Electric Field Induced Permanent Superconductivity in Layered Metal Nitride Chlorides HfNCl and ZrNCl

Devices of electric double-layer transistors (EDLTs) with ionic liquid have been employed as an effective way to dope carriers over a wide range. However, the induced electronic states can hardly survive in the materials after releasing the gate voltage V_\rm G at temperatures higher than the melting point of the selected ionic liquid. Here we show that a permanent superconductivity with transition temperature T_\rm c of 24 and 15 K is realized in single crystals and polycrystalline samples of HfNCl and ZrNCl upon applying proper V_\rm G's at different temperatures. Reversible change between insulating and superconducting states can be obtained by applying positive and negative V_\rm G at low temperature such as 220 K, whereas V_\rm G's applied at 250 K induce the irreversible superconducting transition. The upper critical field H_\rm c2 of the superconducting states obtained at different gating temperatures shows similar temperature dependence. We propose a reasonable scenario that partial vacancy of Cl ions could be caused by applying proper V_\rm G's at slightly higher processing temperatures, which consequently results in a permanent electron doping in the system. Such a technique shows great potential to systematically tune the bulk electronic state in the similar two-dimensional systems.