  Measurements of the DC resistivity of under-doped cuprate superconductors
have revealed a metal--insulator transition at low temperatures when
superconductivity is suppressed by a very large magnetic field, with the
resistivity growing logarithmically in the low temperature limit. This
insulating behaviour has been associated not only with the large magnetic
fields, but also with the under-doped composition and intrinsic sample
inhomogeneity, and it is important to establish whether these factors are
essential to it. Here we report high resolution microwave measurements of the
flux-flow resistivity of optimally doped YBa_(2)Cu_(3)O_(6+x) in the mixed
state at temperatures down to 1.2 K. We find that the effective resistivity of
the vortex cores exhibits a metal-insulator transition, with a minimum at 13 K
and a logarithmically growing form below 5 K. The transition is seen in samples
of the highest quality and in magnetic fields as low as 1 T. Our work is the
first report of a metal-insulator transition in optimally doped
YBa_(2)Cu_(3)O_(6+x), and the first such transition to be seen in a system in
which superconductivity has not been globally suppressed.
