Tunable Superconductivity in 2H-NbSe_2 via \boldsymbol In~Situ Li Intercalation

Using the newly-developed solid ionic gating technique, we measure the electrical transport property of a thin-flake NbSe_2 superconductor (T_\rm c = 6.67 K) under continuous Li intercalation and electron doping. It is found that the charge-density-wave transition is suppressed, while at the same time a carrier density, decreasing from 7\times 10^14 cm^-2 to 2\times 10^14 cm^-2 also occurs. This tunable capability in relation to carrier density is 70%, which is 5 times larger than that found using the liquid ionic gating method Phys. Rev. Lett. 117 (2016) 106801. Meanwhile, we find that the scattering type of conduction electrons transits to the s–d process, which may be caused by the change of the occupied states of 4d-electrons in Nb under the condition of Li intercalation. Simultaneously, we observe a certain decrement of electron-phonon coupling (EPC), based on the electron-phonon scattering model, in the high temperature range. Based on data gathered from in situ measurements, we construct a full phase diagram of carrier density, EPC and T_\rm c in the intercalated NbSe_2 sample, and qualitatively explain the variation of T_\rm c within the BCS framework. It is our opinion that the in situ solid ionic gating method provides a direct route to describing the relationship between carrier density and superconductivity, which is helpful in promoting a clearer understanding of electronic phase competition in transition metal dichalcogenides.