The electrical conductance at different locations on individual atoms has been measured by scientists in Japan. The experiment involved adding and removing electrons from atoms on a surface by using a scanning tunnelling microscope (STM). The technique could also be used to measure the distribution of charge in superconductors, as well as to study magnetic interactions on very small length scales.

An STM builds up an image of a surface by using an atomically sharp tip to inject or withdraw electronic charge. The tip is scanned across the surface at a height of less than 1 nm, and the position of the tip can be controlled in 3D with picometre precision. As a result, STMs can produce images of individual atoms on surfaces, and have also been used to measure atomic-scale phenomena such as the quantization of conductance.

In this latest work, Huwon Kim and Yukio Hasegawa of the University of Tokyo used a state-of-the-art STM to study the surface of lead in an ultrahigh vacuum and a just a few degrees above absolute-zero. They found that when the distance between the needle and the surface was relatively large (about 100 pm), the conductance decayed exponentially with this distance. This is exactly what is expected from an electric current created by quantum tunnelling. Also, the same decay was measured when the needle was directly above an atom and when it was above a gap between two or more atoms.

Atomic skimming

When the needle was brought closer to the surface, the conductance started to increase faster than expected if tunnelling were involved. However, this increase only occurs when the needle is directly above an atom. This, say the researchers, is because the needle skims the surface of the lead, putting it in direct contact with the top of each atom. This allows electrons to move more freely between tip and atom, thereby boosting the conductance. However, when the tip is over a gap between atoms, electrons can only be exchanged via quantum tunnelling.

When the needle is brought closer still – actually pressing into the surface – the reverse effect on conductance is seen. The conductance is highest when the needle is pressed into a gap between atoms, rather than into a single atom. The researchers believe this may be because when in a gap, the tip has access to electrons from all of the neighbouring atoms. However, when the tip is touching a single atom, it can only interact directly with electrons in that one atom. Nevertheless, says Hasegawa, studies by theorists will be required to clarify exactly what is going on. “Our basic conclusion is that we measure the conductance at the sites,” he says.

Probing the superconducting gap

This detailed understanding of the variable concentration of electronic charge around and between atoms may prove to be important in nanotechnology because it should aid the design and optimization of atomic-scale electronic devices. The researchers themselves, however, are interested in superconductivity. Lead becomes a superconductor at temperatures below 7.2 K, and their experiment was carried out at 2.1 K. Although the material would have been a superconductor during the experiment, the observations were made in a region of the energy spectrum where this would have made no difference. Now, however, the researchers are preparing a paper describing how they have used the technique to look at electrons in the so-calledsuperconducting gap, in the hope that this will provide insights into the nanoscale behaviour of electrons in superconductors.

Laurent Limot of the Institute of Physics and Chemistry of Materials of Strasbourg, France, commends the researchers, but points out that the technique is “not completely new”, because two papers in 2011 looked at the charge distribution around atoms, one in gold and one in C₆₀ molecules. Nevertheless, he says, the researchers have shown the differences in conductivity at different sites much more solidly than previous work. As well as studying superconductivity, says Limot, one can envisage using a ferromagnetic tip to look at nanoscale magnetic interactions. This would be useful for developing spintronic devices that use the magnetic spin of the electron to store and process information.

The research is published in Physical Review Letters.