c-axis penetration depth in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single   crystals measured by ac-susceptibility and cavity perturbation technique
  The $c$-axis penetration depth $\Delta\lambda_c$ in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (BSCCO) single crystals as a function of temperature has been determined using two techniques, namely, measurements of the ac-susceptibility at a frequency of 100 kHz and the surface impedance at 9.4 GHz. Both techniques yield an almost linear function $\Delta\lambda_c(T)\propto T$ in the temperature range T<0.5 T_c. Electrodynamic analysis of the impedance anisotropy has allowed us to estimate $\lambda_c(0)\approx 50 \mu$m in BSCCO crystals overdoped with oxygen ($T_c\approx 84$ K) and $\lambda_c(0)\approx 150 \mu$m at the optimal doping level ($T_c\approx 90$ K). 
D.V.Shovkun, M.R.Trunin, A.A.Zhukov, Yu.A.Nefyodov, N.Bontemps,   H.Enriquez, A.Buzdin, M.Daumens, and T.Tamegai