Systematic substitutional studies in Bi 1.7 Pb 0.4 Sr 2− x Ce x Ca 1.1 Cu 2.1 O 8+ δ were carried out by varying the Ce content (0.2 ≤  x  ≤ 0.6) and sintering temperature (845 °C ≤  T s  ≤ 865 °C) to determine the effects of rare earth doping and sintering temperature ( T S ) on their structural and transport properties. Under the preparative conditions employed, X-ray diffraction studies indicate solid solubility of Ce in the (Bi,Pb)-2212 matrix up to x  = 0.6 while the resistivity studies show that superconductor to insulator, insulator to metal and metal to insulator transitions can be realized in the (Bi,Pb)-2212 system by a suitable tuning of temperature, Ce content and T S . The analysis reveals that for each T S , the carrier concentration of (Bi,Pb)-2212 changes progressively with Ce content from the overdoped to the underdoped region and the consequent hole filling leads to decrease in T C and suppression in superconductivity. It is identified that disorder leads to localization of existing carriers and the electronic conduction takes place through thermal activation, variable range and multiphonon hopping at various temperature regimes. The related physical parameters have been evaluated and discussed in detail.

Superconductor–metal–insulator crossover in Bi1.7Pb0.4Sr2−xCexCa1.1Cu2.1O8+δ (0.2 ≤ x ≤ 0.6) sintered between 845 °C ≤ Ts ≤ 865 °C

Unlike the correlation-induced Mott transition and disorder-induced Anderson one which appear to hold for a weak localization limit, we have studied the new types of the metal-insulator transitions (MITs) induced by strong defect-phonon-carrier and electron -phonon interactions in doped compounds. We formulate a strong localization limit of the MIT problem by applying the continuum theories of extrinsic (or defect-phonon-assisted) and intrinsic (or phonon-assisted) self-trapping of carriers to the doped systems. We find that th e criterion for the doping induced MIT gives the critical doping concentrations x c = 0.06 for La 2− x Sr x CuO 4 in accordance with observations. Key differences between the MITs in amorphous and crystalline doped systems are discussed.

New metal-insulator transitions induced by strong defect-phonon- carrier and electron-phonon interactions in doped high-Tc oxides and other non-metallic solids

R. Yusupov

Papers

New metal-insulator transitions induced by strong defect-phonon- carrier and electron-phonon interactions in doped high-Tc oxides and other non-metallic solids