Investigation of the superconductor-insulator transition in YBaCuNbO material

YBa2Cu3-xNbxO7 material synthesized has been investigated within a wide range of niobium concentration 0 <= x <= 0.9. A superconductor-insulator transition has been observed in this material under Nb content variation. For small x values corresponding to the superconducting state the resistivity of the compound increased with the Nb concentration while the critical temperature remained at about 90 K. In the insulating state the conductivity was controlled with the variable-range-hopping (VRH) mechanism. The analysis of the experimental results along with the available publication data made it possible to assess the value of the carrier localization length and the density of state s at the Fermi level for the VRH regime