Tuning antiferromagnetic fluctuations and superconductivity insingle layer FeSeS by chemical pressure

Single layer FeX (X = S, Se, Te) epitaxially grown on SrTiO₃ (STO) substrate represents a model system for probing a host of quantum phenomena due to the interplay of topology, magnetism, and superconductivity. In this work, we investigate the impact of chemical pressure on the antiferromagnetic (AFM) fluctuations and superconductivity in single layer FeSe_1-xS_x/STO.

Using in situ angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy, we observe that with increasing S concentration (x), the Fermi surface at M point becomes more anisotropic, and the effective mass decreases from 4.3 to 1.2 m_e, while the hole pocket at Γ point stays at 60 meV below the Fermi level. In addition, tunneling spectra acquired at 4.2 K on single layer FeS are mostly V-shaped. The superconducting coherence peak emerges when the Se concentration is greater than 30%. When Se concentration is 100% (FeSe), a fully gaped U-shaped tunneling spectrum is observed with two coherence peaks at 12 and 15 meV, comparable to earlier studies. Our spin spiral calculations indicate that the FeS system is closer to the collinear AFM phase boundary. With the incorporation of Se,  spin fluctuations in FeS is tuned toward  the checkerboard (CB)  AFM fluctuations region. Our findings demonstrate that the CB AFM fluctuations play an essential role in the enhanced superconductivity in epitaxial single layer iron chalcogenide superconductors on STO.