@article{DuanXieBraaketal.2016,
  author    = {Liwei Duan and You-Fei Xie and Daniel Braak and Qing-Hu Chen},
  title     = {Two-photon Rabi model: analytic solutions and spectral collapse},
  journal   = {Journal of Physics A: Mathematical and Theoretical},
  volume    = {49},
  number    = {46},
  pages     = {464002},
  doi       = {10.1088/1751-8113/49/46/464002},
  year      = {2016},
}