Abstract
We have used the triple-axis neutron-scattering technique to study and which undergo charge-density wave transitions at K and K, respectively. The transitions in both compounds appear to be second-order and involve atomic displacements of symmetry. At inception, the superlattices in both compounds have nearly identical incommensurate wave vectors with magnitude , with . The Nb superlattice remains incommensurate to 5 K but Ta undergoes a first-order lock-in transition where at 90 K. The temperature dependence of the superlattice wave vector in the incommensurate phase and the lock-in transition are discussed using a free energy involving third-order "umklapp" terms and a secondary order parameter. The secondary lattice distortion which is predicted in this model is observed experimentally. Most phonon branches having energies less than 10 meV with propagation vectors in the [] and [] directions have been measured at 300 K. Strong anomalies are found in the phonon branches in both materials near the wave vector characteristic of the low-temperature superlattices. Substantial softening of this phonon is observed as the transition is approached. In addition, the spectral profile exhibits a central peak which is not measurably inelastic.
- Received 20 December 1976
DOI:https://doi.org/10.1103/PhysRevB.16.801
©1977 American Physical Society