ИСТИНА

The superconducting transition temperature (TC) of tetragonal Fe1+δSe was enhanced from 8.5 K to 44 K by chemical structure modification. With inserted large alkaline cations like K or solvated lithium and iron cations in the interlayer space, the [Fe2Se2] interlayer separation increases significantly from 5.5 Å in native Fe1+δSe to > 7 Å in KxFe1–ySe and to > 9 Å in Li1−xFex(OH)Fe1–ySe. An electrochemical route to modify the superconducting properties of Fe1+δSe is presented. In contrast to conventional chemical (solution) techniques, the electrochemical approach allows to insert non-solvated Li+ into the Fe1+δSe structure which preserves the native arrangement of [Fe2Se2] layers and their small separation. The amount of intercalated lithium is extremely small (about 0.07 Li+ per f.u.), however, its incorporation results in the enhancement of TC up to ~ 44 K. The quantum-mechanical calculations show that Li occupies the octahedrally coordinated position, while the [Fe2Se2] layers remain basically unmodified. The obtained enhancement of the electronic density of states at the Fermi level clearly exceeds the effect expected on basis of rigid band behavior [1]. 1. A.M. Alekseeva et al., Sci. Rep. accepted (2016).