High-temperature superconductivity in two-dimensional hydrogenated titanium diboride: Ti2B2H4

In recent years, superconductivity in two-dimensional (2D) layered hexagonal metal borides has aroused much interests. Here, based on first-principles calculations, we theoretically report a new type of 2D hydrogenated transition-metal diboride: Ti2B2H4. Results reveal that the introduction of hydrogen atoms expands the frequency range of phonon spectrum of monolayer Ti2B2, and significantly increases the electron-phonon coupling (EPC). The calculated EPC constant λ of Ti2B2H4 is 1.18, and the corresponding superconducting critical temperature (Tc) is 48.6 K, exceeding the McMillan limit (39 K). The EPC mainly originates from the coupling between electrons in Ti-3d orbitals and the low-frequency vibration modes of Ti atoms as well as high-frequency vibration modes of H atoms. Additionally, the coupling strength can be further increased to 1.61 by applying appropriate biaxial compressive strain, and the corresponding Tc can be boosted to 69.4 K, approaching to liquid nitrogen (N2) temperature. The predicted Ti2B2H4 provides a new platform for 2D superconductivity and may have potential applications in 2D nanodevices.