In-plane magnetic penetration depth in NbS2

Abstract

We report on the temperature dependence of the in-plane magnetic penetration depth (${\ensuremath{\lambda}}_{ab}$) and first penetration field [${H}_{f}\ensuremath{\propto}1/{\ensuremath{\lambda}}_{ab}^{2}(T)$ for $H\ensuremath{\parallel}c$] in 2H-NbS${}_{2}$ single crystals. An exponential temperature dependence is clearly observed in ${\ensuremath{\lambda}}_{ab}$(T) at low temperature, marking the presence of a fully open superconducting gap. This compound is the only superconducting 2H-dichalcogenide which does not develop a charge density wave (CDW). However, as previously observed in 2H-NbSe${}_{2}$, this gap (${\ensuremath{\Delta}}_{1}=1.1\phantom{\rule{0.16em}{0ex}}{k}_{B}{T}_{c}$) is significantly smaller than the standard BCS weak-coupling value. At higher temperature, a larger gap (${\ensuremath{\Delta}}_{2}=1.8\phantom{\rule{0.16em}{0ex}}{k}_{B}{T}_{c}$) has to be introduced to describe the data which are compatible with a two-gap model. The superconducting gaps are hence very similar in NbS${}_{2}$ and NbSe${}_{2}$ and we show here that both of them open in the strongly coupled Nb tubular sheets independent of whether or not a CDW is present.