Controversy clarified: Why two insulators together can transport electricity

How can two materials which do not conduct electricity create an electrically conducting layer when they are joined together? Since this effect was discovered in 2004, researchers have developed various hypotheses to answer this question - each with its own advocates, who defend it and try to prove its validity. Now, an international team under the leadership of researchers at the Paul Scherrer Institute has probably settled the controversy. They have shown that it is the combination of the properties of both materials that produces the effect, and therefore disproved an alternative hypothesis, which proposes that the materials mix at the interface to create a new, conducting material. The materials under study are so-called perovskites, members of a large class of materials with interesting electrical or magnetic properties that could play a significant role in electronics and computing in the future. In 2004, researchers discovered something amazing: If the two materials, SrTiO₃ and LaAlO₃, both of which are electrical insulators, are joined together, a thin, electrically conducting region is formed at their interface. Over the subsequent three years, a number of hypotheses were proposed to explain this effect, which has led to controversy ever since. "There are conference sessions at which the majority of contributions are about this effect," says Mathilde Reinle-Schmitt, a researcher at PSI and the first author of the work presented here. Of the various explanations forwarded, effectively only two competing hypotheses have survived. In order to compare their relative merits, an appropriate experiment was performed by scientists at PSI, together with colleagues from the University of Geneva. In addition, scientists from the University of Liège have contributed important theoretical insights. Intermixing or polar catastrophe?