Observation of Phasons in Metallic Rubidium

The heat-capacity anomaly caused by phasons—the collective excitations of an incommensurate charge-density wave—is identified in rubidium metal at 0.8 ${}_{}{}^{\mathrm{o}}{}_{}{}^{}\mathrm{K}$. Data analysis leads to a value ${\Theta }_{\varphi }=4.5\mathrm{}°$K for the phason cutoff frequency. The velocity ratio of a purely longitudinal to a purely transverse phason is found to be ∼11.