Emergent Spin Supersolids in Frustrated Quantum Materials

Abstract: Recent years have witnessed the emergence of spin supersolids in frustrated quantum magnets, establishing a material-based platform for supersolidity beyond its original context in solid helium. A spin supersolid is characterized by the coexistence of longitudinal spin order that breaks lattice translational symmetry and transverse spin order associated with the spontaneous breaking of a spin U(1) symmetry. Extensive experimental investigations, together with advanced numerical studies, have now revealed a coherent and internally consistent picture of these phases, substantially deepening our understanding of supersolidity in quantum magnetic materials. Beyond their fundamental interest as exotic quantum states, potential applications in highly efficient demagnetization cooling have been supported by a giant magnetocaloric effect observed in candidate materials. Moreover, the possible dissipationless spin supercurrents could open promising perspectives for spin transport and spintronic applications. In this Review, we summarize recent progress on emergent spin supersolids in frustrated triangular-lattice quantum antiferromagnets. We survey experimental evidence from thermodynamic and spectroscopic measurements and compare these results with theoretical studies of minimal models addressing global phase diagrams, ground state properties, and collective excitations. In addition, we discuss characteristic spin-transport phenomena and outline future directions for exploring spin supersolids as functional quantum materials.