Coexistence of Superconductivity and Topological States in 2D Hydride Monolayers TMH₄ at Ambient Pressure

Topological and superconductive materials are a focus of current research on quantum materials. Herein, we demonstrate the coexistence of nontrivial topological band and phonon-mediated superconductivity in the two-dimensional transition metal hydride monolayers TMH₄ (TM = Ni, Co, Rh, Ir, Ru). A striking experimental feasibility is highlighted by the NiH₄ monolayer, which exhibits a T_c of 23.3 K at ambient pressure─a value exceeding liquid-hydrogen temperature. Crucially, we reveal that both the conventional electron-phonon coupling superconductivity and nontrivial normal-state topology in this system are tunable: applying 2% tensile strain enhances T_c to 29.4 K, while hole doping at 0.04 hole/cell yields a T_c of 26.1 K, accompanied by controllable modulation of topological bands near the Fermi level. This work establishes TMH₄ monolayers as dual-functional materials with dual-tunable parameters (T_c and topological states), offering a platform for investigating the topological states and superconductivity in a single material.