New method unveils electronic properties at interfaces19.09.2016
Physicists from IFW Dresden and coworkers from Würzburg University, MPI Stuttgart, MPI CPfS Dresden, Canada, the U.S.A. and Korea developed a new method to uncover important charge properties of correlated oxide interfaces with unprecedented atomic scale resolution.
Conventional electronic devices like chips are based on networks of so-called p-n junctions, interfaces between a semiconductor carrying positive charges (p-doped) and a semiconductor carrying negative charges (n-doped). The interplay of the junctions can be used to realize logical calculations or memory units. However, current efforts to miniaturize and increase the functionality of such devices, calls for a change of paradigm in which the information transfered by these devices is not only based on the motion of charges, such as that given by electrons or holes (electronics), but also on the manipulation of inherent electron properties like their tiny magnetic moment called spin (spintronics), and electron orbitals (orbitronics), i.e., the arrangement of the electron clouds around the atoms.
Transition-metal oxides exhibit many different properties ranging from magnetism to high temperature superconductivity with very unconventional properties. Forming interfaces between such materials creates a plethora of phenomena, which hold promise for novel applications such as sensors, actuators, lossless computer memory and ultrafast processors. Notwithstanding, study of such heterointerfaces is an experimental challenge, not only due to the fact that such interfaces are buried below the device surface, but also due to the variety of phenomena and due to the much shorter length scale, over which the properties of oxides change at such heterointerfaces, which is often just a few atomic spaces. Of crucial importance is the behavior of electrons at the interface: Do they tend to accumulate? Which orbitals do they occupy? Is there magnetic order?
Based on resonant x-ray reflectometry, a technique exploiting x-ray light created at a synchrotron, researchers from the IFW-Dresden and coworkers from Germany, Canada, the U.S.A. and Korea have developed methods and analysis tools to answer exactly these questions, with the atomic-scale resolution of less than a nanometer. They apply the technique on thin films of lanthanum cobalt oxide, LaCoO3, a material which has interesting magnetic properties due to the fact that cobalt has many electrons in the outer shell, which can arrange in different ways and with different orientations of the spins.
In the present publication, however, the research team has concentrated on another aspect: the polar character of the material. Like many other materials, such as simple table salt, NaCl, or the semiconductor GaAs, LaCoO3 consists of ions. These ions form a sequence of positively and negatively charged layers: In our case, these are LaO(+) and CoO2(-) layers, stacked to a 15 nanometer thin film. This stacking of charges produces an electrostatic field that costs a lot of energy to the system. Nature, however, is economical and avoids these field energy costs: It brings positive and negative charges to the opposite faces of the film, respectively, in such a way, that, just like between the plates of a capacitor, a new field is formed. This field is opposite to the original one and cancels it.
This accumulation of pure electronic charge at the film surfaces is a very elegant solution, since it preserves their smoothness. The mechanism is called electronic reconstruction. When electronic reconstruction is not possible, the compensating charge is provided by comparatively large ions, which results in corrugated film faces. Such corrugations would be obviously detrimental for devices based on film interfaces, especially when, like in transition-metal oxides, the material properties change on an atomic scale at the interface.
The present publication is important because it shows microscopic evidence that electronic reconstruction is indeed realized at transition-metal oxide interfaces. It also provides a tool to study the microscopic properties of such interfaces, which are not limited to electronic reconstruction, but encompass the arrangement of chemical elements, the electronic occupation of atomic orbitals and the spin orientation.
Publication: Valence-state reflectometry of complex oxide heterointerfaces. Jorge E Hamann-Borrero, Sebastian Macke, Woo Seok Choi, Ronny Sutarto, Feizhou He, Abdullah Radi, Ilya Elfimov, Robert J Green, Maurits W Haverkort, Volodymyr B Zabolotnyy, Ho Nyung Lee, George A Sawatzky & Vladimir Hinkov, npj Quantum Materials 1, Article number: 16013 (2016), doi:10.1038/npjquantmats.2016.13
Dr. Jorge Enrique Hamann Borrero
Leibniz Institute for Solid State and Materials Research Dresden (IFW Dresden)