Coexistence of nontrivial topological properties and strong ferromagnetic fluctuations in $A_2$Cr$_3$As$_3$ ($A$=Na, K, Rb and Cs)

TL;DR

This study reveals the coexistence of topologically protected triply degenerate points and strong ferromagnetic fluctuations in $A_2$Cr$_3$As$_3$ superconductors, suggesting potential nontrivial topological superconducting states.

Contribution

First-principles calculations uncover topological features and magnetic fluctuations in noncentrosymmetric $A_2$Cr$_3$As$_3$ superconductors, highlighting their complex electronic and magnetic properties.

Findings

Triply degenerate points connected by 1D arcs along $C_{3}$ axis.

Strong ferromagnetic fluctuations in K$_2$Cr$_3$As$_3$ and Rb$_2$Cr$_3$As$_3$.

Arc surface states may remain stable in the superconducting state.

Abstract

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 spin-orbit coupling $g_{k}$ suggests that the arc surface states might remain stable in the superconducting state, giving rise to possible nontrivial topological properties.