Unusual Charge Localization in Zn-doped and Heavily Underdoped YBa2Cu3O7-d at Low Temperatures

The in-plane normal-state resistivity of Zn-doped YBa2Cu3O7-d and heavily underdoped pure YBCO single crystals is measured down to low temperatures under magnetic fields up to 18 T. We found that the temperature dependence of the normal-state rho_ab does not obey log(1/T) and tends not to diverge in the low temperature limit. The result suggests that the ``ground state’’ of the normal state of YBCO is metallic.