Magnetically coupled pancake vortex molecules in HgBa 2 Ca n − 1 Cu n O y ( n ≥ 6 )

As recently shown, ${\text{HgBa}}_{2}{\text{Ca}}_{n\ensuremath{-}1}{\text{Cu}}_{n}{\text{O}}_{y}$ $(n\ensuremath{\ge}6)$ 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 $(n\ensuremath{-}2)$ ${\text{CuO}}_{2}$ nonsuperconducting inner planes are weakly (magnetically) coupled, forming magnetically coupled pancake vortex molecules.