Universal temperature dependence of the London penetration depth in κ − ( ET ) 2 X 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 − (D 8 ET)2 Cu [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 η sc varies logarithmically with the cooling rate over nearly five decades. The average size of superconducting regions is estimated to vary from 5 to 40 μ m , 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)2 Cu (NCS)2 [A. Carrington et al., Phys. Rev. Lett 83, 4172 (1999)]. 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.