As recently shown, ${\text{HgBa}}_2{\text{Ca}}_{n-1}{\text{Cu}}_n{\text{O}}_y$ $\left(n\ge 6\right)$ cuprate superconductors have a critical temperature of about 100 K independent of $n$. This remarkable property can be explained by the very imbalanced distribution of carriers among the inequivalent ${\text{CuO}}_2$ planes in the unit cell. We discovered that these materials also have a common vortex melting line that resembles the theoretical melting lines of magnetically coupled pancake vortices. We suggest that there are two types of pancake pairs situated in the superconducting ${\text{CuO}}_2$ outer planes: those separated by the thin charge reservoir layer are strongly (Josephson) coupled, while those separated by the thick block of $\left(n-2\right)$ ${\text{CuO}}_2$ nonsuperconducting inner planes are weakly (magnetically) coupled, forming magnetically coupled pancake vortex molecules.

• Received 18 July 2007

DOI:https://doi.org/10.1103/PhysRevB.77.144518