A powerful new tool for electronic structure studies is resonant inelastic x-ray scattering, a coherent process similar to Raman scattering in which the incident x ray scatters inelastically, creating electronic excitations. The spectra provide information about the electronic density of states in weakly correlated systems and about local interactions in strongly correlated systems such as rare earths and transition-metal oxides. This colloquium surveys the different kinds of experiments providing such resolution, the various sources of intense x rays, and the current experimental constraints on the processes that may be studied. It then describes some of the first experimental results. He when the flow is induced by a heat source is analyzed here theoretically as a critical transition under nonequilibrium conditions.
Gravity is shown to play an important role.
The analysis has application to planned experiments in space, where the microgravity environment makes the transition region between normal and superfluid flow much more accessible to experimental measurements.