Disorder and $c$-axis quasiparticle dynamics in underdoped Bi2Sr2CaCu2O8

Contribution à LT25; International audience; We present measurements of the Josephson plasma frequency and the in-plane penetration depth of underdoped single crystalline Bi2Sr2CaCu2O8 with varying degrees of disorder introduced by irradiation with 2.3 MeV electrons. Increasing disorder drives T_c down, in agreement with in all model descriptions of high T_c superconductivity. However, the manner in which the JPR frequency, the square of which represents the zero-frequency spectral weight of the c-axis conductivity in the superconducting state, is driven down by disorder depends more strongly on the model description. We show that only the model of impurity assisted quasiparticle hopping in a d-wave superconductor, together with strongly scattering point defects in the superconducting layers, can explain the disorder dependence of the c-axis plasma frequency, the in-plane penetration depth, and T_c consistently. From the data, we extract the energy scale governing nodal quasiparticle excitations, Delta_0 ~ 2.5 k_BT_c.