What have we learnt from the highest-Tc superconducting Hg-based cuprates?

Hg-based high-temperature superconductors (Hg-HTSs) have a similar layered structure to that in most other superconducting cuprates. The weakly attached Hg cations can be swapped with Tl ones in a cation exchange process, which allows exploration of the physical properties associated closely to the charge reservoir block change as a consequence of the Hg-Tl cation exchange. This paper intends to provide an overview of over a decade long investigation of Hg(Tl)-HTSs thin films processed in forward and reverse cation exchange, and from which to elucidate the interesting aspects that are related to the superconductivity of these fascinating materials.