Chemistry of an Interface
It sounds implausible, but two poor electrical conductors can be the building blocks for an excellent one. In 2004, experimentalists observed that thin films of LaAlO3 and SrTiO3, both wide-band-gap insulators, formed a metal at their interface. Later experiments showed this two-dimensional metal could support unusual transport properties, including superconductivity. Explanations for these and other properties of the interface often invoke (among other things) oxygen vacancies, but a new systematic study cautions that even in the absence of such oxygen defects, small changes in the amount of lanthanum ( La) in the LaAlO3 layer can have large effects on the interfacial electrical resistance. As reported in Physical Review Letters, whether there is an excess or deficiency of La depends on how the samples are fabricated, but the difference can be easily missed by conventional sample characterization.
Eric Breckenfeld and colleagues at the University of Illinois, Urbana-Champaign followed a standard recipe for preparing their LaAlO3- SrTiO3 heterostructures: they used a laser to ablate La and Al from a LaAlO3 target and deposited the atoms onto a SrTiO3 substrate, controlling for oxygen content with a variable oxygen partial pressure. Careful elemental analysis of the samples with x rays revealed the ratio of La to Al depended on the laser energy density: The deviations were small enough to be missed by characterization probes like electron diffraction and atomic force microscopy, but even a few percent La deficiency led to a one-million-fold drop in the resistance of the interface. Changing the partial pressure of oxygen, however, had a much smaller effect. Based on the authors’ findings, future work aimed at understanding the physics of this interesting interface will have to take into account the role of the chemistry of the materials. – Jessica Thomas