Phase transition at H/sub c1/ for superconducting Nb and V

Magnetization curves for V and Nb have been studied to look for a first-order transition at $H_{c1}$ and to determine the temperature interval in which the interaction between quantized vortices is attractive. Both the Nb sample with resistivity ratio of Γ≈1000 and the V sample with Γ≈1500 show a very sharp rise in magnetic induction at $H_{c1}$ which is reminiscent of a first-order transition. The values of vortex spacing derived from the magnitude of this abrupt rise agree well with the decoration technique at 1.2 K and the neutron-diffraction technique at 4 K. As the temperature increases, the magnetic-induction curves near $H_{c1}$ retain the same shape for all temperatures up to reduced temperatures above 0.95. Hence there is no evidence in the magnetization data for a changeover from an attractive to a repulsive interaction in this temperature range. The detailed shape of the phase-transition curves near $H_{c1}$ agrees fairly well with model calculations which include only bulk pinning.