Fundamental Similarities between Organic and Cuprate Superconductors Shown by Muon Spin Relaxation Studies

Muon Spin Relaxation (μSR) measurements on (BEDT - TTF)2Cu(NCS)2 show that the magnetic field penetration depth λbc within the conductive b-c plane is about 8000 Å at T → 0. This value corresponds to the superconducting carrier density ns = 1.6 × 1020/cm3 for an in-plane effective mass m* = 3.5me. Comparisons with μSR results on high-Tc cuprate superconductors demonstrate that both the present organic superconductor and the hole-doped cuprate systems are characterized by a small number of superconducting pairs, i. e. 5 to 10 pairs, in the coherence area ξ2 within the 2-dimensional conductive plane. Implications of this feature on theories of superconductivity in these systems are discussed.