Abstract
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 2H-TaS$_2$ (P6$_3$/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 2H-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 2H-TaS$_2$-type compounds, a reduced discontinuity ΔC$_e$/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(EF) 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 2H-Li$_x$(H$_2$O)$_y$TaS$_2$, the simultaneous changes of λ$_{ep}$ and Θ$_D$ may also play a certain role.
Liu, Huanlong