Temperature-Dependent Spin Gap in Multiferroic 2D-XY Antiferromagnet Ba 2 CoGe 2 O 7 Under Applied Magnetic Fields

We report an inelastic neutron scattering (INS) study on the low-energy magnetic excitations of Ba2CoGe2O7 single crystals under applied magnetic fields for several fixed temperatures. We have observed a sizable spin gap of approximate to 0.105 meV for the acoustic mode at 2.2 K under zero magnetic fields, whereas no changes in energy of the optical modes at the magnetic zone center are observed under varying temperatures. However, the spin gap of the acoustic mode at finite fields also evolves under varying temperatures and the gap energy decreases with increasing temperature. Interestingly, the difference between the spin gap at 2.2 and 4.6 K is about approximate to 0.025 meV for all the applied fields of 0.35, 1, and 5 T. Our results enlighten the discussion in detail on understanding the origin of the spin gap in Ba2CoGe2O7.