6.1.2 Organic spin Peierls materials

The spin Peierls transition was observed experimentally in the 1970's, with organic materials such as, TTF-CuBDT, TTF-AuBDT [127,128] and MEM-(TCNQ)₂ [129]. These materials show a structural transition at relatively high temperatures ($T_{\rm L}$=240 K for TTF-CuBDT and 340 K for MEM-(TCNQ)₂), so that a one dimensional chain structure is prepared. The S=1/2 spins are held by a lone-pair on a molecule (TTF⁺ and (TCNQ)₂^-).

In these materials, the spin Peierls transition was identified with an exponential drop of susceptibility below $T_{\rm SP}$ (11 K for TTF-CuBDT [127,128] and 18 K for MEM-(TCNQ)₂ [129]). Contrary to an antiferromagnetic Néel order, the drop of susceptibility is isotropic, suggesting a spin singlet formation. In the following, other experimental signatures of spin Peierls transition are introduced.