Superconductivity – Electrons in Single File Provide New Insights

A team at the University of Innsbruck, Austria has been successful in conducting electrons in metals along predetermined channels. This behaviour, observed for the first time in metals, provides important insights into the interactions of electrons – and on how the phenomenon of the current flow without any resistance loss, termed super-conductivity, can occur. Thereby this project aided by the Austrian Science Fund (FWF) combines fundamental research, at its best, with potential applications in the future.

High-temperature superconductors are ceramic materials that conduct electricity without resistance, and thus without loss, below a certain temperature. At higher temperatures, the behaviour rapidly changes and experiences resistance. Such discontinuous changes due to external influences are typical for the so-called “smart materials”. Their discontinuous behaviour is closely linked with a mutual dependence of spatially confined electrons, giving rise to a commonly coordinated motion pattern. So far this dependence termed as correlation had been observed only in non-metals.

Electrons in Single File…

Now a team under Prof. Erminald Bertel, Institute of Physical Chemistry, University of Innsbruck, Austria, has for the first time succeeded in forcing the electrons in a metal as well into such a mutual dependence. For this purpose, the researchers first of all created nano-structures on the surface of metal single crystals, which are crystals with uniform lattice structure.

Prof. Bertel, the project director, explains: “Normally, the electrons in a metal spread in all three directions in space. But in metal single crystals, some of the electrons are confined to the surface and therefore can move only in two dimensions. Nano-structures can then further restrict their freedom of movement. To produce such structures, the surfaces of copper crystals for instance can be oxidised in such a way that free copper channels of 3 nanometres width lie between ridges of copper oxide. In these channels, the electrons can only move unidimensionally. Also on platinum crystals atom chains can be arranged to run parallel across the surface with approximately 0.8 nanometre spacing. Certain electrons can then only spread along these chains.”

Once the electrons were forced into a controlled motion along the channels or chains, Professor Bertel’s team was able to observe something fascinating – depending on experimental conditions, the electrons move within the individual channels entirely independent of each other, i.e. incoherently, or they align their movements across all channels. In such a state of motion that is described as coherent, the electrons can no longer be assigned to individual channels, but are “de-localised”.

… When the Temperature is Right

For a closer analysis of the states of the electrons, the researchers at Innsbruck also made use of photoelectron spectroscopy. In this method, the energetic distribution of electrons emitted from the surface due to light (photon) absorption is measured. Interestingly, the spectra showed that above a critical temperature, the electrons pass from a coherent into an incoherent state.

A completely similar temperature dependence of photoelectron spectra, however, is already known in superconductors, but was explained differently so far. Thus the observations of the Innsbruck team suggest that the superconductivity in ceramic superconductors is connected to a transition of electrons from an incoherent state into a coherent state.

Prof. Bertel: “The transport of electricity without loss due to electric resistance could mean a significant contribution to energy saving and to the solution of some environmental problems. But at present our comprehension of superconductivity does not allow the synthesis of superconductive materials that can afford a commercial use under economical conditions. Our team has achieved in adding a small chip to the mosaic, which brings us a little closer to such applications.”