Spin-Peierls transition in N-methyl-N-ethyl-morpholinium-ditetracyanoquinodimethanide [MEM-(TCNQ)2]

In this paper we reinterpret the magnetic-susceptibility data and present and discuss specificheat data on MEM-${\mathrm{}\left(\mathrm{TCNQ}\right)\mathrm{}}_2$ in terms of a spin-Peierls transition theory. We find that the data can be described reasonably well by a mean-field spin-Peierls transition theory which suggests that at low temperatures the TCNQ chain should be tetramerized. The magnetic susceptibility above the transition temperature is shown to behave like a one-dimensional Heisenberg antiferromagnet. The consequences of this behavior on the relative magnitude of the on-site Coulomb interaction are discussed.