Hidden $sl(2)$-algebraic structure in Rabi model and its 2-photon and   two-mode generalizations
  It is shown that the (driven) quantum Rabi model and its 2-photon and 2-mode generalizations possess a hidden $sl(2)$-algebraic structure which explains the origin of the quasi-exact solvability of these models. It manifests the first appearance of a hidden algebraic structure in quantum spin-boson systems without $U(1)$ symmetry. 
Yao-Zhong Zhang
quant-ph math-ph math.MP