  Measurements of thermal conductivity versus temperature over a broad range of
doping in YBa$_2$Cu$_3$O$_{6+x}$ and HgBa$_2$Ca$_{n-1}$Cu$_n$O$_{2n+2+\delta}$
($n$=1,2,3) suggest that small domains of localized holes develop for hole
concentrations near $p$=1/8. The data imply a mechanism for localization that
is intrinsic to the CuO$_2$-planes and is enhanced via pinning associated with
oxygen-vacancy clusters.
