Absence of superconductivity and valence bond order in the Hubbard-Heisenberg model for organic charge-transfer solids

A frustrated, effective 1/2-filled band Hubbard-Heisenberg model has been proposed for describing the strongly dimerized charge-transfer solid families κ-(ET)₂X and Z[Pd(dmit)₂]₂. In addition to showing unconventional superconductivity, these materials also exhibit antiferromagnetism, candidate spin-liquid phases, and, in the case of Z = EtMe₃P, a spin-gapped phase that has sometimes been referred to as a valence bond solid. We show that neither superconductivity nor the valence bond order phase occurs within the Hubbard-Heisenberg model. We suggest that a description based on 1 4 -filling, that is reached when the carrier concentration per molecule instead of per dimer is considered, thus may be appropriate.