Superconducting phase diagrams of S-doped 2H−TaSe2 under hydrostatic pressure

We report the hydrostatic pressure effect on the charge density wave (CDW) and superconductivity (SC) of single-crystal $2H\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{Se}}_{2\text{\ensuremath{-}}x}{\mathrm{S}}_{x}$ by measuring the electrical resistivity and ac magnetic susceptibility under various pressures up to 12.5 GPa. Different from the previously reported phase diagrams of CDW and SC as a function of the Se/S substitution and/or the uniaxial pressure, the superconducting transition temperature (${T}_{\mathrm{c}}$) was found to increase monotonously with increasing the pressure at first and then reaches a maximum constant above a critical pressure ${P}_{\mathrm{c}}$. The maximal ${{T}_{\mathrm{c}}}^{\mathrm{max}}$ is nearly two times higher than the optimal ones via the S doping and decreases with further increasing the S-doping level. High-pressure ac magnetic susceptibility demonstrates that the superconducting volume quickly increases along with the gradual suppression of CDW and reaches nearly 1 above ${P}_{\mathrm{c}}$, which provides clear evidences for pressure-induced crossover from CDW to bulk SC. High-pressure x-ray diffraction proves that there is no structural phase transition in $2H\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{Se}}_{2}$ up to 20 GPa; although the dependence of ${T}_{\mathrm{c}}$ on the volume shows distinct behaviors at the initial S doping and the applied physical pressure, they both collapse into a universal curve upon the further volume shrinkage. This indicates that both physical pressure and the S-doping-induced chemical pressure can melt CDW and enhance SC through lattice contraction consistently. In comparison with the effects of hydrostatic pressure, the S doping can destruct the CDW ground state faster due to the presence of chemical disorders, which also restrict the further enhancement of ${T}_{\mathrm{c}}$ after the collapse of CDW.