Structural and electrical properties of Bi1.7Pb0.4Sr2-xHoxCa1.1Cu2.1O8+δ system across the metal to insulator transition

The structural and electrical properties of Bi1.7Pb0.4Sr2-xHoxCa1.1Cu2.1O8+δ (0.5 ≤ x ≤ 1.0) system, synthesized by solid state reaction technique are reported across the metal to insulator transition (MIT). The system shows superconducting transition at x = 0.5 and suppression in superconductivity with increasing x. A MIT is found to occur at 0.6 < x ≤ 0.7 and for x ≥ 0.7 the samples show insulating behavior. A detailed resistivity analysis shows that Ho substitution localizes the electronic states of (Bi,Pb)-2212 system at the Fermi level and the transport mechanism in insulating samples is the two-dimensional Mott's variable range hopping. The normal state resistivity increases with x and a considerable drop in localization length takes place at the MIT. The results are interpreted on the basis of decrease in charge carrier concentration and disorder induced carrier localization, due to the substitution of Ho atoms at the Sr site of (Bi,Pb)-2212 system.