On the "inverse" isotope effect in organic superconductors: new findings and implications

The "inverse" deuterium isotope effect, previously found in kappa-(BEDT-TTF )(2)Cu(NCS)(2), is also found in two other BEDT-TTF based superconductors w ith different packing motifs and different types of anions. Remarkably, the magnitude of the isotope shift is essentially identical in all three super conductors, ca. +0.26 +/- 0.06 K. These results, when taken together with t he recent results of Lang et al. on the uniaxial pressure derivatives of T- c, suggest that the "inverse" isotope effect may not have a direct relation ship to the pairing mechanism but instead is a reflection of the change in the "internal lattice pressure".