Magnetism in BEDT-TTF materials

Strong commensurate antiferromagnetism proximate to superconductivity is found in some members of the kappa-(ET) family, while a spin gap (SG) is found in the theta-(ET). Both kappa- and theta-(ET) materials have frustrated triangular lattice structures. We show from calculations of spin-spin correlations within the effective half-filled band triangular lattice proposed for the kappa-ET, as well as for the real lattice, that long range AFM order is not obtained as a consequence of this frustration. We argue that some other mechanism reduces the magnetic frustration in these systems. We show that the low temperature magnetic states in these materials can only be understood if the effects of the {\it cooperative} charge and bond ordering transitions occurring at higher temperatures in these systems are taken into account. In the kappa-ET, this co-operative transition leads to unequal hole populations on the ET dimers that form the triangular lattice.