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 YbRh2Si2, 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.
More than enough possibilities in the lifetime of one researcher
It is precisely the interpretation of the signal at – 6 millivolts where the Dresden physicists are not quite certain; calculations which they have done in parallel to their experiments predict the signal will be at around – 2 millivolts. The researchers find the signals of the crystal field splitting at exactly the position where they should be in theory. “It is therefore not clear whether we are really seeing the band of the Kondo lattice,” says Wirth.
The physicists now intend to investigate the discrepancy in more detail; however, this is not the only direction they can now follow. “After our initial investigation with a scanning tunnelling microscope provided such a detailed image of the electronic structure of heavy fermion metals, we now have more possibilities than can be covered in the lifetime of one researcher,” says Steffen Wirth. He and his colleagues therefore want to find out what the current-voltage curve looks like when ytterbium rather than silicon forms the top atomic layer. The researchers also want to add traces of further elements to the ytterbium rhodium silicide, which would probably change its electronic properties considerably. In addition, the researchers want to measure the tunnelling current at temperatures far below minus 268 degrees Celsius. This is where the Kondo effect is assumed to break down, and the 4f electrons take on an anti-ferromagnetic order.
“If we expand the method further, we may also be able to make a contribution to explaining the unconventional superconductivity in heavy fermion metals,” says Steffen Wirth. And that would provide a relatively strong clue as to how so-called high-temperature superconductivity comes about. This is promising in technical terms, although it still begins at temperatures far below freezing. Only when physicists accurately understand its causes can they look for materials that lose their resistance under ordinary conditions.
Background: purity law for a sample surface
In order for the experiments with the scanning tunnelling microscope to succeed, the physicists require samples with almost flawless surfaces: the atoms must also align themselves on the surface to form the same, completely regular lattice as is found in the bulk of the sample. The Dresden-based researchers produce these sample surfaces by bonding a tiny stamp smaller than the head of a knitting needle onto their samples. They then knock off the stamp with a precisely executed blow from a punch – and hope that the stamp takes a portion of the sample with it, leaving behind a clean surface with a regular composition. This is now a matter of routine for the researchers.
The only question that remains concerns the site at which the sample breaks. This determines the composition of the surface created. In scanning tunnelling microscope all atoms appear the same. Although there are a large number of methods to determine the type of atom, they are not available in the ultra-high vacuum chamber of the scanning tunnelling microscope, and to bring the sample from one instrument into another one would immediately contaminate the surface. The researchers in Dresden therefore use indirect means to deduce the composition of the surface. The crystal structure tells them that above an ytterbium layer there is a silicon layer, then a rhodium layer, another silicon layer and then an ytterbium layer again, after which the stacking sequence repeats itself. The bonds between the rhodium atoms and the silicon atoms which surround them do not break open, and the researchers know this. This means that the top atomic layer either consists of silicon or ytterbium atoms. In order to differentiate between the two layers, the researchers are assisted by the fact that even the best crystal is not perfect, and that ytterbium atoms are slightly bigger than silicon and rhodium atoms. Therefore, if a silicon or rhodium atom, which really does not belong there, is incorporated into an ytterbium layer every now and then, the defect can always be seen as a dent. A silicon layer, in contrast, has dents, as well as humps. And this was precisely the case in most of the samples examined by the scientists in Dresden. On a surface of around 30,000 atoms, the number of defects was limited to a mere 70 – the material was therefore ideal for determining the electronic properties of pure ytterbium rhodium silicide.