Superconductivity, magnetic susceptibility and electrical resistivity of the double-doped La1.85-xSr0.15+xCu1-xCoxO4 (0 ≤ x ≤ 0.4)

The infrared spectra, magnetic susceptibility, and electrical resistivity for double-doped polycrystalline samples La1.85-xSr0.15+xCu1-xCoxO4 (0 ≤ x ≤0.4) are reported. The critical Co3+ doping level above which the superconductivity is suppressed is higher than that of the earlier results on the single-doped series, which is attributed to the charge carrier compensation effect; in the vicinity of the superconducting transition for samples with lower Co3+ concentration, the coexistence of superconductivity and antiferromagnetic interaction was observed. The magnetic susceptibilities for samples with Co3+ doping deviate from the Curie-Weiss law due to the evolvement of the spin state of Co3+. The coincidence of the in-plane O-Cu bond-stretching mode induced by the heavier Co3+ substitution for Cu2+ and the metal-insulator transition is understood in terms of the electron-phonon coupling and the electronic screening; for the nonsuperconducting samples, although the resistivity is dominated by the variable range hoping mechanism in random magnetic potential background introduced by Co3+ spins, this mechanism fails to give an accurate description for samples with higher Co3+ concentration. The local electrons of Co3+ rather than its spin are suggested as the reason for the suppression of superconductivity