Fractionalization, emergent SU($N$) symmetries, and fragmentation in layered quantum spin-orbital models

Abstract

We propose a family of layered quantum spin-orbital models as a platform to study fractionalization, unconventional forms of symmetry-breaking order, and their possible coexistence. The models are built by stacking $N$ layers of a square-lattice system in which Kitaev-type interactions promote the formation of a $\mathbb{Z}_2$ quantum spin-orbital liquid and coupling the different layers via Ising spin interactions. Using a parton construction, we show how, at low energies, these Hamiltonians can be mapped to $N$-component Fermi Hubbard models on a $\pi$-flux square lattice at half filling. We also demonstrate that the models acquire an emergent SU($N$) symmetry in the limit of equal all-to-all interlayer couplings and argue that, for $N>2$, the proximity to this limit offers the potential to realize an array of competing phases. To illustrate this point, we compute the zero-temperature phase diagram of the effective $N=3$ Hubbard model within mean-field theory and uncover rich phenomena, including intertwined orders and flavor-selective localization. Mapping back to the original degrees of freedom reveals that the ground states realize distinct forms of magnetic fragmentation, wherein the orbitals remain in a quantum liquid state whereas the spins can present conventional long-range order or nonlocal order characterized by a nontrivial string order parameters. We highlight possible extensions of our construction as well as its potential to provide concrete microscopic models for different fractionalized quantum critical points.