Supra meets Nano
Superconductors like it cold - otherwise normally conductive, they only demonstrate their loss-free current conduction below a transition temperature. Now, however, scientists have managed to create microscopically small normal and superconducting areas in one and the same material at the same temperature, which can be specifically modified. This opens up new possibilities for the construction of electronic components. There are some research areas in physics in which an intensive search is currently being made for new findings that should be ready for application as soon as possible. Superconductivity is just as important as nanotechnology. In one branch, the developers are trying to develop new substances that can transport electricity without losses even at room temperature, while others are miniaturizing more or less complex structures in order to equip devices or materials with previously unknown functional properties. The first free-floating high-temperature superconducting axle bearing for industrial applications recently turned at the Hanover Fair. And you learn something new about developments in nanotechnology almost every week.
Now Jean-Marc Triscone from the University of Geneva, together with colleagues from France and Japan, has tried to bring both worlds together, although he obviously feels connected to electronics. He is experimenting with so-called perovskites. These are compounds in which the various atoms that make up the solid are in precisely defined positions in the crystal lattice. The Triscone team uses a material made of strontium titanium oxide in its experiments. The pure composite material is an insulator, so it does not conduct electricity. However, by doping – the introduction of foreign atoms to add or remove charge carriers – a semiconductor or even a conductor can be made from it. Adding a bit of the chemical element niobium creates a metal that also becomes a superconductor at low temperatures.
The working group has now applied a mere 26 nanometer thin film of this material with niobium to a carrier made of pure perovskite. On top of this they laid a 50 nanometer thick layer consisting of a compound of the chemical elements lead (Pb), zirconium (Zr), titanium (T) and oxygen. The substance, which is called PZT after the abbreviation for its chemical composition, shows ferroelectric properties. The solid material thus possesses – similar to water molecules – an inherent electric dipole moment: it appears negatively charged at designated points on the substrate, positively charged at others, which makes it extremely sensitive to external electric fields affecting the atomic structure. Piezoelectric crystals, which react to pressure with sparks, for example, and are therefore used in some lighters, are typical representatives of this group of substances.
Another property of ferroelectric compounds is that their polarity can be reversed by applying an electrical voltage to spatially limited areas. In this way, alternately positively and negatively charged areas can be generated. The scientists working with Triscone have now produced such tiny structures using the fine tip of an atomic force microscope, which is normally used to scan surfaces with nanometer precision.
The different distribution of charges in the top layer affects the underlying niobium-doped perovskite. The electron density also shifts there, to which the material reacts very sensitively. Depending on the temperature – superconductivity is always temperature-dependent – sooner or later the substance will lose its ability to conduct electricity without loss. Areas over positively polarized areas already switched to the normally conducting state at 236 millikelvin, those over the negatively polarized areas only at 296 millikelvin.
Although the difference isn't huge. Nevertheless, the researchers believe they have developed a technique that can be used to create completely new electronic components, just a few nanometers in size, that can alternately act as superconductors and then as normal conductors again.