In-plane magnetic penetration depth λab in Ca2-xNaxCuO2Cl2:: Role of the apical sites

A study of the in-plane magnetic penetration depth lambda(ab) in a series of the cuprate superconductors Ca2-xNaxCuO2Cl2 (Na-CCOC) with Na content x similar or equal to 0.11, 0.12, 0.15, 0.18, and 0.19 is reported. The zero-temperature values of lambda(ab)(0) were obtained by means of the muon-spin rotation technique, as well as from measurements of the intrinsic susceptibility chi(int)(0) by using the procedure developed by Kanigel [Phys. Rev. B 71, 224511 (2005)]. lambda(ab) at T = 0 K was found to increase with decreasing doping from lambda(ab)(0) = 316(19 nm) for the x similar or equal to 0.19 sample to lambda(ab)(0) = 430(26) nm for the x similar or equal to 0.11 one. From a comparison of the present Na-CCOC data with those of Br2Sr2-xLaxCuO6+delta and La2-xSrxCuO4 cuprate superconductors, it is concluded that substitution of the apical oxygen by chlorine decreases the coupling between the superconducting CuO2 planes, leading to an enhancement of the two-dimensional properties of Na-CCOC.