Heavy or light electrons depending on the movement in two-dimensional material

The findings, published in Nature Physics, reveal an intrinsic way in which electron mass is governed by the shape of electron clouds, known as orbitals, around atoms. Just as a flower has petals pointing in specific directions, these electron clouds have patterns that make some directions through the material fundamentally different from others, opening a new platform for exploring and designing future quantum materials.

In everyday life, objects weigh the same no matter how you move them, as long as their speed does not approach the speed of light. In the quantum world of materials, however, electrons can appear to have different effective masses depending on how they interact with their surroundings. In so-called heavy-fermion materials, mobile electrons interact strongly with electrons tightly bound to magnetic atoms. Through this interaction, the mobile electrons become heavy fermions, which are a new type of charge carrier that moves dramatically more slowly and behaves as if it had an enormous mass.

Until now, heavy-fermion behavior has mainly been studied in three-dimensional (3D) materials, where electrons can move throughout the crystal in all directions. In these systems, the interaction that gives rise to heavy fermions is also present in every direction, so the electrons become heavy regardless of how they move through the material. External parameters such as pressure or chemical substitution can modulate the strength of this interaction in an uneven way, but they typically affect the material as a whole and never eliminate heavy-fermion behavior along specific directions.

Direction-dependent interaction

But CeSiI is different. It is made of weakly connected layers that can be peeled into sheets only a few atoms thick. Within each 2D sheet, electrons are largely confined to move in a flat plane, creating an electronic environment that is fundamentally different from that of conventional 3D heavy-fermion materials: it is strongly direction dependent.

To explore how this 2D structure affects electron behavior, the team used scanning tunnelling microscopy, a technique that images electronic behavior with atomic-scale resolution, together with theoretical calculations. The experiments revealed clear signatures of direction-dependent electron behavior, while the calculations showed how the 2D structure and atomic pattern of CeSiI give rise to this effect.

“We discovered that the interaction between electrons tightly bound to the magnetic cerium atoms and electrons moving through the material depends strongly on the direction of motion,” explains Chin Shen Ong, researcher at the Department of Physics and Astronomy at Uppsala University. “This interaction, known as hybridization, disappears along specific directions within the material because of the shape of the cerium electron cloud. As a result, the same electrons can behave as heavy or light depending on how they move through the material.”

In CeSiI, the electronic environment isolates a particular electron cloud around the magnetic cerium atoms. This electron cloud is tightly bound to those atoms and does not look the same in all directions. Its shape can be visualized as a flower surrounding the atom. Specifically, mobile electrons moving through the material that pass through the petal regions interact strongly, are slowed down and become heavy, while electrons moving between the petals, known as nodes, do not interact and remain light.

A new control mechanism

This effect arises naturally from the atomic structure of CeSiI and does not rely on external tuning or material disorder. It introduces an intrinsic way to distinguish between heavy and light electronic behavior within a single material. Instead of being controlled from outside the material, the behavior is built directly into how electrons are arranged and interact inside CeSiI.

This stands in contrast to ordinary metals such as copper or aluminum, where external conditions can slightly change how electrons respond to forces, but often by no more than a factor of two compared with a free electron. It also differs from traditional heavy-fermion materials, where electrons can become orders of magnitude heavier, but typically do so uniformly throughout the material rather than in a direction-dependent way.

Future applications in quantum technology

Understanding that the same electrons can act as heavy or light depending on direction in a 2D material opens a new platform for exploring quantum phenomena. One striking consequence appears in how CeSiI conducts electricity and heat. The material shows an unusually low electrical resistance despite storing a large amount of heat. This occurs because electrical current is carried mainly by the light electrons, while the heavy electrons dominate the ability of the material to store thermal energy.

“Cerium-based systems continue to surprise us with their unique physical and chemical properties,” says Olle Eriksson, Professor at Uppsala University and co-director of the Wallenberg Initiative Materials Science for Sustainability (WISE). “This insight may, in the long term, help guide the design of materials with tailored electronic properties.”

Beyond transport properties, CeSiI becomes magnetic below 7 kelvin (−266 °C), a temperature only a few kelvin above that of deep space. At this point, neighboring atomic magnetic moments align in opposite directions. The present measurements were performed just above this transition. A key next step is to explore even lower temperatures, where heavy-electron behavior and magnetism begin to influence one another. Studying this interplay in a 2D system could reveal new collective quantum states and deepen our understanding of how strongly interacting electrons behave when competing electronic tendencies are closely balanced. More broadly, these results highlight 2D heavy-fermion materials as a promising platform for discovering and controlling new quantum behaviors in materials.