Gapped and gapless quantum spin liquids on the ruby lattice

Abstract: The ruby lattice has been the subject of much interest recently due its realization in Rydberg atom arrays, where its rich variety of frustrated interactions gives rise to topologically ordered quantum spin liquids. Similarly, numerical studies of ruby-lattice spin models, with both isotropic and anisotropic interactions, have provided evidence of gapped and gapless spin-liquid ground states with different low-energy gauge structures. Motivated by these findings, we perform a projective symmetry group (PSG) classification of U(1) and $\mathbb{Z}{2}$ fermionic spinon mean-field theories$\unicode{x2014}$respecting space-group and time-reversal symmetries$\unicode{x2014}$for $S=1/2$ spins. We obtain a total of 50 U(1) and 64 $\mathbb{Z}{2}$ PSGs, and upon restricting their realization via mean-field $\textit{Ans\"atze}$ with up to second-nearest-neighbor singlet amplitudes (relevant to the models studied here), only 8 U(1) and 18 $\mathbb{Z}{2}$ distinct states are obtained. We present the singlet fields for all $\textit{Ans\"atze}$ up to third-nearest-neighboring bonds and discuss their spinon dispersions as well as their dynamical spin structure factors. Building on this information, we also obtain the phase diagram of the Heisenberg model in the presence of first ($J{1}$), second ($J_{1}'$), and third ($J_{2}$) neighbor antiferromagnetic couplings within a self-consistent mean-field approximation.