Strain and carrier-induced coexistence of topologically insulating and superconducting phase in iodized Si(111) films

The importance of silicon in modern electronic devices has led to considerable interest in exploring the unconventional electronic properties of Si-based materials for future applications in spintronics and quantum computing. Here, using density functional theory, we present the results of a systematic study of the effect of strain on Si(111) thin films whose surfaces are functionalized with iodine. Films with an odd number of layers under biaxial strain are found to undergo a phase transition from a normal insulator to a topological insulator and ultimately to a metal. The spin-orbit coupling-induced topologically nontrivial band gap at the Γ point is found to be as large as 0.50 eV, which not only surpasses that of other Si-based topological materials, it is also large enough for practical realization of quantum spin Hall states at room temperature. No such nontrivial states are found in films with an even number of layers. Mechanisms for such a strain-induced transition are illustrated by a tight-binding model composed of s, p_x, and p_y orbitals. Equally important, we predict that iodized silicene, when stretched and hole-doped, would be a phonon-mediated superconductor with a critical temperature of 9.2 K. This coexistence of a topological insulator and a superconducting phase in a single material is unusual; it has the potential for applications in electronic circuits and for the realization of Majorana fermions in quantum computations.