High- Tc 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≈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 χs 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, Tc is approximately constant, in agreement with experiments. At x=0.055 we found that Tc 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-Tc superconductor.