High-TcT_c superconductivity in weakly electron-doped HfNCl


We investigate the magnetic and superconducting properties in electron-doped Lix_xHfNCl. HfNCl is a band insulator that undergoes an insulator to superconductor transition upon doping at x0.13x\approx0.13. The persistence of the insulating state for x<0.13x<0.13 is due to an Anderson transition probably related to Li disorder. In the metallic and superconducting phase, Lix_xHfNCl 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\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<0.15x<0.15. On the contrary, for x>0.15x>0.15, TcT_c is approximately constant, in agreement with experiments. At x=0.055x=0.055 we found that TcT_c can be as large as 40 K, suggesting that the synthesis of cleaner samples of Lix_xHfNCl could remove the Anderson insulating state competing with superconductivity and generate a high-TcT_c superconductor.Comment: 8 pages, 6 figure

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