Potassium-tetracyanoquinodimethane (K-TCNQ): A spin-Peierls system

The paramagnetic susceptibility of potassium-tetracyanoquinodimethane (K-TCNQ) as a function of temperature is analyzed. K-TCNQ is considered to be in the strong to intermediate correlation regime of the Hubbard model. The magnetic properties are described in terms of a Heisenberg exchange Hamiltonian between localized spins. The magnetic susceptibility of the high-temperature phase is fitted using the Heisenberg uniform-chain theory of Bonner and Fisher with an exchange constant J which is temperature dependent as a result of the thermal expansion along the stacking axis. The low-temperature phase is described using a nonuniform Heisenberg chain with a temperature-dependent order parameter δ(T) having the BCS dependence as in the spin-Peierls transition. The phase transition comes from a complex displacive structural change between two locally stable overlapping modes of the TCNQ^- anions which is driven by a dimerization instability of the spin-Peierls type.