Electric field induced permanent superconductivity in layered metal nitride chloride MNCl(M: Hf and Zr)

The device of electric double-layer transistor (EDLT) with ionic liquid has been employed as an effective way to dope carriers over a wide range, which can induce metallic state, magnetic reconstruction and superconducting transition. However, the induced electronic state can hardly survive in the materials after releasing the gate voltage, strongly restricting the experimental study for discovery of physics. Here, we show that a permanent superconductivity with transition temperature $T_c$ of 24 and 15 K is realized in single crystals and polycrystalline samples of HfNCl and ZrNCl upon applying proper gate voltage $V_{\rm G}$, respectively. Reversible change between insulating and superconducting state can be obtained through applying positive and negative $V_{\rm G}$ at low temperature such as 220 K, whereas $V_{\rm G}$ applied at high temperatures ($\geq$ 250 K) could induce partial deintercalation of Cl ions and result in irreversible superconducting transition. The present results clarify a connection between traditional chemical synthesis and the electrochemical mechanism of the EDLT induced superconductivity. Such a technique shows a great potential to systematically tune the bulk electronic state in the similar two-dimensional (2D) systems.