Magneto-optical Kerr spectroscopy of exciton Rydberg states in a magnetic van der Waals heterostructure

In the world of tiny materials, scientists are constantly looking for ways to control how light and electricity interact. A key to this control lies in symmetry, a concept that describes how a system looks when you flip it or turn it around. When this symmetry is broken, strange and useful things happen, such as electricity flowing in only one direction or light behaving differently depending on its spin. For decades, researchers have known that placing a non-magnetic material next to a magnetic one can break this symmetry without needing a giant external magnet. This happens because the magnetic material whispers its magnetic influence to its neighbor across a tiny gap, a process called proximity coupling. This effect is particularly powerful in atomically thin semiconductors, where electrons and holes (the absence of an electron) bind together to form particles called excitons. These excitons are the primary way these materials absorb and emit light, making them the perfect candidates for future optical technologies. However, while scientists have seen how this magnetic whisper affects the most basic excitons, it has remained a mystery whether this influence reaches the more complex, higher-energy versions of these particles.

A team of researchers has now solved this puzzle by creating a sandwich of two different atomically thin materials: a layer of molybdenum diselenide and a layer of a magnetic material called iron gallium telluride. They built this structure with extreme care, wrapping it in a protective shell of boron nitride to keep it clean and stable. By shining light of different colors onto this sandwich and measuring how the light's polarization rotated upon reflection, they were able to listen to the magnetic whisper inside the material. This technique, known as magneto-optical Kerr spectroscopy, acts like a sensitive ear that detects how the magnetic environment changes the way light interacts with the material. The researchers found that the magnetic influence did not stop at the ground level; it extended all the way up to the excited, higher-energy states of the excitons.

The study revealed a clear and surprising pattern in how the light responded. The material has two main families of excitons, which the researchers call the A-family and the B-family. When the magnetic field was applied, the light signals from these two families flipped in opposite directions. One family showed a positive signal, while the other showed a negative one. This is a crucial finding because it proves that the magnetic effect is coming from the specific way the magnetic material talks to the spins of the electrons inside the semiconductor, rather than just a general push from an external magnetic field. If it were just an external push, both families would have reacted the same way. The fact that they reacted oppositely confirms that the magnetic layer is directly influencing the internal spin structure of the material.

To be absolutely sure they were seeing what they thought they were seeing, the researchers performed a clever trick with their data. They measured the material with the magnetic field pointing up, and then again with it pointing down. By comparing these two sets of measurements, they could separate the true magnetic signal from any background noise or static effects that didn't change when the field flipped. The magnetic signal inverted its polarity when the field was reversed, while the background remained the same. This confirmed that the resonances they observed were indeed caused by the magnetic interaction. Furthermore, they were able to distinguish not just the basic excitons, but also their excited states, which are like higher rungs on a ladder of energy. These higher states appeared as distinct features in the light spectrum, showing that the magnetic influence permeates the entire structure of the exciton, from its lowest energy state to its more energetic, excited forms.

The researchers also used powerful computer simulations to understand why the energy levels of these excitons looked the way they did. They modeled the effect of the magnetic layer inducing an exchange coupling to the spins of the electrons and holes. This interaction alters how the electrons and holes feel each other's pull. In a normal vacuum, these particles feel a strong attraction, but the metallic magnetic layer screens this force, making it weaker. The simulations showed that this screening effect is so strong that it actually rearranges the order of the energy levels. In a standard material, the second excited state of one family would sit higher in energy than the ground state of the other. However, in this specific sandwich, the screening was strong enough to push the second excited state of the first family below the ground state of the second family. This theoretical prediction matched the experimental data perfectly, giving the team confidence that they had correctly identified the different energy states they were observing.

This work does more than just map out energy levels; it demonstrates a new way to probe the hidden magnetic properties of these tiny materials. By using light to listen to the magnetic whispers, the researchers showed that they can detect how magnetism affects the most delicate quantum states in a material. The ability to resolve these higher-energy states is a significant step forward, as previous methods often missed them or could not distinguish them clearly. The findings suggest that by stacking different magnetic and non-magnetic materials, scientists can tune the optical properties of these systems with great precision. This opens the door to designing new types of electronic and optical devices that rely on the spin of electrons, potentially leading to faster and more efficient technologies in the future. The study stands as a clear demonstration that the magnetic influence of a neighboring layer can reshape the entire landscape of light-matter interaction in atomically thin materials.