Electron paramagnetic resonance with parallel pumping in quantum spin chains with field-induced spin gap

Recently several compounds were observed to reveal properties of a one-dimensional spin-1/2 antiferromagnetic Heisenberg chain with gapless low-lying spin excitations in an absence of an external magnetic field. However even a very weak external magnetic field yields an onset of a spin gap for those low-lying excitations. Usually this onset of a field-induced spin gap is related to the presence of a staggered Dzyaloshinskii-Moriya interaction, or to the staggered effective g factors of magnetic ions. Experiments on thermodynamic properties cannot distinguish between those two origins for the spin-gap formation. Here we propose the setup for an experiment, in which one can clearly distinguish between the staggered Dzyaloshinskii-Moriya coupling and staggered external magnetic field: an electronic paramagnetic resonance in the parallel pumping geometry. The characteristics of the system under conditions of such an experiment, i.e., the renormalization of the homogeneous and staggered magnetizations, power of the microwave magnetic field, absorbed by the system, are calculated. Several important features of the solution are discussed.