Soft phonons and the charge-density-wave transition in alpha uranium

Abstract

It has been known since the measurement of the phonon modes in the alpha phase of the element uranium (Cmcm orthogonal structure) that the Σ 4 phonon softens appreciably near the transition to the charge-density-wave (CDW) state. In explaining the driving mechanism for the CDW transition, which has three components in q-space and occurs at T CDW = 43 K, Yamada suggested that the most important parameter is q x (∼ 0.5) and that the small q x and q z components are a consequence of coupling to the strain. Thus, classical soft-mode behavior should be found for the Σ 4 phonon for T > T CDW with its frequency squared being proportional to ( T − T CDW). Experiments on the IN8 triple-axis spectrometer at the ILL support this idea and show that the frequency does not depend on q y or q z , but they also show that E 2 ∝ T, which is unsual.