Interplay of electronic and lattice degrees of freedom in Fe Se 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 a nanodiamond anvil pressure cell has permitted us to reveal a steplike decrease in the Fe-Se bond distance at p 11 GPa and, at the same pressure, the local atomic structure of Rb1−xFe2−ySe2 becomes more ordered. Thus, at this pressure the phase separation might be suppressed, which has a large influence on the electronic structure, as evidenced by changes in the Fe K-edge prepeak. The present results provide compelling evidence that the reemerging superconductivity in A1−xFe2−ySe2 is closely related to the transition to a locally more ordered state, which has a different electronic structure.