Dependence of T_c on the Hole Concentration in the High‐T_c Superconductors Bi_1.5Pb_0.5Sr₂Ca_1−xGd_xCu₂O_8+z

Abstract

0 5 z 5 5 1.0) is accompanied by a decrease in hole concentration with increasing x. Nevertheless, T, = 90 K remains nearly constant in t h e interval 0 (= z 5 0.25. Compounds with x = 0.35 and 0.5 are still superconducting; the increased width of the transition points on a percolation of superconductivity. Compounds with x 2 0.75 are semiconductors. The presence of variable range hopping in the insulating regime strongly suggests that the metal-insulator transition is of Anderson type. (0 5 x 5 1,O) fuhrt zu einer Abnahme der Defektelektronenkonzentration. T, = 90 K bleibt im Bereich 0 5 5 0,25 trotzdem praktisch konstant. Verbindungen mit x = 0,35 nnd 0,5 sind noch supraleitend; die zunehmende Breite des ubergangs weist auf eine Perkolation der Supraleitung hin. Verbindungen mit x 2 0,75 und 1,0 sind Halbleiter. Das Vorliegen von variable range hopping im Isolator-Gebiet legt nahe, daB der Metall-Isolator-Ubergang vom Anderson-Typ ist.

Influence of Small Ln Fractions on the Physical Properties of Bi-2212 in the System Bi1.75Pb0.25Sr2Ca1−xLnxCu2O8+z (Ln = Gd, Yb)

“…On this scale the incorporation of Ln for Ca offers the possibility of the local formation of insulating structural segments within a superconducting matrix and it is well known that the superconducting properties disappear with increasing Ln substitution. The threshold value for the development of superconductivity is influenced by the hole concentration and, for example, in the system Bil,,Pb,,,Sr,Cal -xGdxCu,O,+z a value of about 0.2 holes/Cu ion has been found [3] which is in agreement with other investigations. For a possible action as pinning centers the average Ln-Ln distance of the local insulating (Bi, Pb),Sr,LnCu,O, + = inclusion should fall in the range of the coherence length.…”

Efectos de substitución en materiales magnéticos y superconductores

“…Important modifications in the hole carrier concentration take place due to the additional electrons and to the subsequent changes in the oxidation states of copper and bismuth (15,16). In this way, a significant increase of T c occurs at low rare-earth dopings ; at intermediate substitutions (0.3 ² x ² 0.5) the critical temperature decreases very fast and it reaches zero at about x = 0.5.…”