Triplet superconductivity in coupled odd-gon rings.

Shedding light on the nature of spin-triplet superconductivity has been a long-standing quest of solid-state physics since the discovery of superfluidity in liquid $^3$He. Nevertheless, the mechanism of spin-triplet pairing is much less understood than that of spin-singlet pairing explained by the Bardeen-Cooper-Schrieffer theory or even observed in high-temperature superconductors. Here we propose a versatile mechanism for spin-triplet superconductivity, which is mediated through a melting of macroscopic spin polarization in weakly coupled odd-gon-unit system (e.g., triangular unit, pentagon unit, etc). We demonstrate the application of this mechanism by considering a new class of quasi-one-dimensional superconductors A$_2$Cr$_3$As$_3$ (A=K, Rb, and Cs). Furthermore, we derive a simple effective Hamiltonian to easily illustrate the adaptability of the mechanism to general coupled odd-gon-unit systems. We thus argue that materials consisting of odd-numbered geometric units would be a prospect of spin-triplet superconductivity.