Phonon softening in alpha-uranium associated with the CDW transition

Uranium is the only element known so far to exhibit a charge-density wave (CDW) instability (alpha phase; orthorhombic Cmcm space group, Z = 4, T-o = 43 K). The structure below the transition is complex, consisting of small displacements of the atoms along all three of the orthorhombic axis. The main displacement is associated with a doubling of the a-axis, and it has been known for many years that the transition may be regarded as one in which a soft phonon condenses at this point half way across the Brillouin zone. The theory of Yamada (1993, Phys. Rev. B 47, 5641) suggests that the only important component of the phonon wave vector is that along [100]. We report here high-resolution neutron scattering experiments at low temperature which show that the principal softening occurs at the exact CDW points in the Brillouin zone. This result is in disagreement with the suggestions of Yamada but in better agreement with a new theoretical approach by Fast(1998, Ph.D. Thesis, Uppsala University, Sweden). (C) 1999 Elsevier Science B.V. All rights reserved.