Symmetry-selective nonrelativistic spin splitting in antiferromagnets driven by coherent phonons

In the world of modern electronics, controlling the flow of information often relies on magnetism. Traditional devices use materials like iron, where tiny atomic magnets all point in the same direction, creating a net magnetic field that can be easily manipulated. However, this magnetic field is also a liability; it creates interference and requires significant energy to switch states. Scientists have long sought a way to harness the spin of electrons—their intrinsic angular momentum, which acts like a tiny internal compass—without the baggage of a net magnetic field. This is the promise of antiferromagnets, a class of materials where neighboring atomic magnets point in opposite directions, canceling each other out perfectly. While these materials are invisible to external magnetic fields and ideal for ultrafast, low-power computing, they have historically been difficult to control because their internal order is so stable and hidden. A recent breakthrough involves finding a way to split the energy levels of electrons with opposite spins in these materials, a process known as nonrelativistic spin splitting, which allows for the manipulation of electron flow without needing the heavy machinery of traditional magnetism.

A team of researchers has now demonstrated a method to dynamically induce and control this splitting in antiferromagnets using sound waves at the atomic scale. In a study focused on a layered crystal called manganese phosphorus trisulfide, the scientists showed that by vibrating the material with specific patterns of light, they could temporarily break the symmetry that keeps the electrons' spins equal. This process does not rely on the material's natural magnetic state but is instead driven by the precise way the atoms are shaken. The researchers found that the direction of the light used to shake the atoms acts like a switch, selecting which type of spin behavior emerges. By aiming the light in one direction, they created a state where the energy difference between spin-up and spin-down electrons is present everywhere, including at the center of the material's momentum map. By aiming the light in a perpendicular direction, they created a different state where this energy difference vanishes at the center but appears strongly in other directions, a pattern characteristic of a newly discovered class of magnetic materials called altermagnets.

The study relied on the fact that the atoms in this crystal can vibrate in specific, coordinated ways when hit by light. The researchers focused on two distinct types of vibrations that occur at a frequency of roughly 4.5 terahertz, a speed far too fast for the human ear to hear but perfectly matched to the natural rhythm of the crystal's atoms. These vibrations are "infrared-active," meaning the atoms move in a way that allows them to absorb light energy directly. The team used first-principles calculations to model what happens when these vibrations are frozen in place, effectively creating a snapshot of the crystal while it is distorted. They discovered that the two vibrations, though similar in speed, move the atoms in fundamentally different patterns. One vibration pushes the manganese atoms in a direction that breaks all the symmetrical connections between the opposing magnetic layers. The other vibration moves the atoms in a way that preserves a specific mirror-like symmetry between those layers.

This difference in symmetry dictates the behavior of the electrons. When the vibration breaks all connections between the opposing magnetic layers, the electrons with opposite spins separate in energy across the entire material, creating a uniform split. This is described as an s-wave state, where the effect is consistent in all directions. In contrast, when the vibration preserves the mirror symmetry, the energy split behaves differently. It is zero at the very center of the momentum map but grows larger as you move away, changing sign as you cross certain lines. This creates a d-wave pattern, which is the signature of the altermagnetic state. The researchers calculated that for a vibration amplitude of 0.05 angstroms, the energy difference between the spins reaches about 50 to 100 millielectronvolts, a significant shift that could be detected and utilized in electronic devices. Crucially, the size of this energy split grows linearly with the strength of the vibration, meaning the effect can be tuned simply by adjusting the intensity of the light.

The power of this approach lies in its selectivity and speed. Because the two vibrations respond to light polarized in different directions, a scientist can choose which electronic state to create simply by rotating the polarization of the incoming light beam. If the light is polarized one way, the uniform s-wave state appears; if it is rotated ninety degrees, the directional d-wave state takes over. Furthermore, because the vibration is driven by a coherent light pulse, the direction of the atomic displacement flips back and forth rapidly. This causes the induced spin splitting to reverse its sign in sync with the vibration, oscillating at the frequency of the light. This suggests that it is possible to switch between different spin states at terahertz speeds, far faster than current electronic switches can operate. The study confirms that these effects are primarily due to the mechanical distortion of the crystal lattice rather than relativistic effects, making the phenomenon robust and predictable.

These findings establish a new pathway for controlling spin in antiferromagnets without the need for static magnetic fields or chemical changes. By using light to drive specific atomic vibrations, researchers can dynamically engineer the electronic properties of a material, turning it into a switchable spin filter. The ability to select between a uniform spin-split state and a directional altermagnetic state simply by changing the angle of the light offers a versatile tool for future spintronic devices. The work suggests that the symmetry of a material's internal structure can be temporarily rewritten by sound waves, opening the door to ultrafast, low-power electronics that operate on principles previously thought to be fixed by the material's natural state. This method provides a direct route to manipulating the flow of information in the next generation of computing hardware, where speed and energy efficiency are paramount.