Angle-resolved photoemission spectroscopy is a powerful experimental technique that can measure the energy-momentum relation of electron removal excitation from interacting electrons in a solid. This relation is often referred to as band dispersion, and it is split into spin-up and spin-down branches in ferromagnets and ferrimagnets with net magnetization. Such band splitting in a collinear magnet without net magnetization is a unique signature of altermagnetism. This state has recently attracted considerable interest due to its excellent suitability for   applications in spintronics.

In this study, we observed such band splitting in the magnetic phase of BaFe₂S₃ with a Fe spin ladder structure with no net magnetization by means of angle-resolved photoemission spectroscopy. In the magnetic and insulating   state of BaFe₂S₃, each Fe ion takes the d⁶ high-spin electronic configuration. In each Fe spin ladder, the spin-up and spin-down sublattices are not connected through translation or inversion because of the tetrahedral coordination of S around Fe. Rather, the sublattices are connected through reflection and form a collinear arrangement without net magnetization. As a result, each Fe spin ladder satisfies the condition of altermagnetism once the magnetic order is established below 120 K. However, the magnetic state of BaFe₂S₃ differs from that expected for a typical altermagnet because the neighboring ladders are connected through translation. Therefore, the observed splitting in the magnetic phase of BaFe₂S₃ at 80 K indicates a novel mechanism of magnetic band splitting that should be explored through   experimental and theoretical investigations in future research.

In addition, BaFe₂S₃ exhibits pressure-induced superconductivity below 24 K. Possible interplay between magnetic band splitting and superconductivity makes future applications of BaFe₂S₃ even more attractive.

(Written by T. Mizokawa on behalf of all authors.)

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