Superconducting properties of indium in the restricted geometry of porous Vycor glass

The superconducting transition of indium in 56-Å-pore Vycor glass has been studied through resistance and magnetic-susceptibility measurements. The confined indium behaves like a ‘‘dirty’’ type-II superconductor, and the superconducting transition consists of ItwoR transitions, which become well separated in applied magnetic fields. While the lower critical field ${\mathit{H}}_{\mathit{c}}^{\mathit{l}}$ follows de Gennes’s prediction of ${\mathit{H}}_{\mathit{c}2}$(T), the higher critical field ${\mathit{H}}_{\mathit{c}}^{\mathit{u}}$ has an unusual dependence of (1-T/${\mathit{T}}_{\mathit{c}}$ ${)}^{1/2}$. The possible origin of this transition is discussed in terms of the microstructure of the composite.