C -axis coupling in underdoped Bi2 Sr2 Ca Cu2 O8+δ with varying degrees of disorder

The dependence of the Josephson plasma resonance (JPR) frequency in heavily underdoped Bi2 Sr2 Ca Cu2 O8+δ on temperature and controlled pointlike disorder, introduced by high-energy electron irradiation, is cross-correlated and compared to the behavior of the ab -plane penetration depth. It is found that the square of the zero-temperature plasma frequency, representative of the superfluid component of the c -axis spectral weight, decreases proportionally with Tc when the disorder is increased. The temperature dependence of the JPR frequency is the same for all disorder levels, including pristine crystals. The reduction of the c -axis superfluid density as function of disorder is accounted for by pair breaking induced by impurity scattering in the Cu O2 planes, rather than by quantum fluctuations of the superconducting phase. The reduction of the c -axis superfluid density as function of temperature follows a T2 law and is accounted for by quasiparticle hopping through impurity-induced interlayer states.