Incommensurate broken-helix and broken-fanlike states in axion insulator candidate EuIn_2As_2

Zintl phase EuIn_2As_2 has garnered growing attention as an axion insulator candidate, triggered by the identification of a commensurate double-𝐐 broken-helix state in previous studies, however, its periodicity and symmetry remain subjects of debate. Here, we perform resonant x-ray scattering experiments on EuIn_2As_2, revealing an incommensurate nature of the broken-helix state, where both the wave number and the amplitude of the helical modulation exhibit systematic sample dependence. Furthermore, the application of an in-plane magnetic field brings about a broken-fanlike state preserving the double-𝐐 nature, which could be attributed to multiple-spin interactions in momentum space. We propose that the itinerant character of EuIn_2As_2, possibly induced by Eu deficiency, gives rise to the helical modulation and impedes the realization of a theoretically-predicted axion state with the collinear antiferromagnetic order.