2H NbS2 Research Articles

Anisotropy and temperature dependence of the first critical field in 2H–NbS2

We report on μ Hall probe measurements on single crystals of 2H–NbS2. This compound is the only superconducting 2H-dichalcogenide which does not develop a charge density wave. At low temperature and low magnetic field, a Bean profile is observed, allowing to evaluate the critical current. Moreover, the anisotropy and temperature dependence of the first critical field in 2H–NbS2 was measured down to 1.2K. A linear temperature dependence of the first penetration field is clearly observed. The absolute magnetic penetration depth is found to be 83nm which is slightly reduced compared to the iso-structural compound 2H–NbSe2.

Studies on two-gap superconductivity in 2H–NbS2

We present the ac-calorimetry measurements of superconducting 2H–NbS2 in the temperature range down to 0.6K and magnetic fields up to 8T. The temperature and magnetic field dependence of the electronic specific heat consistently indicate existence of two superconducting energy gaps in the system – one of them with the coupling ratio below the BCS weak-coupling limit and the other above that value. These results support previous findings by scanning tunneling microscopy and spectroscopy measurements [I. Guillamón, H. Suderow, S. Vieira, L. Cario, et al., Phys. Rev. Lett. 101 (2008) 166407] of two pronounced features in density of states related to a two-gap superconductivity in this system.

Superconducting parameters of a CDW compound Lu5Ir4Si10

Superconductivity is established below 3.9K in a high quality single crystal of a charge density wave compound (TCDW=83K)Lu5Ir4Si10. The present studies show that Lu5Ir4Si10 has low anisotropy unlike that of the charge density wave (CDW) superconductor such as 2H–NbS2. The analysis of the critical current suggests that pinning is stronger in Lu5Ir4Si10 by three orders of magnitude at 1.7K as compared to that of chalcogenide superconductors. Moreover, pinning along the c-axis is stronger where the CDW transition takes place.

Mechanical properties of niobium disulfide and its hydrated sodium cation intercalation compound

Mechanical properties of 2H–NbS2 and its intercalation derivative Nax(H2O)yNbS2 were measured by using nanoindentation techniques. The intercalation chemical process was conducted in solution and the cation–hydrated derivative produced was [Nax(H2O)yNbS2]. It was observed that the intercalation process occurs through the crystal edges producing a wave intercalation's front that moved as the reaction proceeded. The hardness and elastic modulus presented very low values in the intercalated region. The load × displacement curves from nanoindentation tests suggested that intercalation of hydrated sodium ions through the edges caused layer separation even in the nonintercalated region at the center of the crystal. It is important to emphasize that no similar studies were found in the literature about this theme. Intercalation process is very important in several areas, like solid-state batteries, and mechanical properties of these kinds of materials are not completely understood. This study is a new approach to understanding the mechanical behavior of layered materials submitted to an intercalation process.

Superconducting energy gap of NbS2

The superconducting energy gaps of small particles (1–10 μm) of 2H–NbS2 and 2H–NbSe2 have been measured by far-infrared (FIR) transmission spectroscopy. Comparison of these results with previous FIR measurements made on millimetre-size single crystals of 2H–NbSe2 validates the use of the novel method of sample preparation used here. Comparison of the gap frequencies (νg) with the superconducting transition temperature (Tc) determined by magnetic susceptibility measurements indicates that 2H–NbS2, unlike 2H–NbSe2, exhibits non-Bardeen–Cooper–Schrieffer (BCS)-like behaviour, with hνg/kTc = 2.3.

Long-wavelength phonons in 2H–NbS2, 2H–TaS2, and AgxTaS2

The Raman spectra of 2H–NbS2 and 2H–TaS2 have been analyzed in terms of a valence-force-field (VFF) model that incorporates four parameters. Analytic expressions for the Brillouin-zone-centre phonon frequencies have been obtained from the model and used in conjunction with the Raman frequencies to evaluate the four force constants for the two compounds. The results are compared with the known phonon spectra of similar compounds. The Raman spectrum of AgxTaS2 has also been investigated as a function of silver concentration x. Three new phonon modes appear in the Raman spectrum of AgxTaS2 with x < 1/3 and these new modes are attributed to a zone-folding mechanism. The plausibility of this interpretation has been investigated using phonon dispersion curves calculated from the VFF model.

3d transition metal intercalates of the niobium and trantalum dichalcogenides

Abstract Optical reflectivity spectra of the first-row transition metal intercalation complexes, M1/3NbS2 and M1/3TaS2 (M = V, Cr, Mn, Fe, Co, Ni), Mn1/4 TaS2 and Fe1/4 NbSe2 have been measured at room temperature and compared with those for the host compounds, 2H NbS2, 2H TaS2 and 2H NbSe2. It is found that a simple rigid-band model is inconsistent with the data and that it is necessary to include both broadening of the valence and the lowest conduction band through increased interlayer interactions and broadening of the 3d energy levels on the intercalate ions through interaction with the Nb or Ta d-band conduction electrons. Quantitative analysis of the free-carrier reflectivity edge is performed by fitting to a simple Drude model from which the free-carrier effective mass and scattering time can be deduced.

Pressure-enhanced superconductivity and superlattice structures in transition metal dichalcogenide layer crystals

Abstract The pressure dependence of the superconducting transition temperature T c is investigated for the metallic layer crystals 2H NbS2 and 2H NbSe2. In 2H NbS2 T c is pressure-independent, as in the 3R polytype. It is suggested that the pressure-enhanced T c in 2H NbSe2 may be related to recent observations in electron diffraction and N.M.R. of a phase change to a superlattice structure in this material at low temperatures. Our results would tend to rule out the model of pressure-dependent Josephson tunnelling between layers suggested by previous authors. The aniso-tropy and pressure dependence of the critical field H c2 are described.