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    Band structure of a coupled Bose-Einstein-condensate–cavity system: Effects of dissipation and geometry

    David Baur, Simon Hertlein, Alexander Baumgärtner, Justyna Stefaniak, Tilman Esslinger, Gabriele Natale*, and Tobias Donner

    • *Contact author: gnatale@phys.ethz.ch

    Phys. Rev. A 113, 043307 – Published 6 April, 2026

    DOI: https://doi.org/10.1103/l71r-m9l6

    Abstract

    We present a theoretical model for a transversally driven Bose-Einstein condensate coupled to an optical cavity. We focus on the interplay between different coherent couplings, which can trigger a structural phase transition, known as the superradiant phase transition. Our approach, based on band-structure theory and a mean-field description, enables a comprehensive analysis of the nature of the system's excited modes, precursing the phase transitions. By incorporating dissipative couplings, intrinsic to these systems, we find non-Hermitian phenomena such as the coalescence of crossing precursor modes and the emergence of exceptional points. The general formulation of our model allows us to explain the role of an angle between the transverse pump and the cavity deviating from 90∘. This offers us a unified perspective on the plethora of different implementations of such systems.

    Physics Subject Headings (PhySH)

    See Also

    Synchronization of Quasiparticle Excitations in a Quantum Gas with Cavity-Mediated Interactions

    Gabriele Natale, Alexander Baumgärtner, Justyna Stefaniak, David Baur, Simon Hertlein, Dalila Rivero, Tilman Esslinger, and Tobias Donner
    Phys. Rev. Lett. 136, 140401 (2026)

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