Special Colloquium
Department of Physics, NCU
Impact of nonreciprocity on electromagnetic fluctuation induced phenomena: the case of an atom near a Chern insulator
Speaker
Dr. Bing-Sui Lu
Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore
Date 2021.3.04 (Thu)
Time 10:30am
Place S4-625
Abstract
In my talk, I describe certain electromagnetic fluctuation induced phenomena associated with a system that breaks electromagnetic reciprocity. Electromagnetic reciprocity implies that an interaction is symmetric if the space-time positions of a source and its field response are interchanged. However, there are systems in which such reciprocity is broken. As perhaps one of the simpler examples of a nonreciprocal system, we consider an atom interacting with a Chern insulator. A Chern insulator (CI) is a two-dimensional planar system which exhibits the quantum anomalous Hall effect, i.e., the static limit of its Hall conductance is quantised in units of $e^2/h$. Owing to the Hall effect it becomes nonreciprocal. For the phenomena associated with electromagnetic fluctuations, we consider the Casimir-Polder interaction and the spontaneous emission behavior of an atom near the CI, with the atom being modelled by a two-level system. To adequately address the fluctuational problem, we derive the full frequency dispersion of the conductivity tensor for a CI using the Kubo formula, finding for the case of real frequencies that the spectrum of possible behaviors of a CI can be classified into three possible regimes, namely, the low frequency regime, where the CI (being probed by photons with frequencies smaller than the CI band gap) behaves more akin to a topological Hall conductor; the intermediate frequency regime, where the frequency of photons is large enough to promote valence electrons to the conduction band and the CI acquires a real longitudinal conductance; and lastly, the high frequency regime, where the conductivity asymptotically vanishes at large frequencies. We look at how each frequency regime influences both the behavior of the lifetime and also the resonant contribution to the Casimir-Polder shift in the transition frequency of an excited atom, finding for instance that the presence of Van Hove singularities in the electronic density of states can dramatically enhance both the transition rate and the transition frequency between the atomic states. Electromagnetic nonreciprocity leads to coupling between a circularly polarised state of an atom with the Hall conductance of the CI, and depending on the relative sign of the circular polarisation and Hall conductance, the resonant behavior of the excited atom can manifest itself in rather different outcomes, which can facilitate the discrimination of the sign of the Chern number of the CI or the polarisation state the atom is in. Finally, we explore the possibility that the Casimir-Polder force between a ground-state atom and a Chern insulator can become repulsive for a certain window of atom-surface separations.
References
[1] B.-S. Lu, K. Z. Arifa, and X. R. Hong, “Spontaneous emission of a quantum emitter near a Chern insulator: Interplay of time-reversal symmetry breaking and Van Hove singularity.” Phys. Rev. B 101, 205410 (2020).
[2] K. Z. Arifa, M. Ducloy, D. Wilkowski, and B.-S. Lu, “Casimir-Polder interaction between a two-level quantum emitter and a Chern insulator” (manuscript in preparation).