Soft mode dispersion at the 43 K CDW instability in alpha U

The element uranium, which has an orthorhombic Cmcm structure in its low-temperature alpha phase, exhibits an incommensurate charge density wave (CDW) at T0=43 K. The main component of the atomic displacements is along the a axis. The CDW is driven by a soft optic phonon mode. Neutron scattering experiments show that the phonon mode frequency follows a (T-T0)0.5, as expected in Landau theory. The experiments reported here map the three-dimensional phonon dispersion surface near T0. Comparisons are made to theories discussing the origin of the CDW. (author)