High-pressure X-Ray Absorption and Diffraction Study of the Self-Doped Superconductor EuFBiS2

The BiS2-based layered materials are characterized by a highly susceptible physical state, revealing a large response to external conditions. A particular case is the EuFBiS2 compound, showing a superconducting transition temperature T-c similar to 0.3 K at ambient pressure. Upon increasing external pressure, T-c goes through a large amplification, accompanied by a structural phase transition (SPT) from tetragonal to monoclinic symmetry. Here, we use a combination of Eu L-3 - edge x-ray absorption spectroscopy and synchrotron x-ray diffraction to unveil the evolution of the Eu valence and lattice symmetry under high pressure. We find that the average Eu valence increases gradually with pressure, exhibiting a pressure plateau near the SPT, at which the T-c increases sharply. Since in EuFBiS2 the charge carriers are introduced via self-doping induced by the mixed valence of the Eu ions, our findings clearly indicate that the role of the charge doping is marginal in the T-c enhancement. On the other hand, the structural distortions, taking place at the SPT, play a central role in enhancing the superconducting properties of the EuFBiS2 system.