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Superconductivity in Yb$_x$($Me$)$_y$HfNCl ($Me$ = NH$_3$ and THF)

The intercalated layered nitride $β$-HfNCl has attracted much attention due to the high superconducting transition temperature up to 25.5 K. Electrons can be introduced into $β$-$M$NCl ($M$=Zr and Hf) through alkali-metals intercalation to realize the superconductivity. Here, we report the observation of superconductivity in rare-earth metals cointercalated compounds Yb$_x$($Me$)$_y$HfNCl with $Me$ = NH$_3$ and tetrahydrofuran (THF), which were synthesized by the liquid ammonia method at room temperature. The superconducting transition temperature is about 23 K and 24.6 K for Yb$_{0.2}$(NH$_3$)$_y$HfNCl and Yb$_{0.3}$(NH$_3$)$_y$HfNCl, respectively. Replacing the NH$_3$ with a larger molecule THF, superconducting transition temperature increases to 25.2 K in Yb$_{0.2}$(THF)$_y$HfNCl, which is almost the same as the highest $T_{\rm c}$ reported in the alkali-metals intercalated HfNCl superconductors. The $T_{\rm c}$ of Yb$_{0.2}$(THF)$_y$HfNCl is apparently suppressed by pressure up to 0.5 GPa, while the pressure effect on $T_{\rm c}$ becomes very small above 0.5 GPa. The liquid ammonia method is proved to be an effective synthetic method to intercalate metal ions into HfNCl. Our results suggest that the superconductivity in these layered intercalated superconductors nearly does not rely on the intercalated metal ions, even magnetic ion.