ESR of TMPD–TCNQ: Spin Excitations of the Heisenberg Regular Linear Chain

Abstract

The detailed electron spin resonance of single crystals of the charge‐transfer complex tetramethylphenylenediamine–tetracyanoquinodimethan (TMPD–TCNQ) at X band and Q band are reported. The ESR spectra are interpreted as resulting from the thermally excited (activation energy, 0.068 eV) spin excitations of the Heisenberg regular antiferromagnetically coupled chain. Although the exact solution to the excited states of the Heisenberg chain with antiferromagnetic coupling is not available, an approximate solution treating the quasiparticles as Wannier spin excitons is very successful at predicting the spin resonance properties of the excitations; fundamental to the Wannier spin exciton model is the absence of spin correlation in the excitations. The spin–spin ${\mathrm{(T}}_2^{\text{−1}})$ and spin–lattice ${\mathrm{(T}}_1^{\text{−1}})$ relaxation rates are observed as functions of temperature and are determined mainly by spin exchange between excitons. Exchange is proportional to exciton concentration, and the low activation energy in TMPD–TCNQ allows exchange frequencies on the order of the Larmor frequency to be achieved without decomposition. Nonsecular broadening of the linewidth is observed and interpreted in terms of the Kubo–Tomita formalism, modified for the uncorrelated spins of the excitations. Similar equations describe the spin–lattice relaxation. The form of the correlation function for spin fluctuations in one dimension can be deduced from the $T_1$ data and is shown to be closely Lorentzian, rather than Gaussian. The crystal structure is known and both the $g$ tensor and the principal parts of the dipolar secular moment for the excitations are shown to have the molecular rather than crystal symmetry.