The Lipkin–Meshkov–Glick model as a particular limit of the SU(1,1) Richardson–Gaudin integrable models

The Lipkin–Meshkov–Glick (LMG) model has a Schwinger boson realization in terms of a two-level boson pairing Hamiltonian. Through this realization, it has been shown that the LMG model is a particular case of the SU(1,1) Richardson–Gaudin (RG) integrable models. We exploit the exact solvability of the model to study the behavior of the spectral parameters (pairons) that completely determine the wave function in the different phases, and across the phase transitions. Based on the relation between the Richardson equations and the Lamé ordinary differential equation we develop a method to obtain numerically the pairons. The dynamics of pairons in the ground and excited states provide new insights into the first, second and third order phase transitions, as well as into the crossings taking place in the LMG spectrum