In-plane magnetic penetration depth in NbS$_{2}$

We report on the temperature dependence of the in plane magnetic penetration depth ($λ_{ab}$) and first penetration field ($H_f\propto 1/λ_{ab}^2(T)$ for $H\|c$) in 2H-NbS$_2$ single crystals. An exponential temperature dependence is clearly observed in $λ_{ab}$(T) at low temperature, signing 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 ($Δ_{1}=1.1\, k_{B}T_{c}$) is significantly smaller than the standard BCS weak coupling value. At higher temperature, a larger gap ($Δ_{2}=1.8\, k_{B}T_{c}$) 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 independently of the presence of a CDW or not.

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