Abstract
Results of resistivity ( ϱ( T)), thermoelectric-power (TEP) and magnetic-susceptibility ( χ( T)) measurements are reported for Sr/Ba and Ca/Y substituted YBa 2Cu 3O 7 (YBCO) with various substitutions (Co, Cr or Ga) on the Cu(1) chain site. From the TEP and the superconducting transition temperature ( T c) we conclude that Sr/Ba substitution decreases the carrier concentration of the Cr doped samples and seems to cause some pair breaking, and also that Ca substitution partially restores the hole density in agreement with the conclusions of Babu and Greaves [Physica C 207 (1993) 44]. Using the room-temperature TEP as a guide, we find that Cr doping causes less change in the carrier concentration than Co doping and suggest that its vortex pinning properties should be investigated. Resistivity and TEP for the Ga doped samples give evidence for a gap in the spin-excitation spectrum. This gap is not visible in the static susceptibility because of the presence of a significant Curie term, but electron spin resonance (ESR) measurements show that these spins are not from impurity phases. We propose that the unpaired spins form on the Cu(1) sites when the CuO chains are broken into short segments by Ga substitution.
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