Spin correlations in the nematic quantum disordered state of FeSe

The quantum-disordered magnetic ground state in FeSe, intertwined with superconductivity and nematicity has been a research focus in iron-based superconductors. However, the intrinsic spin excitations across the entire Brillouin zone in detwinned FeSe, crucial for understanding its magnetism and superconductivity, have remained unresolved. Using inelastic neutron scattering , we reveal that stripe spin excitations (Q=(1, 0)/(0, 1)) exhibit the C2 symmetry, while Néel spin excitations (Q=(1, 1)) retain C4 symmetry with in the nematic state. Temperature-dependent differences between Q=(1, 0) and (0, 1) spin excitations above the structural phase transition unambiguously reveals the nematic quantum disordered state. The similarity of the Néel excitations in FeSe and NaFeAs suggests that the Néel excitations are driven by enhanced 3dxy orbital correlations. By fitting the dispersion of stripe excitations using a Heisenberg Hamiltonian with J1-K-J2, we establish a spininteraction phase diagram, positioning FeSe near a crossover region between the antiferroquadrupolar, Néel, and stripe ordering.The results provide an experimental basis for microscopic theoretical of the intertwined orders in iron-based superconductors.