The ${\mathrm{BiS}}_2$-based layered materials are characterized by a highly susceptible physical state, revealing a large response to external conditions. A particular case is the ${\mathrm{EuFBiS}}_2$ compound, showing a superconducting transition temperature $T_c\sim 0.3$ 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 ${\mathrm{EuFBiS}}_2$ 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 ${\mathrm{EuFBiS}}_2$ system.

中文翻译：

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自掺杂超导体EuFBiS2的高压X射线吸收和衍射研究

的 ${\mathrm{BiS}}_2$基层状材料的特征在于高度易感的物理状态，显示出对外部条件的强烈响应。一个特殊的情况是${\mathrm{欧盟}}_2$ 化合物，显示出超导转变温度 ${Ť}_C〜0.3$ K在环境压力下。随着外界压力的增加，${Ť}_C$经历大的放大，伴随着从四方对称到单斜对称的结构相变（SPT）。在这里，我们结合使用Eu${\mathrm{大号}}_3$-边缘x射线吸收光谱和同步加速器x射线衍射，揭示了在高压下Eu价和晶格对称性的演变。我们发现，平均Eu价随压力逐渐增加，在SPT附近表现出压力平稳状态，${Ť}_C$急剧增加。自从${\mathrm{欧盟}}_2$ carriers离子的混合价引起的自掺杂引入了载流子。 ${Ť}_C$增强。另一方面，发生在SPT上的结构畸变在增强SPT的超导性能方面起着核心作用。${\mathrm{欧盟}}_2$ 系统。