Staged emergence of anomalous Hall transport in a correlated uranium Weyl semimetal

In the world of quantum materials, scientists are constantly trying to understand how electrons behave when they are forced to interact intensely with one another. Usually, electrons in a metal move freely, like a crowd of people walking through a wide hallway. However, in certain exotic materials, these electrons become so crowded and reactive that they start to influence each other's paths in complex ways, creating a state of matter where their collective behavior is far more important than their individual motion. At the same time, physicists have discovered that the paths electrons take can be twisted by the fundamental geometry of the material itself, creating a kind of internal map that guides their flow without any external push. This phenomenon, known as topology, can cause electrons to move in a specific direction even without a voltage pushing them sideways, a behavior called the anomalous Hall effect. The big question for researchers has been whether this special transport happens immediately when the material becomes magnetic, or if it requires the electrons to slowly reorganize themselves first.

A team of researchers has now answered this question by studying a specific crystal made of uranium, phosphorus, and sulfur, known as UPS. This material is a ferromagnet, meaning its internal magnetic moments align in the same direction below a certain temperature, but it also contains heavy uranium atoms with electrons that are both free to move and strongly correlated with one another. The scientists wanted to see if the strange sideways flow of electricity appeared the moment the material became magnetic, or if it emerged later as the electrons settled into a new, more complex arrangement. To find out, they cooled the crystal down from room temperature while measuring its electrical resistance, its magnetic strength, and the energy levels of its electrons using a technique called angle-resolved photoemission spectroscopy, which uses high-energy light to knock electrons out of the material so their properties can be measured.

The researchers discovered that the story of this material unfolds in distinct stages rather than all at once. When the sample was cooled, it first became magnetic at a temperature of 118 Kelvin, which is about minus 250 degrees Fahrenheit. At this point, the magnetic order was established, but the electrical properties did not immediately settle into their final form. Instead, the ability of the material to generate a sideways electrical current, known as the anomalous Hall effect, continued to grow and change as the temperature dropped further. The signal for this effect did not simply follow the strength of the magnetism; instead, it kept evolving well after the magnetic order had formed, reaching a peak in its development around 90 Kelvin. This suggested that something else was happening inside the material, a reconstruction of the electron landscape that was happening on a different timeline than the magnetism itself.

To understand what was happening to the electrons, the team used resonant photoemission spectroscopy, a method that allows them to specifically tune in to the uranium atoms within the crystal. They found that the electrons near the surface of the material had a dual nature: some were tightly bound and localized, while others were free to travel through the crystal. As the temperature dropped below 90 Kelvin, the researchers observed a significant shift in how these electrons were distributed. The energy levels of the uranium electrons rearranged themselves, creating a new, more organized structure deep inside the magnetic state. This rearrangement was not a sudden jump but a gradual process that continued to evolve even though the material was already fully magnetic. The data showed that the electrons were not just sitting still in their magnetic state; they were actively reshaping their own energy landscape, a process that took place over a range of temperatures distinct from the initial magnetic transition.

Theoretical calculations performed by the team supported these observations by revealing the hidden geometry of the electron paths. The models showed that the magnetic order in the crystal created a special point in the electron energy structure where different paths crossed in a way that is protected by the symmetry of the material. This crossing point acts like a source of intense curvature in the electron's path, which is what drives the large sideways current observed in the experiments. The calculations predicted a value for this current that matched the experimental measurements very closely, confirming that the effect is intrinsic to the material's structure. However, the theory also highlighted that the real-world electrons were undergoing a continuous transformation that the simple magnetic order alone could not explain. The strength of the sideways current was tied not just to the fact that the material was magnetic, but to the ongoing, temperature-dependent reorganization of the uranium electrons themselves.

This work reveals that in this uranium compound, the magnetic order, the electronic correlations, and the topological transport properties do not all switch on at the same time. Instead, they emerge in a staged sequence. First, the material becomes magnetic. Then, as it cools further, the electrons undergo a profound reconstruction, shifting their energy states and creating the conditions necessary for the large anomalous Hall effect to fully develop. The researchers found that the electrical response continues to evolve long after the magnetic order is established, proving that the two phenomena are linked but operate on different timescales. This discovery challenges the idea that magnetic order alone dictates the transport properties of such materials, showing instead that the complex dance of correlated electrons continues to reshape the material's behavior even after the magnetic state is locked in. The study establishes uranium pnictochalcogenides as a unique platform for exploring how strong electronic interactions can reshape topological phenomena, offering a clearer picture of how nature builds these complex quantum states.