Doping Variation of Optical Properties in ZrNCl Superconductors

Systematic optical reflectivity measurements were performed on a series of electron-doped layered nitride LixZrNCl superconductors with a variable carrier density. A Drude band grew up with increasing Li doping level x, providing optical evidence of a transition from a band insulator to a metal. The doping variation of the plasma edge exhibited excellent agreement with the results of first-principles calculations. Furthermore, the good agreement among the optical, thermal, and theoretical effective masses highlights the uniqueness of the present materials, indicating that relatively high T-c superconductivity with T-c similar to 15 K can emerge from a light effective mass m* similar to 0.9 m(0) with minimal renormalization.