New High-Temperature Superconductor (THA)_xFeSe: a Bridge Between Bulk and Monolayer FeSe

The nature of enhanced superconductivity in monolayer FeSe still waits for consensus. Finding FeSe-derived bulk superconductors to mimic the monolayer is critical for elucidating the essence of high T_\rm c. Here we report a new high-temperature superconductor, (THA)_xFeSe, synthesized by electrochemical intercalation with an expanded c-axis parameter of approximately 18.4 Å. Bulk superconductivity is characterized by superconducting diamagnetization with the transition temperature T_\rm c=42.0 K as well as resistance transition with T_\rm c^\rm onset=50.7 K. Detailed analysis of the electronic transport behavior reveals that (THA)_xFeSe shares nearly identical strong two-dimensional characteristics and significant superconducting fluctuations with monolayer FeSe. Combining the density functional theory calculations with experimental results, this study constructs the phase diagram for FeSe-derived superconductors and analyzes the evolution of T_\rm c with the electron doping levels. These findings bridge the gap between monolayer and bulk FeSe-derived superconductors, opening a new avenue for enhancing T_\rm c in the FeSe system.