Spin dynamics in the expanded lattice molecule intercalated iron selenide Lix(C5D5N)yFe2-zSe2

Strongly correlated electron systems can facilitate an exotic interplay between magnetism and superconductivity, bringing them at the center of interest in the condensed matter physics community. Two-dimensional iron (Fe)-selenides display an intriguing enhancement of the superconducting transition temperature (Tc) when the Fe-square planes are spaced apart through the intercalation of electron donating species in the host FeSe lattice. Specifically, the Tc of FeSe is enhanced up to five times when large organic molecular spacers (pyridine; C5H5N) are co-intercalated with alkali metal (Li) cations in the host lattice, consequently producing a material with significant c-axis lengthening. In unconventional Fe-based systems, a spin resonance (Eres) appears below Tc as a peak in the (Q,ω) space of the inelastic neutron scattering (INS) spectra. This peak is centered at a characteristic wave-vector (Qres), which is close to the propagation vector of the antiferromagnetic correlations that border with the emerging superconducting state, thus suggesting that magnetism may couple to superconductivity. We intend to pursue INS studies to decipher the spin fluctuations in the polycrystalline Lix(C5D5N)yFe2-zSe2 (Tc~ 38 K) material by scanning the (Q,ω) space above and below Tc. The high flux capabilities of the MERLIN spectrometer will enable the assessment of the spin resonance below Tc, thus shedding light in the spin-fluctuation picture of superconductivity in high Tc materials.

强关联电子系统（Strongly correlated electron systems）能够促成磁性与超导之间的奇异相互作用，使其成为凝聚态物理（condensed matter physics）学界的研究热点。二维铁硒化物（Two-dimensional iron (Fe)-selenides）在主体FeSe晶格中嵌入给电子物种以分隔铁正方形平面时，其超导转变温度（superconducting transition temperature, Tc）会出现引人关注的提升。具体而言，当大有机分子间隔体（吡啶；C5H5N）与碱金属（Li）阳离子共插层进入主体晶格时，FeSe的Tc可提升至原值的五倍，最终得到c轴显著伸长的材料。在非常规铁基体系中，自旋共振（Eres，spin resonance）会在Tc以下出现，表现为非弹性中子散射（inelastic neutron scattering, INS）谱的(Q,ω)空间中的一个特征峰。该峰的中心位于特征波矢（Qres）处，该波矢接近与新兴超导态相邻的反铁磁关联的传播矢量，这表明磁性或与超导存在耦合。本研究计划开展INS实验，通过在Tc上下扫描(Q,ω)空间，解析多晶态Lix(C5D5N)yFe2-zSe2（Tc≈38 K）材料中的自旋涨落。MERLIN谱仪（MERLIN spectrometer）具备高通量性能，可支持对Tc以下自旋共振的观测，从而为高温超导材料的自旋涨落图景提供阐释。