The 1111-type iron-based superconductor LnFeAsO1-xFx (Ln stands for lanthanide) is the first material with a Tc above 50 K, other than cuprate superconductors. Electron doping into LaFeAsO by H, rather than F, revealed a double-dome-shaped Tc-x diagram, with a first dome (SC1, 0.05<x<0.20) and a second dome (SC2, 0.2<x<0.5). Here, we report the Tc for the whole hydrogen-doping range in LaFeAsO1-xHx under pressures of up to 19 GPa. Tc rises to 52 K at 6 GPa for the Tc-valley composition between the two Tc domes. This is the first instance of the Tc exceeding 50 K in La-1111-type iron-based superconductors. On the other hand, the Tc of SmFeAsO1-xHx decreased continually, keeping its single-dome structure up to 15 GPa. The present findings strongly suggest that the main reason for realization of the Tc >50 K observed in RE-1111 compounds (RE: Pr, Sm, and Gd) at ambient pressure is the merging of SC1 and SC2.
@article{arxiv.1501.04544,
title = {Superconductivity at 52 K in hydrogen-substituted LaFeAsO1-xHx under high pressure},
author = {Hiroki Takahashi and Hideto Soeda and Muga Nukii and Chizuru Kawashima and Takeshi Nakanishi and Soshi Iimura and Yoshinori Muraba and Satoru Matsuishi and Hideo Hosono},
journal= {arXiv preprint arXiv:1501.04544},
year = {2015}
}