In-plane magnetic penetration depth in NbS2

We report on the temperature dependence of the in-plane magnetic penetration depth (lambda(ab)) and first penetration field [H-f proportional to 1/lambda(2)(ab)(T) for H parallel to c] in 2H-NbS2 single crystals. An exponential temperature dependence is clearly observed in 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-NbSe2, this gap (Delta(1) = 1.1k(B)T(c)) is significantly smaller than the standard BCS weak-coupling value. At higher temperature, a larger gap (Delta(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 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.