Researchers at Oak Ridge National Laboratory combined laser pumping with inelastic neutron scattering (INS) to experimentally observe long-lived nonequilibrium magnons in a 2D square-lattice Heisenberg antiferromagnet (Rb₂MnF₄). By periodically exciting the material with nanosecond laser pulses and probing with neutrons, they detected a clear violation of the detailed balance principle — a hallmark of systems driven out of thermodynamic equilibrium. The nonequilibrium magnon population accumulates at low energies and reaches a nonequilibrium steady state (NESS), sustained by conservation laws and slow magnon-phonon relaxation. Using quantum transport theory and a quantum Langevin equation framework, the team shows that out-of-time-ordered correlations between creation and annihilation operators fail to commute, providing a quantum mechanical explanation for the breakdown of microreversibility. This in operando INS technique opens new avenues for momentum- and energy-resolved studies of dissipative quantum dynamics in spin systems.
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