Prediction of nontrivial band topology and superconductivity in metallic Si allotropes stable at ambient pressure*

Silicon is one of the most abundant elements and plays a key role in modern electronic devices. Various metallic superconducting phases of Si have been reported, but most retain their crystal structure at high pressure. Thus, it remains a challenge to search for potential superconducting Si allotropes. In this work, we report the existence of both nontrivial band topology and superconductivity in three metallic Si allotropes, termed Cmcm-Si₄, Cmmm-Si₄, and I4/mmm-Si₄. Cmcm-Si₄ and I4/mmm-Si₄ are newly predicted using a data-based structure search method whereas Cmmm-Si₄ was proposed to be obtained by removing Li from a Cmmm-LiSi₄ precursor synthesized at high pressure. These metallic allotropes exhibit superconductivity at the critical temperatures of 1.2−11.4 K. We investigate the topological characteristics of the electronic states and find the weak topological nature for all three allotropes. In particular, for Cmcm-Si₄, we confirm the formation of Dirac point in the surface electronic band. Our result provides a promising platform for realizing a nontrivial band topology and superconductivity in all-Si systems at ambient pressure.