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Journal of Physics Condensed Matter

Coulomb interactions, gauge invariance, and phase transitions of the Dicke model

Jonathan Keeling

Published Jun 11, 2007• 95 citations
Publisher

Abstract

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.

DOI
10.1088/0953-8984/19/29/295213

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•Jan 1, 2017•0 citations
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.
The Dicke model, which describes the dipolar coupling between NNN two-level atoms and a quantized electromagnetic field, seemingly violates gauge invariance in the presence of ultrastrong light-matter coupling, a regime that is now experimentally accessible in many physical systems. Specifically, it has been shown that, while the two-level approximation can work well in the dipole gauge, the Coulomb gauge fails to provide the correct spectra in the ultrastrong coupling regime. Here we show that, taking into account the nonlocality of the atomic potential induced by the two-level approximation, gauge invariance is fully restored for arbitrary interaction strengths, even in the $N\ensuremath{\rightarrow}\ensuremath{\infty}$ limit. Finally, we express the Hopfield model, a general description based on the quantization of a linear dielectric medium, in a manifestly gauge-invariant form, and show that the Dicke model in the dilute regime can be regarded as a particular case of the more general Hopfield model.
The long-standing debate whether the phase transition in the Dicke model can be realized with dipoles in electromagnetic fields is still an unsettled one. The well-known statement often referred to as the ``no-go theorem'' asserts that the so-called A2{A}^{2}A2 term, missing from the Dicke model, is relevant and prevents the system from undergoing a phase transition. At variance with this common belief, in this paper we prove that the Dicke model does give a proper description of the interaction of light field with the internal excitation of atoms. The phase transition and the spontaneous formation of a transverse-electric mean field cannot be excluded by principle. We point out that the proper treatment of the single-mode approximation is crucial, wherefore we present a systematic derivation of the dipole gauge inside a perfect Fabry-P\'erot cavity from first principles. Besides the impact on the debate around the Dicke phase transition, such a cleanup of the theoretical ground of cavity QED is important because currently there are many emerging experimental approaches to reach strong or even ultrastrong coupling between dipoles and photons, which demand a correct treatment of the Dicke model parameters.
In a gauge invariant formulation of the molecular electric dipole-photon interaction, the rigorous coupling is strictly linear in the photon creation and photon annihilation operators. The linear coupling allows for a super-radiant phase transition as in the Hepp-Lieb formulation. A previous notion of a quadratic-coupling ``no-go theorem'' for super-radiance is incorrect. Also incorrect is a previous assertion that the dipole-photon coupling has absolutely no effect on the thermal equations of state. These dubious assertions were based on incorrect canonical transformations which eliminated the electric field (and thereby eliminated the dipole-photon interaction) which is neither mathematically nor physically consistent. The correct form of the canonical transformations are given in this work which allows for the physical reality of super-radiant condensed matter phases.
A non-relativistic quantum theory of matter-photon interaction is formulated, within the framework of the dipole approximation, by employing a canonical transformation to diago­ nalize the A' term plus the free photon Hamiltonian. The matter-photon interaction is there­ by expressed rigorously in a renormalized A·p form. Using this form for the Dicke model of two-level atoms interacting with a single-mode radiation field, we examine a possibility of the occurrence of stationary coherent states which could arise for a photon and atomic polarization as the ground state of the system, deducing a condition for such coherent states. The condition, expressed as an inequality which imposes a stable nontrivial Bloch angle of the uniform atomic polarization, is identical with that for the occurrence of a second order phase transition, and indicates that the non-existence of a second order phase transition pointed out by Rza2ewski et al. io; in fact a consequence of the present treatment of the A'­ term renormalization. However, a fulfillment of the condition is recovered, if exchange­ type atom-atom interaction is taken into account in the matter system. We also present a method to construct an effective Hamiltonian of atom-atom interaction which is equivalent to the above Dicke model in the thermodynamic limit.
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.