Phonon Softening in the CDW Superconductor U6Fe

U6Fe is an interesting superconductor with an exceptionally high critical field of 12 T and a superconducting transition temperature of 4K, the highest known for a uranium-based heavy-fermion superconductor, where neither the Pauli nor usual BCS orbital limit is apparently respected. Previous studies of bulk U6Fe provide evidence that the material may undergo a change or possible phase transition at around 110 K, while X-rays showed a CDW that appeared below 10 K. The interplay between CDW order and superconductivity is highly topical, both in the context of heavy fermions but also for high-temperature superconductivity. For U6Fe a CDW may promote the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, an unusual state in which the superconducting order parameter is modulated in real space.
We propose measuring the phonon dispersion in U6Fe to identify the soft phonon associated with the CDW and determine whether this same phonon or another mode is responsible for a second transition at 110 K. The measurements are also expected to detect elastic CDW peaks for the first time with neutrons.