Triple approach to determination of the c-axis penetration depth in BSCCO crystals

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The c-axis penetration depth λc in Bi₂Sr₂CaCu₂O₈₊δ (BSCCO) single crystals as a function of temperature has been determined using three high-frequency techniques, namely: (i) measurements of the ac-susceptibility at a frequency of 100 kHz for different sample alignments with respect to the ac magnetic field; (ii) measurements of the surface impedance in both superconducting and normal states of BSCCO crystals at 9.4 GHz; (iii) measurements of the surface barrier field Hⱼ(T)∝ 1/λc(T) at which Josephson vortices penetrate into the sample. Careful analysis of these measurements, including both numerical solution of the electrodynamic problem of the magnetic field distribution in an anisotropic plate at an arbitrary temperature and influence of defects in the sample, has allowed us to estimate λc(0)≈ 50 μm in BSCCO crystals overdoped with oxygen (Tc≈ 84 K) and λc(0)≈ 150 μm at the optimal doping level (Tc≈ 90 K). The results obtained by different techniques are in reasonable agreement.