Renormalization group analysis of short-range correlations in Shastry-Sutherland model

We study Shastry-Sutherland model and its generalization using pseudofermion Functional Renormalization Group theory. Quantum material SrCu2 (BO3)2 is a very accurate realization of this model where recent experiments reported evidence of phase transitions from a singlet valence bond ground state to a plaquette valence bond state and later to a long-range ordered antiferromagnetic Neel state under tunable pressure. Earlier numerical studies argued possibility of deconfined quantum criticality versus a gapless spin liquid phase in the vicinity of transition from plaquette singlet to Neel state. We develop renormalization flow equations consistent with underlying coupled plaquettes in this system and solve them numerically for large system sizes. We identify a dimer fixed point in the exactly solvable region and show that FRG accurately describes the well established dimer valence bond to plaquette valence bond transition in agreement with the results of the state-of-the-art numerical techniques. Introducing a correlation function to describe the intermediate phase in terms of vertex functions, we show the plaquette ordered state consistent with DRMG results. In the region between plaquette and Neel states, we find continuous flow down to lowest numerical renormalization scale, peak degeneracy in momentum space, and close competition between two distinct plaquettes suggesting suppression of all long-range as well as short-range singlet orders. By analyzing the correlations in this region we demonstrate that exchange couplings map to J1–J2 Heisenberg model around it maximally frustrated regime supporting the exciting possibility of a quantum paramagnet in a small window.