Possibility of superradiant phase transitions in coupled two-level atoms

TL;DR

This paper explores the potential for superradiant phase transitions in coupled two-level atoms within a cavity, challenging the traditional sum rule constraints by considering atom-atom interactions.

Contribution

It demonstrates the possibility of a superradiant phase transition in coupled two-level atoms, accounting for atom-atom interactions and violating the sum rule.

Findings

Superradiant phase transition occurs in the thermodynamic limit.

Atom-atom attractive interactions enable the phase transition.

Finite-size analysis accurately locates the quantum critical point.

Abstract

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 superadiant phase transition as the number of atoms increases, satisfying all the constraints imposed by the sum rule.