Variable Charge State, Magnetic Excitations, and Kondo Effect of Sm/g/Ir(111)
Using low-temperature scanning tunneling microscopy, this study reveals that individual Sm adatoms on graphene/Ir(111) exhibit reversible variable charge states, distinct magnetic excitations confirming a Sm ground state with retained 4f filling, and a novel Kondo resonance with a large Zeeman splitting.
Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
In the quest to store more information in smaller spaces, scientists have long looked toward the ultimate limit: the single atom. Imagine a magnetic switch so tiny that it occupies the space of just one atom on a surface. If such switches could be made stable and controlled, they would allow data storage densities far beyond anything current technology can achieve. However, for a single atom to act as a reliable magnetic switch, it must possess specific magnetic properties that depend heavily on its electrical charge. Atoms are not just solid spheres; they are complex systems of electrons, and the number of electrons they hold determines how they spin and interact with their neighbors. While scientists can often see where atoms sit on a surface, figuring out exactly how many electrons they have, and how those electrons behave when the atom is isolated or crowded, remains a difficult challenge. Understanding this delicate balance is crucial for turning the idea of single-atom magnets into a reality.
A team of researchers has now taken a significant step forward by studying individual atoms of the rare-earth element samarium placed on a sheet of graphene, which itself rests on a crystal of iridium. Using a powerful microscope that can see and measure individual atoms at temperatures near absolute zero, the scientists discovered that these samarium atoms can exist in two different electrical states, and they can switch between them. The state of an atom depends on how close its neighbors are. When a samarium atom stands alone, it behaves one way, showing no magnetic activity in the measurements. But when it is brought within about one and a half nanometers of another samarium atom, it enters a new, unstable state. In this state, the atom becomes sensitive to the electric field generated by the microscope's tip. As the tip hovers over the atom, this electric field can push the atom to flip back and forth between its two charge states. This flipping creates a visible ring pattern in the microscope images, a direct signature of the atom changing its electrical personality right before the researcher's eyes.
The researchers found that this ability to switch charge states is not just a curiosity; it is the key to unlocking the atom's magnetic potential. Only the atoms that are close enough to their neighbors to exhibit this switching behavior also show distinct magnetic excitations. These excitations are like tiny energy jumps that occur when the atom's internal electrons rearrange themselves. The team measured these jumps at specific energy levels, finding two prominent steps at 35 and 54 millielectronvolts, along with a higher-energy step at around 165 millielectronvolts. By comparing these measurements to what is known about samarium atoms in a vacuum, the scientists concluded that the atoms on the surface have lost one electron to the underlying material, leaving them with a specific configuration of electrons that makes them magnetic. This configuration is rare and valuable because it gives the atom a large magnetic response, a property that is essential for it to function as a stable memory bit.
Perhaps the most surprising discovery was the observation of a phenomenon known as the Kondo effect. This effect occurs when a magnetic atom interacts so strongly with the electrons in the material beneath it that they form a temporary, collective cloud around the atom. In this study, the researchers saw a sharp peak in their measurements at the energy level where electrons usually flow freely, indicating this strong interaction. When they applied a magnetic field, this peak split in two, a behavior that allowed them to calculate a specific number called the g-factor, which describes how strongly the atom responds to a magnetic field. The value they found was very large, confirming that the samarium atoms retain their powerful magnetic character even while sitting on the surface. This large response suggests that the atoms are excellent candidates for future magnetic storage devices.
The study also clarified how the distance between atoms influences their behavior. By carefully moving individual atoms closer together with the microscope tip, the team showed that the magnetic steps and the charge-switching rings appear only when the atoms are within a critical distance of each other. If they are too far apart, the atoms remain quiet and non-magnetic. If they are close, they wake up and begin to interact. The researchers ruled out the idea that the atoms were simply moving to different spots on the surface to cause these changes; the concentric rings they saw were too perfect and centered to be caused by physical movement. Instead, the changes were purely electrical, driven by the interplay between the atoms and the electric fields around them. This work provides a clear picture of how to control the charge and magnetic state of single atoms, a fundamental requirement for building the ultra-dense storage systems of the future.
Drowning in papers in your field?
Get daily digests of the most novel papers matching your research keywords — with technical summaries, in your language.