Temperature dependent spin gap in multiferroic 2D-XY antiferromagnet Ba2CoGe2O7 under applied magnetic field

We report an inelastic neutron scattering study on the low-energy magnetic excitations of Ba 2 CoGe 2 O 7 single crystals under applied magnetic fields for several fixed temperatures. Recently, a spin gap of about 0.1 meV has been observed under zero magnetic field and interpreted as an effect of spin-nematic interaction [1]. However, we have predicted such gap via linear spin wave theory by introducing the Dzyaloshinskii–Moriya interaction in the spin Hamiltonian. We have observed a spin gap of 0.105 meV in energy at 2 K under zero magnetic field and the gap energy decreases with increasing temperature. Temperature dependent spin gap also evolves under an applied magnetic field. Our results enlighten the discussion on understanding the origin of the zero-field spin gap in details.