Dimensional crossover from quasi-2D to 3D superconductivity in (Li,Fe)OHFeSe_1–xS_x driven by chemical pressure

The interplay between dimensionality and superconductivity is a central theme in understanding the behavior of low-dimensional superconductors. In this work, we investigate the dimensional crossover from quasi-two-dimensional (quasi-2D) to three-dimensional (3D) superconductivity in (Li,Fe)OHFeSe_1−xS_x single crystals driven by sulfur doping. Through detailed structural, electrical, and magnetic characterization, we identify a critical doping level (x = 0.53) where the system transitions from quasi-2D to 3D superconducting behavior. Reduced superconducting fluctuations and non-Fermi liquid behavior near this critical point suggest the presence of competition between intralayer and interlayer pairing mechanisms. Fluctuation conductivity analysis reveals that the coherence length along the c-axis, ζ_c(0), and the interlayer coupling strength, Γ, increase significantly at x = 0.53, marking the onset of 3D superconductivity. These findings provide new insights into the role of dimensionality and interlayer coupling in modulating superconducting properties, positioning (Li,Fe)OHFeSe_1−xS_x as a unique platform for exploring crossover physics in iron-based superconductors.