Electron tunnelling evidence for fluxon entry into thin type II superconducting films

Electron tunnelling from a normal metal into thin type II superconducting films (Pb-In 20 wt%) has been studied as a function of magnetic field H and film thickness (600-7000 Å) at 4.2 and 1.5 °K. The behaviour at high field confirms the work of Tomasch: surface superconductivity is quenched at H c2 , the upper critical field, when H is perpendicular to the film surface, but persists to H c3 , the surface sheath critical field, in a parallel field ( H || ). The degree of surface conductivity is characterized by a current profile Δ I , defined by the tunnel junctions' ( I, V ) characteristics. For films thicker than 1000 Å, a plot of Δ I against H || exhibits a series of dips which is dependent on film thickness and temperature. At fields where this structure is present, hysteresis is found in decreasing H . The structure and hysteresis are attributed to entry and exit of fluxons from the films. Penetration of H || into a thin film causes fluxon entry to be delayed to a field H e0 , greater than H c1 , the Abrikosov lower critical field. It is argued that subsequent fluxon entry into a thin film will occur mainly at fields close to H en , at which the spacing of fluxons is such as to permit their hexagonal packing into the film's finite thickness. Reasonable agreement is found between the calculated values for H en and the fields at which the minima of the dips occur in the (Δ I, H || ) characteristics. An image calculation of the surface barrier to fluxon entry and escape from thin films shows that their effect becomes less pronounced as film thickness is reduced. It is thought that, in the films used in these experiments, barriers to fluxon entry are rendered ineffective by surface irregularities so that fluxon entry is not delayed above H e0 . In a decreasing field fluxons are thought to be trapped by the surface barrier to fluxon escape and cause hysteresis in a plot of Δ I against H || . A mechanism is proposed by which the variation in the rate of fluxon entry into the films produces the structure in the (Δ I, H || ) characteristics. The argument is based on dubious assumptions as to the microscopic factors governing Δ I .