Spin excitations of the Shastry-Sutherland model -- altermagnetism and proximate deconfined quantum criticality

Symmetry plays a crucial role in condensed matter physics. In quantum magnetism, it dictates a number of exotic phenomena, including deconfined quantum criticality and altermagnetism. Here, by studying the spin excitations of the $S=1/2$ antiferromagnetic Shastry-Sutherland model, we show that the N\'eel antiferromagnetic state in this model is an altermagnet featuring a non-relativistic splitting between two chiral magnon bands. Moreover, we identify a Higgs mode in the longitudinal excitation channel, whose gap softens when approaching the antiferromagnetic to plaquette valence bond solid transition, implying the appearance of nearly deconfined excitations. However, the splitting between the two magnon bands (Goldstone modes) remains finite at the transition. These results indicate that the transition is weakly first-order and proximate to a putative deconfined quantum critical point. We find that the altermagnetism provides a sensitive means to probe the deconfined quantum criticality.