Muon-Spin Rotation Measurements of the Magnetic Field Dependence of the Vortex-Core Radius and Magnetic Penetration Depth inNbSe2

Muon-spin rotation spectroscopy $\left(\mu \mathrm{SR}\right)$ has been used to measure the internal magnetic field distribution in ${\mathrm{NbSe}}_2$ for $H_{c1\mathrm{}}\ll H<\mathrm{}{0.25H}_{c2\mathrm{}}$. 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 ${\lambda }_{\mathrm{ab}}$ increases linearly with $H$ in the vortex state of a conventional superconductor.