  In the most studied family of organic superconductors kappa-(BEDT-TTF)_2X,
the BEDT-TTF molecules that make up the conducting planes are coupled as
dimers. For some anions X, an antiferromagnetic insulator is found at low
temperatures adjacent to superconductivity. With an average of one hole carrier
per dimer, the BEDT-TTF band is effectively 1/2-filled. Numerous theories have
suggested that fluctuations of the magnetic order can drive superconducting
pairing in these models, even as direct calculations of superconducting pairing
in monomer 1/2-filled band models find no superconductivity. Here we present
accurate zero-temperature Density Matrix Renormalization Group (DMRG)
calculations of a dimerized lattice with one hole per dimer. While we do find
an antiferromagnetic state in our results, we find no evidence for
superconducting pairing. This further demonstrates that magnetic fluctuations
in the effective 1/2-filled band approach do not drive superconductivity in
these and related materials.
