New Imaging Technique Illuminates the Direction of Atomic Motion

In the new study, the researchers succeeded in visualising the microscopic movements of atoms in a way that was previously not possible. Atomic motion varies depending on energy, and while earlier research has allowed scientists to measure average atomic movement, this study marks the first time they have been able to observe how atoms move at specific energies.

“The motion of atoms at different energies can be compared to different dances with varying rhythms. You could say we’ve now been able to distinguish the choreography of each dance,” says Paul Zeiger, postdoctoral researcher at the Department of Physics and Astronomy at Uppsala University.

The study focused on vibrations in two different crystal structures: strontium titanate and barium titanate. Using a high-resolution scanning transmission electron microscope, researchers at the University of California, Irvine, identified which atoms vibrated at which energies and in which directions. Meanwhile, physicists at Uppsala University carried out simulations on supercomputers that closely matched the experimental data, helping to confirm and explain the results.

In strontium titanate, strontium and titanium atoms vibrated symmetrically in all directions, while the amplitude of oxygen atom vibrations varied depending on energy and whether they moved towards titanium or strontium atoms. Similar vibration patterns were observed in barium titanate, which has a comparable crystal structure but lower symmetry. In barium titanate, the magnitude of oxygen atom vibrations also varied depending on their positions within the crystal. This lower symmetry gives rise to ferroelectricity, a permanent polarisation in the structure caused by the asymmetric placement of atoms.

In measurements in the scanning transmission electron microscope an electron beam is focused on a surface area smaller than the distance between two atoms, about one-tenth of a millionth of a millimetre. The beam repeatedly scans across a grid over the sample. As the electrons pass through the material, they interact with it, and using electron lenses and magnetic fields in the microscope, researchers can measure how the direction and energy of the electrons changed at each scan point as a result of the interaction. Researchers can then deduce from this data the positions of atoms and also a multitude of other properties, such as the tiny energies associated with atomic vibrations, in the study around 10-120 meV.

What makes the study unique, and forms the basis of the new imaging technique, is the way the researchers collected and analysed the data. By selecting two orthogonal directions of the electrons, they were able to determine the dominant directions of atomic motion at different energy levels, fundamentally based on how the electron beam interacted with the sample.

In simulations of this interaction, the researchers used a computational method known as FRFPMS, previously developed at Uppsala University, and observed that the electron beam is affected differently depending on the direction and energy of atomic vibrations.

“Through both experiments and simulations, we were able to demonstrate that it’s possible to image the differences in how oxygen atoms move at various energies as a result of these interactions,” says Ján Rusz, professor at the Department of Physics and Astronomy at Uppsala University.

Understanding the directions in which atoms vibrate at different energies contributes to our knowledge of how heat is conducted at the atomic level and how atomic motion influences electronic, optical, and other material properties. The researchers’ technique therefore accelerates progress in materials science and nanotechnology.

“We’ve already used this new technique in previous experiments to study monolayers of iron selenide grown on strontium titanate, aiming to deepen our understanding of high-temperature superconductivity. This phenomenon is believed to arise from interactions between atomic vibrations and electron motion in the material,” says Ján Rusz.

Camilla Thulin