  The internal field distribution in the vortex state of YBa2Cu3O6.60 is shown
to be a sensitive measure of both the magnetic penetration depth and the
vortex-core radius. The temperature dependence of the vortex core radius is
found to be weaker than in the conventional superconductor NbSe2 and much
weaker than theoretical predictions for an isolated vortex. The effective
vortex-core radius decreases sharply with increasing H, whereas the penetration
depth is found to be much stronger than in NbSe2.
