In-plane magnetic penetration depth in NbS₂

We report on the temperature dependence of the in-plane magnetic penetration depth (λ_ab) and first penetration field [H _f1/λab2(T) for H c] in 2H-NbS₂ single crystals. An exponential temperature dependence is clearly observed in λ _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-NbSe₂, this gap (Δ₁=1.1k _BT_c) is significantly smaller than the standard BCS weak-coupling value. At higher temperature, a larger gap (Δ₂=1. 8k_BT_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 NbS₂ and NbSe₂ 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.