Goldstone mode of a magnon Bose–Einstein condensate in MnCO$_3$

The low-frequency phase modulation of a radio-frequency field is used to excite a low-frequency collective mode in the Bose-Einstein condensate (BEC) of quasinuclear magnons in an antiferromagnetic material with the Suhl–Nakamura interaction. This oscillation mode is similar to the Goldstone torsional mode observed in antiferromagnetic superfluid $^3$He-B. The results clearly demonstrate the existence of the BEC in the systems under study.