Evidence for a nematic phase in La${}_{1.75}$Sr${}_{0.25}$NiO${}_4$

Determining the nature of electronic states in doped Mott insulators remains a challenging task. In the case of tetragonal La${}_{2-x}$Sr${}_x$NiO${}_4$, the occurrence of diagonal charge and spin stripe order in the ground state is now well established. In contrast, the nature of the high-temperature “disordered” state from which the stripe order develops has long been a subject of controversy, with considerable speculation regarding a polaronic liquid. Following on the recent detection of dynamic charge stripes, we use neutron scattering measurements on an $x=0.25$ crystal to demonstrate that the dispersion of the charge stripe excitations is anisotropic. This observation provides compelling evidence for the presence of electronic nematic order.