Single crystal of uranium (blue square) in the center of the diamond anvil pressure cell. The crystal was thinned to 10 µm at Los Alamos National Laboratory. Diameter of the pressure chamber: 250 µm.
Modeling strongly correlated systems is a very delicate task. For instance, the theory for the phonon spectrum of uranium has only been available since 2008. Our recent measurements have confirmed this theory and our results combined with new calculations provide an interpretation for the very original phase diagram of this metal.
Among simple elements, uranium is certainly the most complex one. Not only is its crystal structure singular, its low temperature behavior is unique as well. While metals are generally either superconducting or magnetic, uranium is neither: its conduction electrons are spontaneously distributed inhomogeneously and form what is called a charge density wave. This state is quite unstable though, because a modest pressure destroys this charge density wave and transforms uranium into a superconductor. Since superconductivity is the result of the coupling between lattice vibration modes, i.e. phonons, and the electrons, we studied phonon dispersion. Our measurements at the ESRF from inelastic x-ray scattering as a function of pressure are in perfect agreement with the prediction of the ab initio calculations performed at DAM using the density functional theory. Then, confident in our model, we examined the electron-phonon coupling influence and we have shown that its variations as a function of pressure explain the crossover from charge density wave to superconductivity. These results will be the basis for future progress in our theoretical insight into strongly correlated systems.
The intensity of x-rays inelastically scattered by phonons varies with pressure according to the model prediction.
Further reading: Raymond S et al., Physical Review Letters 107 (2011) 136401
Maj : 17/02/2014 (916)