Deconfined Gapless phases and criticalities in Shastry-Sutherland Antiferromagnet

Abstract: Antiferromagnets on the Shastry-Sutherland lattice has attracted lots of research interest due to the possible existence of deconfined criticality. In the present work, we study the $J_1$-$J_2$-$J_4$ model using Variational Monte Carlo (VMC) method, where $J_1$, $J_2$, $J_4$ stand for the nearest-neighbor, next nearest neighbor and ring exchange interactions respectively. An empty plaquette (EP) phase is found with spontaneous mirror symmetry breaking. However, different from the EP phase in literature, the EP phase in our VMC approach is a $Z_2$ deconfined phase with majorana-type gapless spinons in the excitation spectrum. Additionally, a symmetric gapless $Z_2$ Quantum spin liquid phase is found adjacent to the EP phase, full plaquette (FP) and the Neel antiferromagnetic phases. The phase transition from the QSL phase to these ordered phases are found to be continuous. While the transition to the EP phase is of Landau type, the transitions to the Neel phase and the FP phase can be interpreted as condensation of $Z_2$ gauge charge ($e$ particle) and flux ($m$ particle).