Elastic Anomalies at Charge Density Wave Transitions in TaS3 and NbSe3

The temperature dependence of the Young's modulus and internal friction of TaS3 (orthorhombic) and NbSe3 have been measured by the vibrating reed technique. There is a large minimum 2%, in the modulus of TaS3 at the commensurate Peierls distortion (Tc = 222K) and the internal friction increases below Tc(ΔQ−1 = 5×10−5), although no critical relaxation effects are observed. On the other hand, a very small minimum, ΔE/E ~ 0.06%, is observed at the upper incommensurate transition in NbSe3, and a small broad increase (0.03%) at the lower transition. The measurements allow estimates to be made of the stress dependence of the transition temperature and of the electron-phonon coupling constant.