@article{article259e9546,
title   = {Two interacting spins in external fields. four-level systems},
author = {Bagrov, V G and Baldiotti, Mario Cesar and Gitman, Dmitri Maximovitch and Levin, A D},
year = {2007},
doi = {10.1002/andp.200610231},
journal   = {Annalen der Physik}
}