Panigrahi, Archisman - MIT

Fermion Statistics Induced Spin Chirality and Young's Interference Patterns of Spin Chirality in Topological Superconductors

We demonstrate that in both normal and superconducting metals with broken time-reversal symmetry (TRS), orbital currents in the ground state can induce spin-chirality. Remarkably, non-zero chirality can emerge purely due to Fermion statistics, without the need for spin-dependent interactions, even when the ground state remains spin-unpolarized. This chirality in the carrier band generates a chiral three-spin RKKY interaction between localized spins coupled to the carriers via the s-d Hamiltonian, an effect detectable by local probes like spin-sensitive STM. The spatial distribution of chirality exhibits Young’s interference patterns near localized magnetic adatoms. The interference arises because Bogoliubov quasiparticles are coherent superposition of electrons and holes with opposite spins, allowing coherent and reversible particle-to-hole conversion within the superconductor bulk. Magnetic adatoms act as beam splitters, enabling interference between particle and hole states, leading to spatial patterns similar to those observed in Young’s double-slit experiment. In topological superconductors, these interference patterns are further modulated by nodal lines that encode the winding numbers of the superconducting gap function’s phase. In systems such as topological superconductors where detecting TRS breaking by conventional means is challenging, local detection of spin chirality provides a reliable diagnostic of superconducting topological phases and also provides insight about the nature of pairing.