Physical origin of superconductivity in EuFe2As1.4P0.6 and EuFe2As2: pressure-induce valence change of europium

TL;DR

This study reveals that pressure-induced valence change of europium in EuFe2As2 compounds is the key physical mechanism behind superconductivity in these pnictides, supported by x-ray absorption evidence.

Contribution

It provides the first experimental evidence linking europium valence transition to superconductivity in EuFe2As2 compounds, clarifying the physical origin.

Findings

Valence transition from divalent to trivalent europium observed

Superconductivity associated with europium valence change

Pressure induces similar valence behavior in EuFe2As2

Abstract

Superconductivity can be realized in Eu-containing pnictides by application of chemical (internal) and physical (external) pressure, the intrinsic physical mechanism of which attracts much attention in physics community. Here we present the experimental evidence for the valence change of europium in compounds of EuFe2As1.4P0.6 exposed to ambient pressure and EuFe2As2 to high pressure by x-ray absorption measurements on L3-Eu edge. We find that the absorption spectrum of EuFe2As1.4P0.6 at ambient pressure shows clear spectra weight transfer from a divalent to a trivalent state. Furthermore, application of pressure on EuFe2As2 using a diamond anvil cell shows a similar behavior of valence transition as EuFe2As1.4P0.6. These findings are the first observation of superconductivity mechanized by valence change in pnictides superconductors and elucidate the intrinsic physical origin of superconductivity in EuFe2As1.4P0.6 and compressed EuFe2As2.