Evidence for ferroquadrupole order in YbRu2Ge2 from x-ray diffraction and elastoresistivity measurements

YbRu2Ge2 undergoes a non-magnetic phase transition at 10K, several Kelvin above a phase transition to a magnetically ordered state that is characterized by a unidirectional incommensurate spin density wave. Here, we show via high-resolution x-ray diffraction that the non-magnetic phase transition corresponds to a continuous tetragonal-to-orthorhombic structural phase transition. Elastoresistance measurements in the tetragonal state indicate a divergence of the quadrupolar strain susceptibility in the B1g symmetry channel, implying that the structural phase transition is driven by quadrupolar order.