Topological states and topological phase transition in Cu2SnS3 and Cu2SnSe3

Based on the first-principles calculations and model analysis, we propose that the isostructural compounds Cu2SnS3 and Cu2SnSe3 are both the simplest nodal-line semimetals with only one nodal line in their crystal momentum space when spin-orbit coupling (SOC) is ignored. The inclusion of SOC drives Cu2SnS3 into a Weyl semimetal (WSM) state with only two pairs of Weyl nodes, the minimum number required for a WSM with time-reversal symmetry. In contrast, SOC leads Cu2SnSe3 to a strong topological insulator (STI) state. This difference can be well understood as there is a topological phase transition (TPT). In it, theWeyl nodes are driven by tunable SOC and annihilate in a mirror plane, resulting in a STI. This TPT, together with the evolution of Weyl nodes, the changing of mirror Chern numbers of the mirror plane, and the Z(2) indices protected by time-reversal symmetry, has been demonstrated by the calculation of Cu2Sn(S1-xSex)(3) within virtual crystal approximation and an effective k center dot p model analysis. Though our first-principles calculations have overestimated the topological states in both compounds, we believe that the theoretical demonstration of controlling the TPT and the evolution of Weyl nodes will stimulate further efforts to explore them.