Niobium induced metal-superconductor to metal-insulator transition in the YBCO system

In this work a novel high-T/sub c/ compound of the form YBa/sub 2/Cu/sub 3(1-x)/Nb/sub x/O/sub 7/ as been synthesized and investigated for a broad range of Nb contents O0.9. The compound displays a sharp metal-insulator (M-I) transition at x=0.21 associated with an increase in resistivity in the range x<0.21 of approx. One order of magnitude. The transition temperature T/sub c/ in contrast, remains fairly constant at approx. 90 K. It was shown that Nb added to YBCO results in a formation of dielectric inclusions YBa/sub 2/NbO/sub 6/, Y/sub 2/BaCuO/sub 5/, and CuO, which dilute the superconducting grains. For the case x>0.21 corresponding to a no-superconducting state, the conduction mechanism is adequately described through the variable-range-hopping (VRH) model. Comparison of experimental data with theory provides an estimation of both, carrier localization length and density of states at the Fermi energy.<>