Superconductivity in hydrated Li x (H 2 O) y TaS 2

We have systematically studied the structural and physical properties of the superconducting hydrated Li x (H 2 O) y TaS 2 (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 2 H -TaS 2 ( P 6 3 / mmc ). The transition temperature to superconductivity shows a dome-shape dependence on the lithium content x with a maximum T c of 4.6 K for x ≈ 0.42, which is larger than in corresponding optimally doped 2 H -TaS 2 superconductors without water or organic intercalants ( T c ∼ 4.2 K). There are no signs of a charge-density-wave formation in hydrated Li x (H 2 O) y TaS 2 . While our magnetic data indicate a rather strongly type-II behavior, heat-capacity measurements reveal, like in other 2 H -TaS 2 -type compounds, a reduced discontinuity Δ C e / γT c ≈ 0.8 at T c , which is smaller than the standard BCS value 1.43. From the corresponding Sommerfeld constants γ and Debye temperatures Θ D we can derive the parameter describing the electron–phonon coupling λ ep and the electron density of states DOS( E F ) at the Fermi level as functions of x . While the variation of the DOS( E F ) is consistent with that of T c , indicating that the lithium intercalation is tuning T c via changing the DOS( E F ) in 2 H -Li x (H 2 O) y TaS 2 , the simultaneous changes of λ ep and Θ D may also play a certain role.