Influence of Re substitution on the flux pinning in (Hg,Re)Ba2Ca2Cu3O8+delta single crystals

Flux pinning in Hg0.77Re0.23Ba2Ca2Cu3O8 + delta single crystals (T-c = 130 K) and in HgBa2Ca2Cu3O8+delta crystals with T-c equal to 130 and 120 K was studied. In the investigated crystals Re atoms substitute at the Hg site of HgBa2Ca2Cu3O8 + delta and alter the defect structure of the host layer by pulling in four new oxygen atoms at (0.34, 0.34, 0) position to form an oct ahedron around Re. As a result a significant improvement of the irreversibi lity Line position at the H-T phase diagram for Hg0.77Re0.23Ba2Ca2Cu3O8 + d elta crystal is observed only at low temperatures (below 80 K). However, ma gnetization measurements performed at temperatures above 100 K show insigni ficant change in the irreversibility line position only, as compared to tha t one of the parent compound with T-c = 130 K. This indicates insignificant influence of new pinning centers, introduced by Re substitution, in the te mperature range, where thermal activation energy becomes significant in com parison with pinning energy. Furthermore, obtained data indicate that no im provement of the coupling between superconducting layers was achieved as a result of the expected increase of electrical conductivity in the blocking layer. Since only a small change in the c-axis lattice constant was observe d in Re substituted crystals, we can conclude that a larger shortening of t he blocking layer in the material is necessary to improve the irreversibili ty Line position in HgBa2Ca2Cu3O8 + delta crystals at high temperatures. (C ) 1998 Elsevier Science B.V. All rights reserved.