Resistive Measurements of Structural Effects in Superconducting Niobium

Abstract

Resistive measurements of the critical current density J_c in transverse magnetic fields were made on single crystals of niobium, which had been subjected to systematic thermal, mechanical, and chemical treatments in order to produce a wide range of substructural densities, and variations in the oxygen content. The introduction of plastic deformation into niobium crystals produced a peak in J_c at applied magnetic fields of strength H_c2, the upper critical field. The peaks in J_c were removed by annealing the crystals in the recovery range. The critical current density was increased both below and above H_c2 by producing a dislocation substructure in the crystals. Small additions of oxygen also caused J_c to increase above and below H_c2. The upper critical field, however, was relatively insensitive to structure, varying only through the structure sensitivity of the bulk GLAG parameters.