Interplay of the electronic and lattice degrees of freedom in A_{1-x}Fe_{2-y}Se_{2} superconductors under pressure

TL;DR

This study investigates how pressure influences the local structure and electronic properties of Rb-based iron selenide superconductors, revealing a link between structural changes, electronic structure, and reemerging superconductivity near a quantum critical point.

Contribution

It provides new insights into the pressure-induced structural and electronic transitions that underpin superconductivity in A_{1-x}Fe_{2-y}Se_{2} compounds.

Findings

Step-like decrease in Fe-Se bond distance at ~11 GPa

Chemical potential remains stable until ~6 GPa, then increases

Superconductivity reemerges near a quantum critical transition

Abstract

The local structure and electronic properties of Rb$_{1-x}$Fe$_{2-y}$Se$_2$ 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 $p\simeq11$ 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 $\sim6$ GPa, followed by an increase until $p\simeq 11$ 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 $A_{1-x}$Fe$_{2-y}$Se$_2$ in vicinity of a quantum critical transition is caused mainly by the changes in the electronic structure.