Emergent symmetry and fractionalized Goldstone modes in a bilayer quantum spin liquid

I will present the phase diagram of a bilayer quantum spin liquid model with Kitaev-like interactions

on a square lattice. I will show that the low energy model is described by a π-flux Hubbard model

with an enhanced SO(4) symmetry. The antiferromagnetic Mott transition induced by interlayer

interactions in the Hubbard model amounts to a magnetic fragmentation transition for the original

degrees of freedom where a fluctuating non-local in-plane order coexists with an out-of-plane local

magnetization. The corresponding topological order is characterized by Z2 × Z2 if the Neel order is

along the z direction and Z2 otherwise. I will benchmark these results with a perturbative analysis.

I will elucidate on the low energy collective excitations of these phases and show that the Goldstone boson of the Z2 × Z2 phase is fractionalized and nonlocal.