Influence of Ag additions on the superconducting properties of HgBa₂CuO_{4 + δ} materials - Hole doping and surface barrier effects

A series of superconducting HgBa₂CuO_{4 + δ} materials, containing Ag_x additions with molar fraction x up to 0.5, has been studied. The processing with Ag at elevated temperatures led to changes in superconducting properties; the properties are consistently interpreted in terms of the superconducting hole density, as obtained from the London penetration depth λ measured on the same materials. The irreversible magnetic properties are well described in terms of thermally activated tunneling of pancake vortices through a surface barrier.