New superconductor LixFe1+δSe (x ≤ 0.07, Tc up to 44 K) by an electrochemical route

The superconducting transition temperature (T-c) of tetragonal Fe1+delta Se was enhanced from 8.5 K to 44 K by chemical structure modification. While insertion of large alkaline cations like K or solvated lithium and iron cations in the interlayer space, the [Fe2Se2] interlayer separation increases significantly from 5.5 angstrom in native Fe1+delta Se to > 7 angstrom in KxFe1-ySe and to > 9 angstrom in Li1-xFex(OH)Fe1-ySe, we report on an electrochemical route to modify the superconducting properties of Fe1+ dSe. In contrast to conventional chemical (solution) techniques, the electrochemical approach allows to insert non-solvated Li+ into the Fe1+delta Se 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 T-c up to similar 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.