Dibenzo-TTF-dichloro-TCNQ: a quasi-one-dimensional magnetic semiconductor

An experimental study of the 1:1 charge transfer salt of Delta 2,2' -bi-d-benzo-1,3-dithiol and 2,5-dichloro-7,7',8,8'-tetracyano-p-quinodimethane (DBTTF-TCNQCl 2 ), is presented. Despite a crystal structure with uniform and segregated stacks of molecules, the room temperature conductivity is rather low, approximately 40 Omega -1 cm -1 , and increases only slightly upon cooling. The conductivity decreases rapidly below 200K. The thermopower is found to increase monotonically with decreasing temperature. Anomalies in thermopower as well as in conductivity at T C1 =180K separate conducting and non-conducting regimes. The magnetic susceptibility is high ( chi (300K) approximately=7*10 -4 emu/mole) and increases slowly down to 100K. There is no anomaly near T C1 . A drastic drop in chi below 40K indicates an essentially magnetic phase transition with transition temperature T C2 =38K. Optical measurements of the plasma edge are consistent with a rather small bandwidth approximately 0.3 eV. The behaviour around T C1 is discussed in terms of a Coulomb driven localisation of the carriers.