Explanation of the Cu spin-wave excitation gap in Nd₂CuO₄

Abstract:

By use of a mean-field approach the spin-wave dispersion of the Cu degrees of freedom in the undoped high-T _C material Nd₂CuO₄is investigated. The experimentally observed sharp decrease of the Cu spin-wave gap with increasing temperature in the range ${T^*} < T < {T_N}$ is explained by a paramagnetic-like susceptibility of the Nd spins which couple to the Cu subsystem. The degeneracy of the “in-plane” and “out-of-plane” polarized Cu spin-wave branches is shown to be lifted by the uniaxial anisotropy of the Cu-Cu nearest-neighbor interaction.