c-axis penetration depth in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ single crystals measured by ac-susceptibility and cavity perturbation technique

Abstract

The $c$-axis penetration depth $Δλ_c$ in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (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 $Δλ_c(T)\propto T$ in the temperature range T<0.5 T_c. Electrodynamic analysis of the impedance anisotropy has allowed us to estimate $λ_c(0)\approx 50 μ$m in BSCCO crystals overdoped with oxygen ($T_c\approx 84$ K) and $λ_c(0)\approx 150 μ$m at the optimal doping level ($T_c\approx 90$ K).