Universal temperature dependence of the London penetration depth in κ-(ET)2X superconductors

High-precision radio-frequency magnetic susceptibility measurements were performed on single crystals of fully deuterated κ-(ET)2Cu[N(CN)2]Br, hereafter designated as κ-(D8ET)2Cu[N(CN)2]Br. This material phase separates into superconducting and antiferromagnetic regions, the degree of which depends strongly upon the cooling rate. We show that the screening fraction ηsc varies logarithmically with the cooling rate over nearly five decades. The average size of superconducting regions is estimated to vary from 5 to 40μm, depending upon cooling rate, consistent with previous infrared microscopy measurements. In the region T≲Tc/3, the effective magnetic penetration depth exhibits power-law behavior λ(T)-λ(0)∼Tn with n=1.6, independent of the cooling rate. Changes in cooling rate and the consequent phase separation evidently do not introduce the kind of disorder that would alter the exponent n in a d-wave superconductor. The exponent remains close to n= 1.5, reported in single crystals of κ-(ET)2Cu[N(CN)2]Br and κ-(ET)2Cu(NCS)2 [A. Carrington et al., Phys. Rev. Lett 83, 4172 (1999)PRLTAO0031-900710.1103/PhysRevLett.83.4172]. The transition temperature fell linearly with 1-ηsc. Measurements were also made on κ-(ET)2Cu[N(CN)2]Cl of normal isotopic abundance in which a very small amount of superconducting phase ηsc≈10-4 developed, presumably through the strain-induced sample mounting. This material showed a power-law exponent of n=1.64, independent of the cooling rate.