Field dependence of the vortex core size

We show that the field dependence of the coherence length xi(H) calculated within the weak-coupling BCS theory for clean superconductors at low temperatures and high fields is qualitatively the same as the H dependence of the vortex core size rho(c) recorded experimentally in a number of superconductors with high Ginzburg-Landau parameter kappa. We argue that H dependencies of xi and rho(c) are weakened by scattering and temperature and disappear in the dirty limit and as T -> T-c. We find that the high-field slope d rho(c)/d(H-1/2) for clean materials at low T's is nearly material independent, and we provide an estimate of this nearly universal slope, the prediction supported by the data available.