Expansion of the vortex cores in YBa2CU3O6.95 at low magnetic fields

Muon-spin-rotation spectroscopy has been used to measure the effective size r(0) of the vortex cores in optimally doped YBa2Cu3O6.95 as a function of temperature T and magnetic field H deep in the superconducting state. While r(0) at H=2 T is close to 20 Angstrom and consistent with that measured by scanning tunneling microscopy at 6 T, we find a striking increase in r(0) at lower magnetic fields, where it approaches an extraordinarily large valu e of about 100 Angstrom. This suggests that the average value of the superc onducting coherence length xi(ab) in cuprate superconductors may be much la rger than previously thought at low magnetic fields in the vortex state. [S 0163-1829(99)50702-9].