Ionic-Liquid-Gating Induced Protonation and Superconductivity in FeSe, FeSe_0.93S_0.07, ZrNCl, 1T-TaS₂ and Bi₂Se₃^*

We report protonation in several compounds by an ionic-liquid-gating method, under optimized gating conditions. This leads to single superconducting phases for several compounds. Non-volatility of protons allows post-gating magnetization and transport measurements. The superconducting transition temperature T _c is enhanced to 43.5 K for FeSe _0.93 S _0.07 , and 41 K for FeSe after protonation. Superconducting transitions with T _c ∼ 15 K for ZrNCl, ∼7.2 K for 1T-TaS ₂ , and ∼3.8 K for Bi ₂ Se ₃ are induced after protonation. Electric transport in protonated FeSe _0.93 S _0.07 confirms high-temperature superconductivity. Our ¹ H nuclear magnetic resonance (NMR) measurements on protonated FeSe _1−x S _x reveal enhanced spin-lattice relaxation rate 1/¹ T ₁ with increasing x, which is consistent with the LDA calculations that H ⁺ is located in the interstitial sites close to the anions. (express letter)