Possibility of superradiant phase transitions in coupled two-level atoms

Although the oscillator strength sum rule forbids the phase transition in ideal non-interacting two-level atoms systems, we present the possibility of the quantum phase transition in the coupled two-level atoms in a cavity. The system undergoes the superradiant phase transition in the thermodynamics limit and this transition is account for the atom-atom attractive interaction, exhibiting a violation of the sum rule. The bosonic coherent state technique has been adopted to locate the quantum critical point accurately in the finite-size system. We predict the existence of the superradiant phase transition as the number of atoms increases, satisfying all the constraints imposed by the sum rule. (C) 2012 Elsevier B.V. All rights reserved.