  Muon spin rotation spectroscopy has been used to measure the effective size
of the vortex cores in optimally doped YBa2Cu3O6.95 as a function of
temperature and magnetic field deep in the superconducting state. While the
core size at H=2T is close to 20 angstroms and consistent with that measured by
STM at 6T, we find a striking increase in the core size at lower magnetic
fields, where it approaches an extraordinarily large value of about 100
angstroms. This suggests that the average value of the superconducting
coherence length in cuprate superconductors may be larger than previously
thought at low magnetic fields.
