Coexistence of nontrivial topological properties and strong ferromagnetic fluctuations in quasi-one-dimensional A 2 Cr 3 As 3

Superconductivity in crystals without inversion symmetry has received extensive attention due to its unconventional pairing and possible nontrivial topological properties. Using first-principles calculations, we systemically study the electronic structure of noncentrosymmetric superconductors A 2 Cr 3 As 3 (A = Na, K, Rb, and Cs). Topologically protected triply degenerate points connected by one-dimensional arcs appear along the C 3 axis, coexisting with strong ferromagnetic (FM) fluctuations in the non-superconducting state. Within random phase approximation, our calculations show that strong enhancements of spin fluctuations are present in K 2 Cr 3 As 3 and Rb 2 Cr 3 As 3 and are substantially reduced in Na 2 Cr 3 As 3 and Cs 2 Cr 3 As 3 . Symmetry analysis of pairing gap Δ( k ) and spin–orbit coupling g k suggest that the arc surface states may also exist in the superconducting state, giving rise to possible nontrivial topological properties.