TEMPERATURE-DEPENDENCE OF ANISOTROPIC PENETRATION DEPTH IN UNDERDOPEDAND OVERDOPED BI2SR2CACU2O8+Y

We study the temperature (T) dependence of magnetic penetration depth lambda(T) from microwave surface impedance measurements in single crys tals of Bi2Sr2CaCu2O8+y with various oxygen contents. At relatively lo w oxygen content, both the in-plane (lambda(ab)(T)) and out-of-plane ( lambda(c)(T)) penetration depths show the T-linear behavior at low tem peratures, consistent with a clean d-wave superconductor. In contrast, overdoped samples exhibit T-2 dependence of lambda(ab)(T) and lambda( c)(T). This strongly suggests that the quasiparticle excitation spectr um changes into that of a gapless state with oxygen doping.