Lock-in transition of flux-lines in layered superconductors

Abstract

Abstract In an attempt to prove the existence of a lock-in transition of flux-lines in layered superconductors, we have synthesised Nb/Cu multilayers by means of evaporation under ultra-high vacuum conditions. Magnetic moment measurements by means of a high-sensitivity torque magnetometer show a series of maxima at fields B n = 1.01±0.13 × 10 −15 (n 2 /D 2 ), with B n in Tesla and the total thickness D in nm. Maxima up to n= 7 have been observed in several Nb/Cu multilayers with different single layer thickness (varying from 4 to 20 nm). The effect disappears when the angle between B and the plane of the multilayer is larger than ≈5°. This is in good agreement with the critical angle predicted for lock-in of flux-lines in the layer plane. To our knowledge, this is the first experimental evidence for a lock-in transition and enhanced pinning in a layered superconductor with a coherence length ξ comparable to the single layer thickness.