Itinerant density wave instabilities at classical and quantum critical points

Charge ordering in metals is a fundamental instability of the electron sea, occurring in a host of materials and often linked to other collective ground states such as superconductivity. What is difficult to parse, however, is whether the charge order originates among the itinerant electrons or whether it arises from the ionic lattice. Here we employ high-resolution X-ray diffraction, combined with high-pressure and low-temperature techniques and theoretical modelling, to trace the evolution of the ordering wavevector Q in charge and spin density wave systems at the approach to both thermal and quantum phase transitions. The non-monotonic behaviour of Q with pressure and the limiting sinusoidal form of the density wave point to the dominant role of the itinerant instability in the vicinity of the critical points, with little influence from the lattice. Fluctuations rather than disorder seem to disrupt coherence.