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

The inverse deuterium isotope effect, previously found in κ-(BEDT-TTF) 2 Cu(NCS) 2 , is also found in two other BEDT-TTF based superconductors with different packing motifs and different types of anions. Remarkably, the magnitude of the isotope shift is essentially identical in all three superconductors, ca. +0.26 ±0.06 K. These results, when taken together with the 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 relationship to the pairing mechanism but instead is a reflection of the change in the internal lattice pressure.