Superconductivity at 43 K in an iron-based layered compound LaO_1-xF_xFeAs

Abstract

The iron- and nickel-based layered compounds LaOFeP (refs 1, 2) and LaONiP (ref. 3) have recently been reported to exhibit low-temperature superconducting phases with transition temperatures T_c of 3 and 5 K, respectively. Furthermore, a large increase in the midpoint T_c of up to ∼26 K has been realized⁴ in the isocrystalline compound LaOFeAs on doping of fluoride ions at the O^2- sites (LaO_1-xF_xFeAs). Experimental observations^5,6 and theoretical studies^7,8,9 suggest that these transitions are related to a magnetic instability, as is the case for most superconductors based on transition metals. In the copper-based high-temperature superconductors, as well as in LaOFeAs, an increase in T_c is often observed as a result of carrier doping in the two-dimensional electronic structure through ion substitution in the surrounding insulating layers, suggesting that the application of external pressure should further increase T_c by enhancing charge transfer between the insulating and conducting layers. The effects of pressure on these iron oxypnictide superconductors may be more prominent than those in the copper-based systems, because the As ion has a greater electronic polarizability, owing to the covalency of the Fe–As chemical bond, and, thus, is more compressible than the divalent O^2- ion. Here we report that increasing the pressure causes a steep increase in the onset T_c of F-doped LaOFeAs, to a maximum of ∼43 K at ∼4 GPa. With the exception of the copper-based high-T_c superconductors, this is the highest T_c reported to date. The present result, together with the great freedom available in selecting the constituents of isocrystalline materials with the general formula LnOTMPn (Ln, Y or rare-earth metal; TM, transition metal; Pn, group-V, ‘pnicogen’, element), indicates that the layered iron oxypnictides are promising as a new material platform for further exploration of high-temperature superconductivity.