Thermal decay of two-spinon bound states in quasi-2D triangular antiferromagnets

Abstract

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 phase transition leading to a spin liquid phase. 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 N\'eel 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 $T^* \simeq 0.75 T_N$, defining a {\it terminated Goldstone regime} between $T^*$ and $T_N$, where only the magnons close to the Goldstone modes survive as well-defined excitations, up to the N\'eel temperature. Our results support the idea that the fractionalization of magnons near a transition to a disordered phase can be extended to more realistic quasi-2D frustrated antiferromagnets.