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. (paper)