Studies on two-gap superconductivity in 2H–NbS2

Josephson Effect in NbS2 van der Waals Junctions

“…11,32−34 It is particularly interesting to note that the small gap falls below the BCS value, as confirmed by different measurement techniques including Andreev reflection spectroscopy, 11 scanning tunneling microscopy and spectroscopy, 32 specfic heat measurements, 33 and ac-calorimetry measurements. 34 The reported small gap ranges from 0.53 to 0.56 meV, in good agreement with our results. In NbSe 2 , it is speculated that the discrepancy between the measured gap and the BCS value is originated from the interaction between CDW and superconductivity.…”

Polytypism and superconductivity in the NbS2system

“…21 In the NbS 2 system only the 2H-NbS 2 polytype is known to be superconducting with a critical temperature of T c ≈ 6 K, although it is worth mentioning that there are reports that found also the 3R-NbS 2 polytype to be superconducting with a very similar critical temperature. [22][23][24][25][26][27] As we shall show later in this work, these observations of superconductivity in samples of the 3R-NbS 2 polytype may likely be caused the presence of traces of 2H-NbS 2 , which are challenging to identify by means of X-ray diffraction. The challenge to prepare phase-pure samples of 2H-NbS 2 and 3R-NbS 2 has earlier been recognized by Fisher et al 28 There it was highlighted that the sulfur pressure during synthesis is crucial for the phase formation of the product.…”

“…The value ΔC/γT c for phase-pure 2H-NbS 2 is in excellent agreement with earlier studies on high-quality single crystals of 2H-NbS 2 . 26,27 Since the specific heat is truly a bulk measure, we can state that the mixed sample with ΔC almost being parity to γT c , contains of maximally 75% of the 2H-NbS 2 polytype, but it might nevertheless be easily mistaken for a bulk superconductor. This finding is in agreement with earlier reports for other superconducting systems, where specific heat measurements were also found to be crucial for the identification of bulk superconducting phases.…”

Traces of charge density waves in NbS2

“…Thus, 2H-NbS 2 is just on the verge of a CDW and DFT calculations also hint at the proximity of density waves instabilities [17][18][19] . The soft phonon modes do contribute to another electronic ordering: the metalto-superconductor transition below T c = 6 K 1 , in which they are the dominant contributor to anisotropic two-gap superconductivity [19][20][21][22][23][24] . Yet, no other electronic phase has ever been found experimentally in 2H-NbS 2 using either: very pure crystals (RRR = 105) 25 , low temperature (100 mK) 20 , or high pressure 26 .…”