Broken inversion symmetry in the charge density wave phase in EuAl4

EuAl4 exhibits a complex phase diagram, including the development of a charge density wave (CDW) below TCDW=145K. Below TN=15.4K, a series of antiferromagnetically (AFM) ordered phases appear, while nontrivial topological phases, like skyrmion lattices, are stabilized under an applied magnetic field. The symmetries of the variously ordered phases are a major issue concerning the understanding of the stabilization of the ordered phases as well as concerning the interplay between the various types of order. EuAl4 at room temperature has tetragonal symmetry with space group I4/mmm. The CDW phase has an incommensurately modulated crystal structure described by the modulation wave vector q≈0.17c*. On the basis of various experiments, including elastic and inelastic x-ray scattering, and second-harmonic generation, it has been proposed that the symmetry of the CDW phase of EuAl4 could be centrosymmetric orthorhombic, noncentrosymmetric orthorhombic or noncentrosymmetric tetragonal. Here, we report temperature-dependent, single-crystal x-ray diffraction experiments that show that the CDW is a transverse CDW with phason disorder, and with noncentrosymmetric symmetry according to the orthorhombic superspace group F222(00σ)00s. Essential for this finding is the availability of a sufficient number of second-order (2q) satellite reflections in the x-ray diffraction data set. The broken inversion symmetry implies that skyrmions might form due to Dzyaloshinskii-Moriya (DM) interactions, instead of a more exotic mechanism as it is required for centrosymmetric structures.