Different regions of fluctuation conductivity in unirradiated and alpha-irradiated Bi₂Sr₂CaCu₂O_8+δ and (Bi,Pb)₂Sr₂CaCu₂O_8+δ superconductors

Abstract

Radiation damage in Bi-2212 and (Bi_0.92Pb_0.17)-2212 superconducting pellets due to irradiation by 40 MeV He⁺⁺-beam has been studied, as these two compounds differ with respect to lattice strain in their Bi-O and (Bi,Pb)-O layers. These samples were prepared with excess oxygen that accounts for their lowered superconducting transition temperature, T_c, of 59.5 and 64.5 K, respectively. Irradiation to 8.14×10¹⁵ He⁺⁺/cm²raised T_cby 4.5 and 5.5 K, respectively, and also their normal state resistivities. Excess conductivity analysis for the unirradiated and irradiated samples showed (i) a transition of the conductivity fluctuation from 2D (λ=1.0) to 3D (λ=0.5) behaviour, on cooling across a temperature T₀, and (ii) also the signature (λ=3.0) of critical fluctuations in a network of superconducting microregions, for temperatures below T_cr, where T_cr is seen to be 5 to 10 K lower than T₀. The rise in T_chas been explained from irradiation-induced removal of oxygen taking the hole concentration towards the value for the peak in T_c.