title: Antiferromagnet with two coupled antiferromagnetic sublattices in a magnetic field.
abstract: We discuss the magnon spectrum of an antiferromagnet (AF) in a magnetic field h consisting of two interpenetrating AF sublattices coupled by the exchange interaction at T = 0. One-ion easy-plane anisotropy is also taken into account. We calculate using the 1/S expansion the gap in the spectrum which is a manifestation of the order-by-disorder effect in this system and the optical magnon mode splitting. Both of these phenomena originate from the inter-sublattice interaction. We also calculate the gap value at h≈h(c) in the leading order of the small parameter (h(c) - h)/h(c) using the magnon Bose-Einstein condensation treatment, where h(c) is the saturation field. By comparing results obtained within these two approaches we conclude that the 1/S expansion gives a qualitatively correct result at h ~ h(c) even at large one-ion anisotropy but it overestimates the gap value. The application is discussed of these results to the actively studied AF of the considered type NiCl₂-4SC(NH₂)₂(DTN).
authors.affiliation: Petersburg Nuclear Physics Institute, Gatchina, St Petersburg, Russia. alexey.sizanov@gmail.com
authors.forename: A V
authors.initials: AV
authors.lastname: Sizanov
authors.forename: A V
authors.initials: AV
authors.lastname: Syromyatnikov