Muon-spin rotation measurements of the magnetic field dependence of the vortex-core radius and magnetic penetration depth in NbSe2

Muon-spin rotation spectroscopy (mu SR) has been used to measure the internal magnetic field distribution in NbSe2 for H-c1 much less than H < 0.25H(c2). The deduced profiles of the supercurrent density J(s) indicate that the vortex-core radius rho(0) in the bulk decreases sharply with increasing magnetic field. This effect, which is attributed to increased vortex-vortex interactions, does not agree with the dirty-limit microscopic theory. A simple phenomenological equation in which rho(0) depends on the intervortex spacing is used to model this behavior. In addition, we find for the first time that the in-plane magnetic penetration depth lambda(ab) increases linearly with H in the vortex state of a conventional superconductor.