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Modern Physics Letters B

DICKE HAMILTONIAN AND SUPERRADIANT PHASE TRANSITIONS

E. DEL GIUDICE, Rosa Mele, Guiliano Preparata

Published Dec 10, 1993• 14 citations
Publisher

Abstract

We show that a correct analysis of the gauge-invariance properties of the em coupling of atomic systems with single-mode em radiation fields allows to overcome the “no-go” theorem for superradiant phase transitions in systems governed by the Dicke Hamiltonian
DOI
10.1142/s0217984993001879

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Phase transitions of a generalized Dicke model in the Coulomb gauge-including A(2) terms in the matter-radiation coupling, as well as direct dipole-dipole interaction terms-are studied. After a brief review of previous work on the 'no-go theorem' for phase transitions in the Dicke model, it is shown that a consistent truncation of the radiation modes and of the direct interactions leads to a model that does have a phase transition. When transformed to the electric dipole gauge, such a system takes exactly the form of the original Dicke Hamiltonian, which displays the expected phase transition.
In this doctoral thesis, the thermodynamic phases und phase transitions of a generalised Dicke model are studied and characterised. Both, finite and vanishing temperatures are considered. The Dicke model of quantum optics describes collective phenomena which occur when light interacts with a many-atom system. In the thermodynamic limit, a so-called Hepp–Lieb superradiant phase transition sets in. A superradiant phase develops for low temperatures or strong coupling between light and atoms, which is characterised by a macroscopic excitation of the light field and a spontaneous, collective polarisation of the atoms. This thesis discusses a generalised version of the Dicke model, which is described by a quantum mechanical system consisting of three-level atoms in Lambda-configuration and two modes of a resonator. By means of the Holstein–Primakoff transformation, the Hamiltonian of this system is written in terms of four interacting, non-linear oscillators, which can be linearised in the thermodynamic limit, yielding the ground-state energy as well as the low-energy excitations. The phase diagram consisting of two superradiant phases separated by continuous and first-order phase transitions is derived. In order to clarify the question whether or not the superradiant phase transition of this generalised Dicke model can be observed experimentally for real atoms, the significance of the diamagnetic term is discussed. In contrast to the original Dicke model, superradiant phase transitions are possible in principle. This is due to the first-order phase transitions. In addition, a no-go theorem for continuous superradiant phase transitions is presented. The argument is based on the Thomas–Reiche–Kuhn sum rule. Last, we study the superradiant phase transition of the generalised Dicke model at finite temperatures. Therefore, the partition sum is computed and analysed in the thermodynamic limit using Laplace’s method. At finite temperatures, the properties of the phase diagram and phases remain. However, here all phase transitions are of first order.