Abstract
To address the structural mechanisms associated with T c variations induced by Co doping, two samples of the nominal composition ( Ga 1-x Co x) Sr 2( Tm 0.6 Ca 0.4) Cu 2 O z with x = 0.5 and 0.7 have been prepared and studied by using neutron powder diffraction and transport measurements. In contrast to the x = 0.5 sample, which shows superconductivity around 35 K, the x = 0.7 sample does not exhibit superconductivity. The neutron powder diffraction studies indicate that both samples exhibit the 1212 structure with space group Ima2 and the Co atoms exclusively substitute for the Cu atoms located on the chain sites of the 123 parent structure. The depression in Tc of the x = 0.7 sample, compared to that of the x = 0.5 sample, is discussed in connection with oxygen content, disorder, and change in hole concentration on the superconducting planes.
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