Independent control of carrier concentration and interlayer spacing in Li$_x$HfNCl layered superconductors

Alkali-metal and organic molecule co-intercalated HfNCl is a new class of layered superconductors with relatively high transition temperature ($T_c$) of 25.5 K. Recently, we have succeeded in synthesis of single phase samples of Li$_x$(molecule)$_y$HfNCl with a wide range of doping concentration of 0.10 $\leq$ $x$ $\leq$ 0.50, where we are able to, continuously and independently, control the carrier density and interlayer distance \textit{d} between the conducting Hf-N layers by means of co-intercalation of Li and organic molecule. Without any molecule, superconductivity appears at $x$ $\sim$ 0.15 and $T_c$ is almost constant against $x$ above this critical value. Furthermore, we found that $T_c$ is enhanced ($\sim$30$\%$) from 20 K to 25.5 K with increasing of \textit{d}.