High-pressure studies for hydrogen-doped LaFeAsO$_{\mathrm{1-x}}$H$_{\mathrm{x}}$ and SmFeAsO$_{\mathrm{1-x}}$H$_{\mathrm{x}}$

Iron-based superconductor LaFeAsO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$ shows the conventional superconducting dome in an $x$-$T$ phase diagram with a maximum $T_{\mathrm{c}}$ of 26 K at $x=$0.1. However, the over-doped region has not been investigated, because of the poor solubility of fluorine above $x=$0.2. Recently, hydrogen was doped for LaFeAsO$_{\mathrm{1-x}}$H$_{\mathrm{x}}$ above $x=$0.5. It is interesting that LaFeAsO$_{\mathrm{1-x}}$H$_{\mathrm{x}}$ exhibits the second superconducting dome in the over-doped region (0.2 \textless $x$ \textless 0.5) with a maximum $T_{\mathrm{c}}$ of 36 K, in addition to the conventional dome. Since large enhancement of $T_{\mathrm{c}}$ under high pressure was reported for LaFeAsO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$, it is intriguing to study the superconducting properties in LaFeAsO$_{\mathrm{1-x}}$H$_{\mathrm{x\thinspace }}$ (x \textgreater 0.2) under high pressure. Marvelous results that $T_{\mathrm{c}}$ of $x=$ 0.2, which corresponds to the ravine between two domes, is enhanced largely from 18 K to 52 K with pressure of 6 GPa are obtained from resistivity measurements. These results are compared with the superconducting properties under high pressure of SmFeAsO$_{\mathrm{1-x}}$H$_{\mathrm{x}}$.