Tuning the structures and superconductivity of non-centrosymmetric 4Ha-NbSe2 by sulfur doping

Non-centrosymmetric 4Ha-NbSe2 has recently been reported to exhibit nodeless two-gap s+s-wave pairing and large upper critical field far beyond the Pauli limit. In this paper, we focus on the crystal and electronic structures, and superconductivity of sulfur-doped 4Ha-NbSe2. Upon S doping to 4Ha-NbSe2−xSx (0 ≦ x ≦ 0.4), the unit cell is compressed accompanied by decrease in interlayer spacing. Paramagnetic Meissner effect is observed in the S-doped samples. The onset transition temperature Tc is down from 6.4 K in pristine 4Ha-NbSe2 to 4.9 K in 4Ha-NbSe1.6S0.4, indicating the breakdown of the multi-band superconductivity. The three-dimensional Fermi surface centered at the Γ point shrinks with doping and vanishes at x = 0.4, accounting for the reduced multi-gap character. We ascribe the drop in Tc to the lowered density of states at the Fermi level N(EF) and electron-phonon coupling. The critical current density Jc is enhanced at low S concentration because a slight S as pinning centers strengthens the vortex pinning. Jc at high doping level, upper critical field μ0Hc2 and irreversible field μ0Hirr decrease with increasing the S content, which is attributed to the prominent impurity scatterings. Pauli-limit-violated μ0Hc2 is reduced by S doping arising from the weakening of the spin-orbit coupling. The results of this study represent the first report on doped 4Ha-NbSe2 to our knowledge, we hope that it could motivate investigation in the future on doping effect in 4Ha-NbSe2 to probe the superconductivity of transition-metal-dichalcogenide superconductors.