The quasi-one-dimensional conductors (PE${)}_2$${\mathrm{PF}}_6$⋅2/3THF and (PE${)}_2$${\mathrm{AsF}}_6$⋅2/3THF (PE = perylene, THF = tetrahydrofuran) undergo a Peierls transition at ${\mathit{T}}_{\mathit{P}}$ = 118 K and 102 K, respectively. We present specific-heat data and a detailed electron-spin-resonance (ESR) study of the metallic high-temperature and the semiconducting low-temperature phase (linewidth, intensity, g tensor, 9.5 GHz, and 415 MHz). Above ${\mathit{T}}_{\mathit{P}}$ motional and exchange narrowings in conjunction with the high one dimensionality account for the extremely small width (ΔB≃ 10 mG ) of the Lorentzian conduction electron line. Due to the small linewidth, the g tensor can be determined with high precision. This allows us to follow the reorientation of the perylene stacks in the course of a structural phase transition in the metallic range. Far below ${\mathit{T}}_{\mathit{P}}$ spectra consist of the lines of several defect types. In the intermediate temperature range near below ${\mathit{T}}_{\mathit{P}}$, where defects and conduction electrons contribute to the ESR intensity, the linewidth was analyzed as a function of temperature and orientation. Two of the defect types are found to interact (magnetic dipole-dipole, exchange) with the conduction electrons. Strong exchange coupling leads to a bottleneck situation, which together with a Korringa-type relaxation of the defects accounts for the pronounced increase in linewidth below ${\mathit{T}}_{\mathit{P}}$. We propose the different nature, i.e., localized at a chain end or solitonlike and mobile, of the defect spins to be responsible for the different temperature dependences of the width of the associated lines. © 1996 The American Physical Society.

• Received 16 February 1996

DOI:https://doi.org/10.1103/PhysRevB.54.12272