Theory of Disordered $ν= 5/2$ Quantum Thermal Hall State: Emergent Symmetry and Phase Diagram

Abstract

Fractional quantum Hall (FQH) system at Landau level filling fraction $ν=5/2$ has long been suggested to be non-Abelian, either Pfaffian (Pf) or antiPfaffian (APf) states by numerical studies, both with quantized Hall conductance $σ_{xy}=5e^2/2h$. Thermal Hall conductances of the Pf and APf states are quantized at $κ_{xy}=7/2$ and $κ_{xy}=3/2$ respectively in a proper unit. However, a recent experiment shows the thermal Hall conductance of $ν=5/2$ FQH state is $κ_{xy}=5/2$. It has been speculated that the system contains random Pf and APf domains driven by disorders, and the neutral chiral Majorana modes on the domain walls may undergo a percolation transition to a $κ_{xy}=5/2$ phase. In this work, we do perturbative and non-perturbative analyses on the domain walls between Pf and APf. We show the domain wall theory possesses an emergent SO(4) symmetry at energy scales below a threshold $Λ_1$, which is lowered to an emergent U(1)$\times$U(1) symmetry at energy scales between $Λ_1$ and a higher value $Λ_2$, and is finally lowered to the composite fermion parity symmetry $\mathbb{Z}_2^F$ above $Λ_2$. Based on the emergent symmetries, we propose a phase diagram of the disordered $ν=5/2$ FQH system, and show that a $κ_{xy}=5/2$ phase arises at disorder energy scales $Λ>Λ_1$. Furthermore, we show the gapped double-semion sector of $N_D$ compact domain walls contributes non-local topological degeneracy $2^{N_D-1}$, causing a low-temperature peak in the heat capacity. We implement a non-perturbative method to bootstrap generic topological 1+1D domain walls (2-surface defects) applicable to any 2+1D non-Abelian topological order. We identify potentially relevant spin TQFTs for various $ν= 5/2$ FQH states in terms of fermionic version of U(1)$_{\pm 8}$ Chern-Simons theory $\times \mathbb{Z}_8$-class TQFTs.