Warm Dense Matter (WDM) is generally described as matter that is at densities broadly comparable to the solid state (0.1-10 g/cc) and at temperatures from 1-100eV. A key characteristic of this matter is the strong correlation between the ions due to their Coulomb interaction. This correlation leads to microscopic structure in the plasma that can be described by a pair correlation function. The degree of correlation depends on the ratio of kinetic energy to Coulomb potential energy. For ion-ion correlations we can define a so called strong coupling parameter:
which is in essence the ratio of Coulombic energy to thermal energy. Systems that have such high correlation factors include the interiors of white dwarfs (~20-200) as well as the interior of giant planets like Jupiter (~20-50). In the latter case, the system is more complicated as the electrons are only partially degenerate and thus form a polarisable gas. The correlation can be seen in the image below as a function of the strong coupling parameter. An important point about this kind of matter is that the Coulomb interactions are strong enough that they cannot be considered as a perturbation to the thermal motion as in classical plasma physics. On the other hand, the temperature is usually of the order of or greater than the Fermi energy and so methods relevant to solid state physics can not be applied directly. The correlation affects almost all aspects of WDM properties, it alters the resitivity, conductivity and compressibility. The elucidation of the microscopic structure is therefore a powerful tool in understanding WDM.