Tunnel view of how electrons play

Scanning tunnelling microscopes provide insights into mysterious electronic effects in some metals

Electrons behave like football teams: the match becomes interesting when the teamwork is as good as that conjured up by the players of FC Barcelona. Electrons which interact strongly with each other give rise to superconductivity, the lossless transport of current, for example. A team headed by researchers at the Max Planck Institute for Chemical Physics of Solids in Dresden is now taking a completely new look at the teamwork between electrons. They have used a scanning tunnelling microscope to investigate the Kondo effect in the metal ytterbium rhodium silicide YbRh 2 Si 2 , which contains unpaired electrons and thus magnetic moments. At low temperatures, the strong interactions between the electrons completely shield the magnetic moments from each other. The Dresden-based physicists have now observed how this shielding is created. Their work also shows how well electronic processes in solids can be investigated with scanning tunnelling microscopes.

An almost perfect surface: The image shows a regularly ordered layer of silicon atoms with a defect where a silicon atom is replaced by a bigger ytterbium atom. In the sample surfaces examined by the Dresden physicists there are only 70 defects among around 30,000 atoms – ideal conditions for convincing measurements. © Steffen Wirth / MPI for Chemical Physics of Solids An almost perfect surface: The image shows a regularly ordered layer of silicon atoms with a defect where a silicon atom is replaced by a bigger ytterbium atom. In the sample surfaces examined by the Dresden physicists there are only 70 defects among around 30,000 atoms – ideal conditions for convincing measurements. © Steffen Wirth / MPI for Chemical Physics of Solids

Events in solid bodies only become interesting when the temperature drops far below freezing, as the atoms then oscillate more slowly and interfere less with the motion of the electrons. The charge carriers therefore experience the forces between each other much more intensely and become aware that they have completely different possibilities apart from conducting electricity or not conducting it. In order to gain a better understanding of the electronic processes in solids, which are the basis of chip technology, communications and modern medical engineering, physicists also investigate the extraordinary effects that electrons in solids display at low temperatures. In future, this understanding may possibly help to produce materials with new properties that may be interesting for technical developments.

“With our current work, we are opening a door that provides us with a completely new way of accessing a large number of electronic phenomena in solids,” says Steffen Wirth, who headed the study. The investigation involved not only the researchers of the Max Planck Institute for Chemical Physics of Solids, but also scientists from the Max Planck Institute for the Physics of Complex Systems, also in Dresden, and the Technical University Braunschweig. The team investigated the metal ytterbium rhodium silicide YbRh 2 Si 2 with a scanning tunnelling microscope as they slowly cooled down the metal.

Physicists also call the material heavy fermion metal; they often talk of fermions when they mean certain properties of electrons. The charged particles are naturally very light but become heavy as a result of the extraordinary effects that are based on the strong interplay of the electrons. The main players here are unpaired localized electrons, i.e. electrons which are firmly bound to the rare earth metal ytterbium. These electrons can occupy any of four groups of 4f orbitals, each with a different shape, assist with the transport of current in the material and have magnetic moments – these are the properties that arouse physicists’ interest.

The Dresden-based researchers have now shown that the interplay between the localized 4f electrons and the freely moving conduction electrons of the material can be observed with a scanning tunnelling microscope. This reveals a great deal about the causes of the effects, and thus about the physical laws in such materials.

The investigation involves positioning the tip of the microscope above a sample surface made up of very well ordered silicon atoms situated one next to the other (see Background: Purity law for a sample surface). The researchers then measure how the tunnelling current depends on the voltage applied at different temperatures. The more they cool the sample, the more marked is the appearance of several peaks and a deep dip in the current-voltage curve. These peaks and troughs in the measured current tell the researchers what happens to the electrons in the material. It is not always easy to interpret the characteristics in the current-voltage curve, however.