XPCS is used to study the slow dynamics of various equilibrium and non-equilibrium processes occurring in condensed matter systems.
This technique is based on the generation of a speckle pattern by the scattered coherent light originating from a material where some spatial inhomogeneities are present.
A speckle pattern is a diffraction limited structure factor, and is typically observed when laser light is reflected from a rough surface, or from dust particles performing Brownian motion in air.
This observation is only possible now because of the development of new synchrotron radiation X-ray sources that can provide sufficient coherent flux.
A specific subgroup of these techniques is atomic-scale X-ray photon correlation spectroscopy (aXPCS).