Transport properties of Pb-doped Bi4Sr3Ca3Cu4Ox semiconducting glasses and glass-ceramic superconductors.

Abstract

Electrical conductivity and thermoelectric power (TEP) of the as-quenched and annealed (at 500{degree}C for 10 h and 840{degree}C for 24 h) Bi{sub 4{minus}{ital n}}Pb{sub {ital n}}Sr{sub 3}Ca{sub 3}Cu{sub 4}O{sub {ital x}} ({ital x} = 0{endash}1.0) glasses have been measured. The dc conductivity data of the as-quenched and the partially annealed (at 500{degree}C) glasses can be explained by considering the small-polaron hopping conduction mechanism which is found to change from the nonadiabatic to the adiabatic regime with annealing the glasses at 500{degree}C. This change over is due to the presence of microcrystals in the partially annealed glasses as observed from x-ray-diffraction and scanning electron microscopic studies. This adiabatic behavior is also visualized even for some as-quenched glasses having a very small amount of the more conducting microcrystalline phase. All the 840{degree}C annealed glasses are superconductors with {ital T}{sub {ital c}} between 110 and 115 K. The Seebeck coefficient ({ital S}) of the partially annealed glass system is found to be positive and increases linearly with temperature. The {ital S} values of the corresponding glass-ceramic superconductors showing broad peaks around {ital T}{sub {ital c}}. A change over in the values of {ital S} from positive (below {approximately}290 K) to negative (above {approximately}290more » K) indicates the coexistence of both electrons and holes in these superconductors. The TEP data can be fitted with both the two-band model of Forro {ital et} {ital al}. [Solid State Commun. {bold 73}, 501 (1990)] and the Nagaosa-Lee model [Phys. Rev. Lett. {bold 64}, 2450 (1990)]. Therefore, the bosonic contribution in the transport properties of these superconductors, as suggested by the Nagaosa-Lee model, is supported. {copyright} {ital 1996 The American Physical Society.}« less