Relevance of the Quadratic Diamagnetic and Self-Polarization Terms in Cavity Quantum Electrodynamics.

Christian Schäfer,Vasil Rokaj,Angel Rubio,Michael Ruggenthaler

doi:10.1021/acsphotonics.9b01649

Christian Schäfer, Vasil Rokaj + Show 2 more

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https://doi.org/10.1021/acsphotonics.9b01649

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Abstract

Experiments at the interface of quantum optics and chemistry have revealed that strong coupling between light and matter can substantially modify the chemical and physical properties of molecules and solids. While the theoretical description of such situations is usually based on nonrelativistic quantum electrodynamics, which contains quadratic light–matter coupling terms, it is commonplace to disregard these terms and restrict the treatment to purely bilinear couplings. In this work, we clarify the physical origin and the substantial impact of the most common quadratic terms, the diamagnetic and self-polarization terms, and highlight why neglecting them can lead to rather unphysical results. Specifically, we demonstrate their relevance by showing that neglecting these terms leads to the loss of gauge invariance, basis set dependence, disintegration (loss of bound states) of any system in the basis set limit, unphysical radiation of the ground state, and an artificial dependence on the static dipole. Besides providing important guidance for modeling of strongly coupled light–matter systems, the presented results also indicate conditions under which those effects might become accessible.

Highlights

Experiments at the interface of quantum optics and chemistry have revealed that strong coupling between light and matter can substantially modify the chemical and physical properties of molecules and solids
This will highlight the domain of applicability of common approximations that disregard these quadratic terms and at the same time indicate under which conditions substantial influence can be expected,[12] accessible with ab initio techniques such as quantum-electrodynamic density functional theory (QEDFT).[12,16,22,25,83]
Nonrelativistic quantum electrodynamics (QED) guarantees a set of further intuitive and essential conditions. It guarantees that the physical observables are independent of the chosen coordinate system, and it guarantees the stability of matter, i.e., atoms and molecules are stable if coupled to the vacuum of the electromagnetic field.[73]

Summary

Even if we break the inversion symmetry of the matter

Dependence without Self-Polarization, the photonic symmetry pα ↔ −pα will retain the trivial connection between the two Hamiltonians and their respective observables. Applying the Dicke model to deduce the influence of strong coupling on the local molecular states calls for a very careful analysis of all of the applied approximations and their consequences It permits physical features such as when charge distributions start to overlap, as is often the case in quantum-chemical calculations, leading to a dependence of the local observables on the surrounding (collective) ensemble.[16]. While state-ofthe-art models might provide insightful perturbative results, the development of corrected models should obtain additional interest, and ab initio calculations could prove beneficial to foster this effort

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■ ACKNOWLEDGMENTS

■ REFERENCES