Magnetic Determination of Ginzburg-Landau Coherence Lengthfor Organic Superconductor κ-(BEDT-TTF)2X (X=Cu(NCS)2,Cu[N(CN)2]Br): Effect of Isotope Substitution

The Ginzburg-Landau coherence lengths are determined from the temperature dependence of the magnetization by fitting measured results with the renormalization theory of the fluctuation developed for layered superconductors. The derived interlayer coherence lengths are 6±2 Å and 3.2±0.5 Å for κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br and κ-(BEDT-TTF) 2 Cu(NCS) 2 , respectively, which are remarkably shorter than the interlayer spacing. It is also found that these coherence lengths are virtually unaffected by isotope substitution in BEDT-TTF molecules.