Predication of topological states in the allotropes of group-IV elements

Three-dimensional(3D)topological insulators(TIs)have been studied for approximately fifteen years,but those made from group-IV elements,especially Ge and Sn,seem particularly attractive owing to their nontoxicity,sizable intrinsic spin–orbit coupling(SOC)strength and natural compatibility with the current semiconductor industry.However,group-IV elemental TIs have rarely been reported,except for the low temperature phase ofα-Sn under strain.Here,based on first-principles calculations,we propose new allotropes of Ge and Sn,named T5-Ge/Sn,as desirable TIs.These new allotropes are also highly anisotropic Dirac semimetals if the SOC is turned off.To the best of our knowledge,T5-Ge/Sn are the first 3D allotropes of Ge/Sn that possess topological states in their equilibrium states at room temperature.Additionally,their isostructures of C and Si are metastable indirect and direct semiconductors.Our work not only reveals two promising TIs,but more profoundly,we justify the advantages of group-IV elements as topological quantum materials(TQMs)for fundamental research and potential practical applications,and thus reveal a new direction in the search for desirable TQMs.