Superconductivity in hydrated Lix(H2O)yTaS2

We have systematically studied the structural and physical properties of the superconducting hydrated Lix(H2O)yTaS2 (0.22 ≤ x ≤ 0.58, y ≈ 0.86). The powder X-ray diffraction patterns suggest that all the samples are single-phase compounds, and the crystal structure is similar to that of 2H-TaS2 (P63/mmc). The transition temperature to superconductivity shows a dome-shape dependence on the lithium content x with a maximum Tc of 4.6 K for x ≈ 0.42, which is larger than in corresponding optimally doped 2H-TaS2 superconductors without water or organic intercalants (Tc ∼ 4.2 K). There are no signs of a charge-density-wave formation in hydrated Lix(H2O)yTaS2. While our magnetic data indicate a rather strongly type-II behavior, heat-capacity measurements reveal, like in other 2H-TaS2-type compounds, a reduced discontinuity ΔCe/γTc ≈ 0.8 at Tc, which is smaller than the standard BCS value 1.43. From the corresponding Sommerfeld constants γ and Debye temperatures ΘD we can derive the parameter describing the electron–phonon coupling λep and the electron density of states DOS(EF) at the Fermi level as functions of x. While the variation of the DOS(EF) is consistent with that of Tc, indicating that the lithium intercalation is tuning Tcvia changing the DOS(EF) in 2H-Lix(H2O)yTaS2, the simultaneous changes of λep and ΘD may also play a certain role.