A material design for double honeycomb lattice superconductors

According to our structural studies for double honeycomb lattice superconductors [1], their transition temperatures have been well scaled by their transfer integrals between the nearest neighbor transition metal atoms. Based on this finding, a material design for the bandwidth of a double honeycomb lattice superconductor is proposed. One of the high-$T_{c}$ candidates, A$_{x}$YOCl (A; alkali metal) with narrow bandwidth, was found to be an insulator. This means that the A$_{x}$HfNCl system ($T_{c}^{ max}$=25.5 K) is close to an insulator region. Nevertheless, there remains a high possibility to find high-$T_{c}$ materials in the double honeycomb lattice compounds based on our material design, since there are many compounds with the double honeycomb lattice modified from a popular CdI$_{2}$-type structure. [1] For example, S. Shamoto \textit{et al}., Physica C 402 (2004) 283-292.