Discovery of Ag_xTaS₂ superconductor with stage-3 structure

Through interfacial engineering, such as ion intercalation, we can tune the properties and optimize the performance of transition metal dichalcogenides and their devices. In this study, we present the first-time experimental investigations of stage 3 of Ag_xTaS₂ single crystal, and its superconductivity was discovered at 3.8 K. Highly crystalline stage 1 and 2 were prepared by a proton-driven ion-introduction method. A simple water-soaking process was employed to achieve the stage-3 structure by deintercalation of Ag ions from lower stage structures. Besides, we developed a general stacking rule to determine the crystal structure, and it can predict any higher-order stage structure of Ag_xTaS₂. The superconducting transition temperature was enhanced from 1.7 K for a stage-2 structure to 3.8 K for a stage-3 structure, which is more than four times that of the pristine TaS₂ (0.8 K). This enhancement is attributed to the increase in density of states at the Fermi level, which was calculated by density functional theory. Also, the water-soaking process reconstructs the stage-2 into the stage-3 structure while deteriorating its crystallinity. Such a structural distortion is one of the potential reasons for the suppression of charge density wave in stage-2, resulting in the enhancement of superconductivity despite the structural degradation.