Strain Engineering of Intrinsic Anomalous Hall and Nernst Effects in Altermagnetic MnTe at Realistic Doping Levels

In the hidden world of solid materials, scientists have long been fascinated by a peculiar class of substances known as altermagnets. These materials possess a unique internal order where the magnetic spins of their atoms are arranged in a pattern that breaks time-reversal symmetry, a fundamental property of nature, yet they manage to do so without creating any overall magnetic pull. Imagine a crowd of people where half face north and half face south; from a distance, the crowd has no net direction, but the individual orientations are strictly organized. For decades, researchers believed that such a state could not generate the anomalous Hall effect, a phenomenon where an electric current flowing through a material is deflected sideways, creating a voltage. This sideways deflection was thought to be the exclusive domain of ferromagnets, the magnets that stick to your refrigerator. However, recent theoretical work suggested that altermagnets should be able to produce this effect, but when scientists tested a leading candidate, hexagonal manganese telluride, the signal they found was disappointingly faint. The question remained: was the effect truly there but hidden, or was it absent entirely?

A team of researchers has now provided a clear answer, revealing that the effect was indeed present but was being silently canceled out by the material's own symmetry. By using a mathematical model to simulate the behavior of electrons in hexagonal manganese telluride, they discovered that the material's internal structure acts like a perfect balance scale. Near the energy levels where the material naturally conducts electricity, the electrons generate two opposing sideways pushes that are equal in strength but opposite in direction. Because the material's crystal lattice is highly symmetric, these two pushes cancel each other out completely, leaving a net signal that is nearly zero. This explains why early experiments saw such weak results; the material was not failing to produce the effect, but rather producing two versions of it that erased one another.

To solve this puzzle, the researchers proposed a simple yet powerful strategy: they simulated the effect of stretching the material's crystal lattice in a specific way. By applying a precise, volume-conserving strain—essentially squeezing the material in one direction while letting it expand slightly in another without changing its total volume—they broke the perfect symmetry that caused the cancellation. This distortion reshaped the paths the electrons take, known as the Fermi surface, and tipped the balance. Instead of two equal and opposite pushes, one direction became dominant. The result was a dramatic amplification of the anomalous Hall effect, with the sideways voltage increasing by orders of magnitude. The researchers found that even a tiny amount of stretching, roughly two percent, was enough to unlock this hidden potential, turning a nearly invisible signal into a robust one that could be easily measured.

The study also looked at a related phenomenon called the anomalous Nernst effect, which occurs when a temperature difference across the material generates a sideways voltage instead of an electric current. Just as with the Hall effect, the researchers found that applying this same strain significantly boosted the Nernst signal. At room temperature, the effect grew steadily as the strain increased, suggesting that this method could be used to create highly sensitive temperature sensors. Crucially, the team confirmed that this enhancement happened without turning the material into a traditional magnet. Even under strain, the material retained its defining characteristic of having zero net magnetization, proving that the dramatic increase in electrical signals came from a change in the material's electronic topology rather than the creation of a magnetic moment.

These findings suggest that the weak signals observed in previous experiments were not a failure of the material, but a consequence of its perfect symmetry. By introducing a controlled amount of strain, likely through the use of specialized substrates in a laboratory setting, scientists can now manipulate the material to reveal its true topological capabilities. The work demonstrates that the anomalous transport properties of altermagnets are not just theoretical possibilities but are accessible and tunable in real-world conditions. This opens the door to new types of electronic devices that can operate without the magnetic interference typical of traditional magnets, relying instead on the subtle, strain-tuned geometry of electron paths to generate powerful electrical responses.