🔬 This microscope reveals how electrons are distributed around atoms

An electron microscope can now distinguish, at the scale of columns of atoms, how certain electrons are distributed around them.

To understand this result, we need to forget the image of electrons orbiting the nucleus like planets. In quantum mechanics, we instead describe regions where they are more likely to be found. These regions are called orbitals. Their shape and orientation can vary depending on the atom’s environment.

Here, an orbital represents the region where electrons are most likely to be found. Some orbitals are elongated in a specific direction. In other words, the “cloud” associated with the electrons can extend more vertically, or instead within the plane of the material.

This orientation matters because it influences how electrons can move and interact in a solid. It can therefore contribute to its electrical or magnetic properties. Until now, X-rays could already be used to study this phenomenon, but mainly as an average over many atoms.

The new method examines matter much more locally.

Roger Guzman and his colleagues use a transmission electron microscope. An extremely fine electron beam passes through a very thin sample. Afterward, the scientists measure the energy lost by the electrons in the beam and the direction in which they were deflected.

The trick is to compare the signal obtained in two perpendicular directions. If the material’s electrons occupy an orbital oriented more strongly in a given direction, this difference appears in the measurements. The microscope can repeat the operation while moving its beam from one point to another across the sample.

The result is a map of the material’s electronic organization. Its resolution is below one ångström, or less than 0.1 nanometer. The authors thus achieve a sensitivity corresponding to a single atomic column. Here, a column refers to several atoms aligned in the direction traveled by the microscope’s beam.

To verify the method, the team studied thin layers of an oxide containing manganese. Some were compressed by their substrate, while others were slightly stretched. In both cases, the manganese electrons favored different orbitals. The result matches trends previously observed through X-ray measurements.

This precision becomes especially interesting near a defect or at the boundary between two materials. In these locations, the organization of electrons can change over just a few atoms and disappear in an averaged measurement. The authors now want to use this technique to study these local variations in quantum materials and artificial interfaces.