Edge dynamics of covariant higher-rank Chern-Simons theory of fractons

Abstract

We study the covariant higher-rank Chern-Simons theory of fractons in 2+1 dimensions in the presence of a planar boundary. The differential generator associated with the scalar gauge symmetry singles out a single scalar Ward combination of the functional field equations. Its zeroth and first normal moments give two integrated boundary identities. The zeroth moment yields a traceless rank-two current obeying a fractonic double-divergence conservation law and a higher-rank Kac-Moody algebra. The first moment makes the normal-tangential field $w_i=h_{2i}$ explicit and yields an ordinarily conserved vector current with a standard $U(1)$ Kac-Moody algebra. Functional differentiation also produces a mixed equal-time bulk-boundary commutator between $w_1$ and $h_{11}$: it is non-vanishing at finite normal distance and vanishes in the intrinsic boundary limit. The equal-time commutators of the boundary current with the spatial components of the gauge-invariant field strength vanish, so microcausality is satisfied; when the boundary parameters are such that the two boundary sectors decouple, its retarded response vanishes at all times. Locally, the boundary constraints are solved by $w_i={\partial }_i\chi$ and $h_{ij}={\partial }_i{\partial }_j\xi +{\eta }_{ij}\varphi$. When the boundary parameters are such that the two sectors decouple, the induced action is the direct sum of a chiral boson in the $w_i$ sector and a higher-rank sector in which $\varphi$ is auxiliary and $\psi ={\partial }_1\xi$ is the propagating field. The latter has a non-chiral Luttinger description on its positive-energy branch and admits a chiral reduction. The boundary conditions determine the velocity and positivity branch of the $w_i$ mode, while the compatibility analysis in the $h_{ij}$ sector selects the positive-energy branch. On the positive-energy branch the two chiral modes co-propagate with orientation opposite to the sign of $\kappa$.

arXiv comment37 pages, no figures