Multiple Phases in K 2 Cr 3 As 3 : A Playground for Manipulating Topological Superconductivity

Spin-triplet topological superconductors are rare but of fundamental interest as they can host Majorana bound states that can be used in fault-tolerant quantum computing. Recent efforts have been devoted to searching for spin-triplet states in U-based compounds, but these materials have a low transition temperature (T_{c}) and coexisting competing orders, which creates significant experimental challenges and often leads to contradictory conclusions. The Cr-based candidate K_{2}Cr_{3}As_{3} offers a promising alternative: it has a much higher T_{c}≥6.2 K and no magnetic order. Here we report a hallmark signature of spin-triplet superconductivity arising from the internal spin degrees of freedom via nuclear magnetic resonance measurements, and demonstrate the high tunability of the topological phases. We discovered three distinct superconducting phases and revealed the evolution of the paired-spins direction [d(k)-vector]. At low magnetic fields, K_{2}Cr_{3}As_{3} evolves from a helical (Phase A) to a chiral state (Phase B) with a rotation of the d(k)-vector from in-plane to out-of-plane direction upon cooling, although both phases have point nodes in the gap. A line-nodal gap is realized in the high-field Phase C, where the d(k)-vector lies in the basal plane. These findings establish K_{2}Cr_{3}As_{3} as a model spin-triplet superconductor and a promising platform for manipulating topological phases.