Regensburg 2025 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 43: Correlated Electrons: Other Theoretical Topics

TT 43.13: Talk

Thursday, March 20, 2025, 12:45–13:00, H33

To Infinity and Back - 1/N Graph Expansion of Light-Matter Systems — •Andreas Schellenberger and Kai Phillip Schmidt — FAU Erlangen-Nürnberg, Erlangen, Deutschland

We present a method for performing a full graph expansion for light-matter systems, utilizing the linked-cluster theorem. This enables us to explore 1/N corrections to the thermodynamic limit N→ ∞, giving us access to the mesoscopic regime. This region is yet largely unexplored, as it is challenging to tackle with established solid-state methods. However, it hosts intriguing features, such as entanglement between light and matter that vanishes in the thermodynamic limit [1-3]. We calculate physical quantities of interest for paradigmatic light-matter systems like generalized Dicke models by accompanying the graph expansion by both exact diagonalization (NLCE [4]) and perturbation theory (pcst++ [5]), benchmarking our approach against other techniques.

[1] J.Vidal, S.Dusuel, EPL 74, 817 (2006).

[2] K.Lenk, J.Li, P.Werner, M.Eckstein, arXiv:2205.05559 (2022).

[3] A.Kudos,D.Novokreschenov,I.Iorsh,I.Tokatly,arXiv:2304.00805(2023).

[4] M.Rigol, T.Bryant, R.R.P.Singh, PRL 97, 187202 (2006).

[5] L.Lenke, A.Schellenberger, K.P.Schmidt, PRA, 108 (2023).

Keywords: Light-matter system; 1/N expansion; Series expansion method; Full graph Expansion; Numerical linked cluster expansion