Thermal Decay of Two-Spinon Bound States in Quasi-2D Triangular Antiferromagnets

We analyze the temperature evolution of the anomalous magnetic spectrum of the spin-1/2 triangular quantum Heisenberg antiferromagnet, which is proximate to a quantum critical point. Recently, its low energy excitations have been identified with two-spinon bound states, well defined in an ample region of the Brillouin zone. In this work, we compute the thermal magnetic spectrum within a Schwinger boson approach, incorporating Gaussian fluctuations around the saddle-point approximation. In order to account for a finite Néel temperature T_N, we incorporate an exchange interaction between triangular layers. As temperature rises, the dispersion relation of the two-spinon bound states, representing single-magnon excitations, remains unchanged but becomes mixed with the thermally activated spinon continuum. Consequently, a crossover occurs at a temperature T^* ≃ 0.75 T_N, defining a "terminated Goldstone regime" between T^* and T_N, where only the Goldstone modes survives as well-defined magnon excitations, up to the Néel temperature. Our results support the idea that the fractionalization of magnons near a quantum critical point can be extended to more realistic quasi-2D frustrated antiferromagnets.