The pinning of the charge density wave to irradiation-induced defects

Abstract

Electron diffraction patterns from neutron- and electron-irradiated samples together with in situ observations in a high-voltage electron microscope have been used to follow the effects of disorder on the periodic lattice distortion (PLD) associated with the charge density wave (CDW) in the quasi two-dimensional conductor 1T-TaS₂ and in the quasi one-dimensional conductor TaS₂. The pinning of the CDW to a low concentration of irradiation-induced defects, of the order of 10⁻³ displacements per Ta atom (d.p.Ta), suppresses the commensurate CDW locked-in to the lattice in 1T-TaS₂ and the three-dimensional ordering of the CDW in TaS₃. With increasing disorder the CDW loses more and more of its flexibility, this leads finally to a defect-dominated strongly incoherent distortion which has no temperature dependence. Even at high doses (a few 10⁻¹ d.p.Ta in 1T-TaS₂) the distortion is still present and the position of the diffuse scattering that it produces (q = 2k _F) is the same as in the pure sample at high temperatures, indicating no change in the band filling arising from irradiation-induced defects.