Expansion of the Vortex Cores in YBa2Cu3O6.95 at Low Magnetic Fields

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.