Spectral characterization of pair condensate states in a triangular lattice

When elementary quasiparticles form pairs and undergo Bose-Einstein condensation, they give rise to strongly correlated phenomena such as superconductivity and fermionic superfluidity. In quantum magnetic insulators, the pairing of magnetic quasiparticles, magnons, leads to the formation of the two-magnon bound state (TMBS). The condensation of TMBS induces a unique pair condensate state, known as the spin-nematic phase, which breaks spin-rotation symmetry without dipolar ordering. While direct observation of the order parameters associated with this symmetry breaking is challenging, spin-nematic phases exhibit distinct dynamical responses that can be probed in spectroscopic experiments. In this talk, I will present a high-resolution inelastic neutron-scattering study on the quantum magnet, Na₂BaNi(PO₄)₂, revealing key dynamical features of the spin-nematic phase. Using generalized spin-wave theory and exact diagonalization calculations, we confirmed the observed quasiparticle dynamics are driven by the underlying spin-nematic order. Our study provides a detailed spectral characterization of spin-nematic phases and offers new insights into the many-body interactions that govern their behavior.