Interplay of electronic and lattice degrees of freedom in A(1-x)Fe(2-y)Se(2) superconductors under pressure

The local structure and electronic properties of Rb1-xFe2-ySe2 are investigated by means of site selective polarized x-ray absorption spectroscopy at the iron and selenium K-edges as a function of pressure. A combination of dispersive geometry and novel nanodiamond anvil pressure-cell has permitted to reveal a step-like decrease in the Fe-Se bond distance at p?11 GPa. The position of the Fe K-edge pre-peak, which is directly related to the position of the chemical potential, remains nearly constant until ~6 GPa, followed by an increase until p?11 GPa. Here, as in the local structure, a step-like decrease of the chemical potential is seen. Thus, the present results provide compelling evidence that the origin of the reemerging superconductivity in A1-xFe2-ySe2 in vicinity of a quantum critical transition is caused mainly by the changes in the electronic structure.