Scientist of the University of Regensburg made a step forward in understanding one of the most in intriguing phenomena of nature: superconductivity. As Dr. Angelo Peronio and Prof. Dr. Franz J. Giessibl of the Institut für Experimentelle und Angewandte Physik report in the renowned journal “Physical Review B”, they were able to operate for the first time a scanning force microscope with a superconductive tip (DOI: 10.1103/PhysRevB.94.094503). They used this new instrument to test experimentally a proposed explanation of this not-yet-fully-understood phenomenon.
A superconductor is a material whose electrical resistance disappears completely if cooled close to the absolute zero (−273 °C). In other words, an electrical current can go on flowing in a superconductor potentially forever, even after we “unplug” it from the socket. This property is used today to build powerful magnets with reduced energy consumption, making possible nuclear magnetic resonance imaging. Superconductivity was observed for the first time in frozen mercury in 1911, and puzzled the physicists for more than 40 years before in 1957 Bardeen, Cooper, and Schrieffer were eventually able to understand why this happens. In 1986 however this explanation proved itself to be incomplete: new materials were discovered, whose superconductive behaviour could not be described by the standard theory. This reopened a puzzle which 30 years later remains to be solved: various attempts have been made, but a complete explanation of superconductivity remains one of the most important open questions of physics.
Prof. Jorge E. Hirsch of the University of California in San Diego published in 2004 the “theory of hole superconductivity”, which aims at describing all superconductive phenomena in an unified framework. In order to check the predictions of Hirsch, the scientists in Regensburg fabricated an atomically sharp superconductive tip and measured the tiny force which act between this tip and a copper surface when the two are less than one millionth of a millimetre apart, using an atomic force microscope (AFM) cooled at −270.8 °C, just 2.4 °C above the absolute zero. If Hirsch is right, this force should change when the tip becomes superconductive.
As often happens in science, the experiment did not provide a definitive answer: «Eventually, we understood that our machine is just a bit too warm: the answer is there, but we would need to be 2 °C colder in order to see it.» says Dr. Peronio. However a new path was opened, and further experiments are already on the way. Moreover, the superconductive AFM tip is exciting in itself: «It is a new tool in our toolbox, it will allow us to use our microscope in many new ways. Especially, we want to use it to “listen” to the vibrations of single molecules. We think that by gently pushing the molecule with the tip, we could tune these vibrations like a violinist does with its instrument.» says Prof. Giessibl.
The atomic force microscope has been introduced three decades ago by Gerd Binnig, Christoph Gerber and Calvin Quate. The AFM is today one of the most important tools for nanoscience, allowing to see single atoms and to move them in a controlled manner. Binnig, Quate and Gerber were rewarded with the Kavli Prize in Nanoscience on September 6, 2016 in Oslo by Haakon Magnus, Crown Prince of Norway. The work in Regensburg is the first successful demonstration of an AFM with a superconductive probe tip, a feat that might be important for future studies in nanoscience. The research was funded trough the Sonderforschungsbereich 689 “Spin phenomena in reduced dimensions“ of the DFG (www-app.uni-regensburg.de/Fakultaeten/Physik/sfb689/).
A. Peronio and F. J. Giessibl, “Attempts to test an alternative electrodynamic theory of superconductors by low-temperature scanning tunneling and atomic force microscopy”, Physical Review B 94 094503 (2016).