High-T-c superconductivity in weakly electron-doped HfNCl

We investigate the magnetic and superconducting properties in electron-doped LixHfNCl. HfNCl is a band insulator that undergoes an insulator to superconductor transition upon doping at x approximate to 0.13. The persistence of the insulating state for x < 0.13 is due to an Anderson transition probably related to Li disorder. In the metallic and superconducting phase, LixHfNCl is a prototype two-dimensional two-valley electron gas with parabolic bands. By performing a model random phase approximation approach as well as first-principles range-separated Heyd-Scuseria-Ernzerhof (HSE06) calculations, we find that the spin susceptibility chi(s) is strongly enhanced in the low-doping regime by the electron-electron interaction. Furthermore, in the low-doping limit, the exchange interaction renormalizes the intervalley electron-phonon coupling and results in a strong increase of the superconducting critical temperature for x < 0.15. On the contrary, for x > 0.15, T-c is approximately constant, in agreement with experiments. At x = 0.055 we found that T-c can be as large as 40 K, suggesting that the synthesis of cleaner samples of LixHfNCl could remove the Anderson insulating state competing with superconductivity and generate a high-T-c superconductor.