Abstract
Magneto-optical imaging studies on a high-quality Bi2Sr2CaCu2O8 single crystal partially patterned with a triangular array of holes reveal enhanced flux shielding in the patterned region of the sample. By mapping local magnetic field and shielding current density distributions at different applied magnetic fields and temperatures we determine the regime where pinning from the patterned holes dominates over the intrinsic pinning in the sample. In this regime, the flux density near the center of the patterned region is observed to increase when the applied field is varied from below the matching field to just above it, while significant magnetic field gradients are sustained in the patterned region. Our measurements indicate heterogeneous pinning properties of the vortex population, exhibiting signatures of both weak and strong pinning, in the nanopatterned region of the superconductor.
Highlights
In pinning-free superconductors vortices are arranged in a well-ordered triangular Abrikosov vortex lattice configuration with a lattice period of a0 ~ φ0 B, where φ0 is the magnetic flux quantum and B is the magnetic field
Measurement of the local shielding response using differential MOI (DMOI) We investigate the nature of the field gradients inside the patterned region and how susceptible the gradients are to field changes
Bz(r) and |j(r)| distributions inside the nanopatterned region are seen to vary nonuniformly across the patterned region at low H. These results suggest that the vortex population inside the nanopatterned region of the sample exhibits signatures associated either with weak or strong pinning viz., a heterogeneous pinning response at low H below Bφ
Summary
In pinning-free superconductors vortices are arranged in a well-ordered triangular Abrikosov vortex lattice configuration with a lattice period (inter-vortex separation) of a0 ~ φ0 B , where φ0 is the magnetic flux quantum and B is the magnetic field. The competing tendency favours a uniform constant gradient in the field distribution or a constant bulk critical current density in the superconductor The competition between these tendencies leads to domains of vortices which are ordered around the periodic pinning sites. We restrict our study to low B < Bφ , viz., the dilute vortex density case to keep the inter-vortex interaction low For this situation at first glance it may be expected that due to comparable inter-vortex and vortex - blind hole pin interaction, vortices will be uniformly distributed across the blind hole array with a uniform filling fraction of ~ B/ Bφ. We have mapped the local magnetic field and shielding current density distributions at different applied magnetic fields and temperatures Using these we determine the regime where blind hole pinning dominates over the intrinsic pinning. We suggest a simple model to explain features in our data, as well as compare our results with predictions from more rigorous recent theories, which are partially related to our work
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