Multigap superconductivity in doped $p$-type cuprates

Andreev and tunneling spectroscopy studies of Bi$_{2}$Sr$_{2}$Ca$_{n-1}$Cu$_n$O$_{2n+4+\delta}$, HgBa$_{2}$Ca$_{n-1}$Cu$_{n}$O$_{2n+2+\delta}$ and Tl$_{2}$Ba$_{2}$Ca$_{n-1}$Cu$_{n}$O$_{2n+4+\delta}$ have shown that superconductivity in single-layer ($n=1$) and two-layer ($n=2$) phases has a single-gap character. Qualitatively different results were obtained for three-layer phases. In doped $p$-type Hg-1223, Bi-2223, and Tl-2223 samples two (or three) superconducting gaps were observed. The existence of multigap superconductivity in superconducting cuprates with $n\geq 3$ is explained by a difference in doping levels of outer (OP) and internal (IP) CuO$_{2}$-planes.