Hidden variable $\mathbb{Z}_2$ topological order in a bilayer kagomé antiferromagnet

Abstract

Topological orders in realistic systems are quite rare, raising one fundamental question of whether they might be harbored with hidden degrees of freedom rather than directly exposed in conventional spins or electrons. We here propose a set of hidden variables and their effective interactions in a bilayer kagomé antiferromagnet. By properly organizing four local states of the spin dimer into two pseudospin-$1/2$ variables ($\boldsymbolτ$ and $\boldsymbolμ$), we identify a regime where an effective Balents-Fisher-Girvin model emerges, realizing a $\mathbb{Z}_2$ topological order in the hidden $\boldsymbolτ$ sector while the auxiliary $\boldsymbolμ$ sector remains a trivial paramagnet. This hidden nature drastically alters the experimental manifestations. Physical spins map to composite objects, typically convolving fractionalized anyon continua with non-topological magnon excitations. Nevertheless, specific symmetry-selected channels, such as the longitudinal intralayer response, exclusively reveal the bare two-vison continuum without collective-mode dressing. Our findings demonstrate that the topological order could be buried in certain hidden and emergent degrees of freedom, pointing to a promising pathway for seeking elusive topological states disguised within hidden internal degrees of freedom.