Parallel upper critical field of niobium/graphite bilayers

The influence of the superconducting proximity effect to the upper critical field (HC) in niobium/graphite bilayers was studied and related to the ratio TC∗/TCS: TC∗ and TCS are the transition temperature of Nb/KG film and that of Nb film respectively. The thickness of niobium (Nb) film was controlled about 40 nm, and that of graphite (kish graphite: KG) film ranged from 120 nm to 140 nm. For making a specimen, Nb was deposited on a KG film and a quartz glass substrate simultaneously, and magnetic field was applied parallel to the Nb/KG interface. HC of Nb/KG film (HC∗) lowered from that of Nb film (HCS) at all temperatures within this work, and the value of ΔHC defined as HCS-HC∗ showed different temperature dependence among samples, depending on whether TC∗/TCS1.00 < or ≧ 1.00. This behavior of HC suggests the additional existence of the interference of electrons in the clean KG film.