Investigation of novel cuprates of the TlCa_1-xLn_xSr₂Cu₂O_{7- δ} (Ln=rare earth) series showing electron- or hole-superconductivity depending on the composition

Superconductivity in cuprates of the general formula TlCa_1-xLn_xSr₂Cu₂O_{7+ delta} has been investigated as a function of Ln and x. Compositions with 0.25<or=x<or=0.8 are superconducting for all Ln with T_c in the range 60-90 K (except when Ln=Yb where x=1 is also superconducting); the value of the T_c varies with x as well as Ln. When x approximately=0.25, electrons are the majority charge carriers, while holes are the majority carrier when x >0.25. Thus, these cuprates exhibit a composition-dependent electron- or hole-superconductivity. In the normal state, most of the members of the series traverse compositionally determined metal-insulator transitions. High-energy spectroscopies show the presence of Cu in the 1+ and 2+ states. The Raman frequency around 525 cm^-1 characteristic of the Tl-O2-Cu linkage is sensitive to both Ln and x, indicating a possible involvement in the mechanism of superconductivity.