between the theory group of Professor Philippe Ghosez (University of Liege, Belgium) and the experimental group of Professor Jean-Marc Triscone (University of Geneva, Switzerland). One of the lead researchers on this project, Matthew Dawber, who recently joined the Department of Physics and Astronomy at Stony Brook University, will be at the forefront of the continued effort to make and understand these revolutionary artificial materials in his new lab.

The new material, a superlattice, which has a multilayer structure composed of alternating atomically thin layers of two different oxides (PbTiO3 and SrTiO3), possesses properties radically different to either of the two materials by themselves. These new properties are a direct consequence of the artificially layered structure and are driven by interactions at the atomic scale at the interfaces between the layers.

'Besides the immediate applications that could be generated by this nanomaterial, this discovery opens a completely new field of investigation and the possibility of new functional materials based on a new concept: interface engineering on the atomic scale,' said Dr Dawber.

Transition metal oxides are a class of materials that provoke great interest because of the great diversity of properties which they can present (they can be dielectrics, ferroelectrics, piezoelectrics, magnets or superconductors) and their ability to be integrated into numerous devices. The majority of these oxides possess a similar structure (referred to as 'perovskite') and, recently, researchers have developed the ability to build these kinds of materials up, atomic layer by atomic layer, much as a child plays with Lego bricks, hoping to produce new materials with exceptional properties.

Ferroelectrics are some of the most useful functional materials, with applications ranging from advanced non-volatile computer memories, to micro-electromechanical machines or infrared detectors. 'Improper ferroelectricity,' is a kind of ferroelectricity that occurs only rarely in natural materials and usually the effects are far too small to be useful. The properties of improper ferroelectrics depend on temperature in a totally different way to normal ferroelectrics, meaning they would have significant advantages for many applications where the operation temperature might vary, if only the ferroelectric properties were larger in magnitude. This new superlattice material shows improper ferroelectricity (a property that neither of the parent materials possesses) with a magnitude around 100 times greater than any naturally occurring improper ferroelectric, opening the door for a host of real world applications.

PbTiO3 and SrTiO3 are two well-known and well characterised oxide materials, presenting, in one case, a ferroelectric structural instability, and, in the other, a non-polar structural instability. A theoretical study carried out in Liege (using sophisticated first principles quantum mechanical simulation techniques, referred to as ab initio) predicted that when these materials are combined in a superlattice, an unusual and completely unexpected coupling between the two types of instabilities occur which is what causes the improper ferroelectricity. A parallel experimental study in Geneva, confirmed the improper ferroelectric character in this type of superlattice, and also provided evidence of an exceptionally useful new property: a dielectric constant (a value which describes the response of the material to an electric field) which is, at the same time, very high and independent of temperature, two characteristics that tend to be exclusive of one another but are here combined in the same material.

But indeed the ideas generated by this discovery are much more significant than the immediate applications; this study demonstrates the possibility of creating radically different materials by engineering on the atomic scale and the PbTiO3/SrTiO3 superlattice system is only a first example. The concept of coupling of instabilities at the interfaces in artificially layered structures is a concept transferable to other types of oxides, and could be a particularly interesting strategy in the emerging domain of multiferroic oxides. These results follow hot on the heels of the discovery last year that the interface between a different pair of oxide materials was in fact superconducting, where neither of the natural materials from which it was made had this property. This and other recent progress lead the prestigious journal Science to class the recent discoveries in oxide multilayers as one of the ten most significant scientific breakthroughs of 2007. In the same way that the mastery of the interface properties of semiconductors was crucial for the development of the modern electronics we depend on today, it seems that engineering of new properties at interfaces between oxides could result in an equally significant technological revolution in the years to come.

This research results from a collaboration, which has been funded by the Volkswagen Foundation (Nanosized Ferroelectric Hybrids), the Swiss National Science Foundation (through the National Centre of Competence in Research-MaNEP) and the European Community (FAME-EMMI and MaCoMuFi).