Possible overdoping in TlSrLaCuO[sub 5] and superconductivity in TlBaLaCuO[sub 5] by isovalent-cation substitution

It is known that superconductivity in TlSrLaCuO[sub 5] is due to the overlap of empty Tl 6[ital s] band with the antibonding Cu 3[ital d][sub [ital x]][sup 2][minus][ital y][sup 2] band at the Fermi level. This motivated us to investigate the effects of isovalent-cation substitution in superconducting TlSrLaCuO[sub 5] and semiconducting TlBaLaCuO[sub 5]. We have succeeded in inducing superconductivity in TlBaLaCuO[sub 5] by introducing Ca[sup 2+] at the Ba sites.