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

The superconducting transition temperature (T_c) of tetragonal Fe_1+δSe 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 Å in native Fe_1+δSe to >7 Å in K_xFe_1−ySe and to >9 Å in Li_1−xFe_x(OH)Fe_1−ySe, we report on an electrochemical route to modify the superconducting properties of Fe_1+δSe. In contrast to conventional chemical (solution) techniques, the electrochemical approach allows to insert non-solvated Li⁺ into the Fe_1+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 T_c 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.