Using DESY's bright research light sources, scientists have opened a new door to better solar cells, novel superconductors and smaller hard-drives. The research reported in the scientific journal Nature Communications this week enhances the understanding of the interface of two materials, where completely new properties can arise. With their work, the team of Prof. Andrivo Rusydi from the National University of Singapore and Prof. Michael Rübhausen from the Hamburg Center for Free-Electron Laser Science (CFEL) have solved a long standing mystery in the physics of condensed matter. CFEL is a cooperation of DESY, the University of Hamburg and the Max Planck Society.
"Interfaces are a hot topic in materials research," says Rusydi. "If two dissimilar materials are put together, completely new properties may be generated. For instance, two insulators and non-magnetic materials can become metallic and magnetic at their interface." The reason for this change of personality of the two materials is the broken symmetry at the interface, explains Rübhausen, who is a professor at the University of Hamburg. "The two materials have different characteristics and different structures. If you put them together, they have to make a deal and rearrange, and this leads to new properties."
Making use of these phenomena can lead to smaller hard-drives, for example. "Conventional hard-drives are currently controlled by bulk physical properties of the material, for miniaturization we would like to control their physical properties by the interface structure," says Rusydi. "The problem is that we do not yet fully understand what is happening at the interface." As an example, the team investigated the interface of strontium titanate (SrTiO3) and lanthanum aluminate (LaAlO3), two insulators that become conductors at their interface. "However, based on Maxwell's theory, a tenfold higher conductivity should be observed. So, 90 per cent of the charge carriers, the electrons, have gone missing. That was a complete mystery to us," says Rusydi.
In search of the "missing electrons" the scientists used DESY's bright synchrotron radiation source DORIS III to floodlight the interface of the two materials in a wider ultraviolet energy range than any investigation had used before. "All the electrons in the material are like small antenna that respond to electromagnetic radiation at different wavelengths, depending on their energy state," explains Rusydi. The absorption of synchrotron radiation at specific wavelengths reveals the energy state of the corresponding electrons and thus their hiding place in the crystal lattice.
The investigation showed that only a fraction of the expected electrons actually migrate to the interface to form a conducting layer. Most of the electrons redistribute in sub-layers within the lanthanum aluminate, where they were hidden from techniques used in previous investigations. Also, the scientists observed that the transfer of electrons from the crystal to the interface depends on the number of so-called unit cells of lanthanum aluminate in the crystal lattice. A unit cell is the smallest unit of a crystal, this means a crystal can be described as many identical unit cells. If the lanthanum aluminate layer is less than three unit cells thick, all electrons redistribute within the lanthanum aluminate sub-layers and no electrons transfer to the interface anymore which then remains an insulator.
This explains, why more than just one layer is necessary to fully unfold the interface properties. "If only a part of the electrons migrate to the interface, you need a bigger volume to compensate for the symmetry breaking," explains Rusydi. With their work, the scientists can now better understand the behaviour of this and of other interfaces. "In principle, our experimental technique can be used to study any interface," says Rübhausen. "We have only just begun to investigate the basic interface characteristics with it." Although further investigations will have to wait until the Superlumi experimental station that has been used for this work has been moved from the now retired light source DORIS III to DESY's current machine PETRA III. "There currently is no facility in the world that can measure this," Rübhausen says.
The scientists expect that with a better understanding of interfaces, their properties can be more easily tweaked to desired characteristics. "If we learn to control the interface, we can design completely new properties and control them," says Rübhausen.
Deutsches Elektronen-Synchrotron DESY is the leading German accelerator centre and one of the leading in the world. DESY is a member of the Helmholtz Association and receives its funding from the German Federal Ministry of Education and Research (BMBF) (90 percent) and the German federal states of Hamburg and Brandenburg (10 percent). At its locations in Hamburg and Zeuthen near Berlin, DESY develops, builds and operates large particle accelerators, and uses them to investigate the structure of matter. DESY's combination of photon science and particle physics is unique in Europe.
"Mechanisms of charge transfer and redistribution in LaAlO3/SrTiO3 revealed by high-energy optical conductivity"; T.C. Asmara et al.; Nature Communications (2014); DOI: 10.1038/ncomms4663