Measurement of the Fundamental Length Scales in the Vortex State ofYBa2Cu3O6.60

The internal field distribution in the vortex state of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.60}$ is shown to be a sensitive measure of both the magnetic penetration depth ${\lambda }_{\mathrm{ab}}$ and the vortex-core radius ${\rho }_0$. The temperature dependence of ${\rho }_0$ is found to be weaker than in the conventional superconductor ${\mathrm{NbSe}}_2$ and much weaker than theoretical predictions for an isolated vortex. The effective vortex-core radius decreases sharply with increasing $H$, whereas ${\lambda }_{\mathrm{ab}}\left(H\right)$ is found to be much stronger than in ${\mathrm{NbSe}}_2$.