Magnon gap excitations in van der Waals antiferromagnet MnPSe$_3$

Magneto-spectroscopy methods have been employed to study the zero-wavevector magnon excitations in MnPSe$_3$. Experiments carried out as a function of temperature and the applied magnetic field show that two low-energy magnon branches of MnPSe$_3$ in its antiferromagnetic phase are gapped. The observation of two low-energy magnon gaps (at 14 and 0.7 cm$^{-1}$) implies that MnPSe$_3$ is a biaxial antiferromagnet. A relatively strong out-of-plane anisotropy imposes the spin alignment to be in-plane whereas the spin directionality within the plane is governed by a factor of 2.5 $\times$ 10$^{-3}$ weaker in-plane anisotropy.