Neutron scattering study of the charge-density wave transitions in 2HTaSe2 and 2HNbSe2

D. E. Moncton, J. D. Axe, and F. J. DiSalvo
Phys. Rev. B 16, 801 – Published 15 July 1977
PDFExport Citation

Abstract

We have used the triple-axis neutron-scattering technique to study 2HTaSe2 and 2HNbSe2 which undergo charge-density wave transitions at T0=122.3 K and T0=33.5 K, respectively. The transitions in both compounds appear to be second-order and involve atomic displacements of Σ1 symmetry. At inception, the superlattices in both compounds have nearly identical incommensurate wave vectors with magnitude qδ=13(1δ)a*, with δ0.02. The NbSe2 superlattice remains incommensurate to 5 K but TaSe2 undergoes a first-order lock-in transition where δ0 at 90 K. The temperature dependence of the superlattice wave vector q 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 [ζ00] and [00ζ] directions have been measured at 300 K. Strong anomalies are found in the Σ1[ζ00] phonon branches in both materials near the wave vector qc=(13,0,0) 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

Authors & Affiliations

D. E. Moncton*,†

  • Physics Department, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
  • Brookhaven National Laboratory, Upton, New York 11973

J. D. Axe*

  • Brookhaven National Laboratory, Upton, New York 11973

F. J. DiSalvo

  • Bell Laboratories, Murray Hill, New Jersey 07974

  • *Present Address: Bell Laboratories, Murray Hill, New Jersey, 07974. Worked based in part on the thesis by D. E. Moncton, submitted in partial fulfillment of the requirements for the Ph.D. degree to Massachusetts Institute of Technology, 1975.
  • Work performed under the auspices of the U. S. ERDA.

References (Subscription Required)

Click to Expand
Issue

Vol. 16, Iss. 2 — 15 July 1977

Reuse & Permissions
Access Options
Author publication services for translation and copyediting assistance advertisement

Authorization Required


×
×

Images

×

Sign up to receive regular email alerts from Physical Review B

Log In

Cancel
×

Search


Article Lookup

Paste a citation or DOI

Enter a citation
×