Spin Rotatory Strength as the Equilibrium Observable for Chiral-Induced Spin Selectivity

Abstract

We identify the spin rotatory strength---a chirality-odd, finite-frequency spin--dipole cross response---as the equilibrium observable for chiral-induced spin selectivity. Time-reversal symmetry forces the static response to vanish, while nonzero SU(2) Wilson-loop flux is an independent necessary condition. Exact diagonalization of a Kane--Mele--Hubbard model ($N{=}4,6,8$) reveals that gapped molecules robustly suppress the response, whereas near-degenerate systems show amplification at small $N$ whose persistence at larger sizes remains open.