PERIODIC PIN ARRAY AT THE FLUXON LATTICE SCALE IN A HIGH-FIELD SUPERCONDUCTING WIRE

A periodic pin array has been made at dimensions approaching the diameter and spacing of the fluxon cores in an ''island''-type of artificial pinning center superconducting composite wire. Gun-barrel drilling was used to precisely incorporate a hexagonal array of Nb 1.3 wt. % Ti pinning centers into an ingot of Nb 46.5 wt. % Ti, before extrusion and wire drawing. Transmission electron microscopy images showed little evidence of pin geometry alteration down to 100 nm pin diameter. At approximately 30 nm effective pin diameter, the aspect ratio of the pins was 3-5, and the pin array retained its hexagonal geometry. Bulk pinning force curves had temperature-independent structure down to 15 nm effective pin diameter, suggestive of matching between fluxons and pins. Because of the uniform pin geometry, the maximum bulk pinning force exceeded that of the best conventional composites, even though the upper critical field and the volume fraction of pins were significantly lower.