Attractive Interaction between Vortices in Nb

The equilibrium spacing between vortices $a_0$ at fields just above $H_{C1\mathrm{}}$ has been calculated from previously published magnetization data for Nb. Results show that $a_0$ is proportional to ${\mathrm{}(1-t)\mathrm{}}^{-\frac{1}{2}}$, where $t$ is the reduced temperature. Hence, the vortex lattice spacing diverges at the critical temperature in the same manner as the penetration depth and the coherence distance. The magnitude of the lattice spacing is about four coherence distances.