Exciton-polariton condensate in the van der Waals magnet CrSBr

In the microscopic world of materials science, researchers are constantly searching for substances that can do two things at once: conduct electricity like a semiconductor and respond to magnetic fields like a magnet. For decades, these two properties were often found in separate materials, making it difficult to build devices where light, electricity, and magnetism could work together seamlessly. Recently, a new family of materials called van der Waals magnets has emerged. These are crystals made of thin layers held together by weak forces, allowing scientists to peel them apart like pages in a book until they are just a few atoms thick. Among these, a material called CrSBr has become a particular focus because it remains stable in normal air and behaves as a semiconductor while also possessing a strong internal magnetic order. The central question driving recent research is whether we can make light and matter interact so intensely within these magnetic crystals that they form a new state of matter, one where the magnetic properties of the crystal can directly control how light behaves.

A team of researchers has now demonstrated that this is possible. They took a thin flake of the CrSBr crystal and placed it inside a specially designed optical cavity, which is essentially a trap for light made of two highly reflective mirrors. By cooling the entire setup to a frigid temperature of 3.5 Kelvin and carefully adjusting the distance between the mirrors, they forced the light bouncing inside to mix with the excitations inside the crystal. When light and matter mix in this way, they create new particles called exciton-polaritons. These are hybrid entities that are part light and part matter, behaving like a fluid that can flow and interact. The researchers successfully pushed this fluid into a state of condensation, a phenomenon where a large number of these particles lock together into a single, coherent quantum state, similar to how water vapor turns into a single drop of liquid, but occurring with light and matter at the quantum level.

The most significant discovery in this work is how the magnetic state of the crystal dictates the behavior of this light fluid. The CrSBr crystal has a natural magnetic arrangement where the spins of its atoms point in opposite directions, known as an antiferromagnetic order. However, when the researchers applied an external magnetic field, they could force the atoms to align in the same direction, switching the crystal to a ferromagnetic order. They found that this switch completely changed how the light fluid interacted with itself. In the natural antiferromagnetic state, the particles in the fluid attracted one another, causing the energy of the light to drop as more particles were added. But when the magnetic field forced the crystal into the ferromagnetic state, the interaction flipped: the particles began to repel each other, causing the energy of the light to rise. This ability to toggle between attraction and repulsion simply by changing the magnetic field is a rare and powerful effect, suggesting that the magnetic order of the material directly controls the forces between the light particles.

To prove that they had truly created a condensate and not just a bright light source, the researchers looked for specific signatures of quantum coherence. They measured how the light waves lined up with each other in space and time. Below a certain power level, the light emitted from the crystal was chaotic and disordered. But once the pump power exceeded a specific threshold of about 16 milliwatts, the light suddenly became highly organized. The researchers observed that the light waves began to march in step, creating a coherent beam that extended across the entire sample. They also measured the statistical properties of the photons, finding that they transitioned from a random, thermal distribution to a more orderly pattern characteristic of a coherent quantum state. These measurements confirmed that the system had undergone a phase transition into a macroscopic quantum fluid.

The researchers also investigated the mechanism behind the strange change in interaction. They determined that the shift from attraction to repulsion was driven by the way the magnetic order influenced the movement of electrons and the vibrations of the crystal lattice. In the antiferromagnetic state, the presence of magnetic waves, or magnons, within the crystal created an attractive force between the light-matter particles. When the magnetic field switched the crystal to the ferromagnetic state, these magnetic waves were suppressed, and the natural repulsive forces between the particles took over. This finding provides a direct link between the magnetic order of a solid and the collective behavior of a light fluid, showing that the internal magnetism of the material can be used as a dial to tune the fundamental forces acting on the condensate.

This work opens a new path for developing devices where magnetism and light are deeply intertwined. By using a magnetic field to control the interactions within a light fluid, scientists could potentially build optical switches or logic gates that are far more efficient than current technologies. The ability to create a Bose-Einstein condensate, a state of matter where particles act as a single quantum entity, within a magnetic semiconductor suggests that we are entering an era of "optomagnonic" devices. These future tools could couple the spin of electrons directly to on-chip light sources, enabling new ways to process information or sense magnetic fields with unprecedented sensitivity. The study confirms that the van der Waals magnet CrSBr is a unique playground where the complex dance of photons, magnetic waves, and excitons can be observed and controlled, offering a tangible route toward manipulating quantum fluids with magnetic fields.