ÖPG-CMD Joint Meeting 2026

Examining the accuracy of the cumulant expansion method in electron-phonon systems

Sep 23, 2026, 11:45 AM

15m

HS 15.04 (University of Graz)

3) Contributed talk M12 - Recent Developments of the Polaron Theory Mini-Colloquium

Speaker

Petar Mitrić (Institute of Physics Belgrade)

Description

The cumulant expansion (CE) method provides an alternative to the conventional Dyson equation approach for the calculation of spectral functions and quasiparticle properties in interacting quantum many-particle systems. Furthermore, when combined with the independent particle approximation (IPA), the CE can also be used to calculate charge mobility.

We examine the range of validity of this method by implementing it in simplified model systems—the Holstein, Peierls, and Fröhlich models[1,2]—for which accurate or numerically exact benchmarks are available. Based on these numerical results, supported by analytical arguments using spectral sum rules, we find that for weak to moderate coupling strengths and not-too-low temperatures, the CE within the IPA framework yields accurate results. We also discuss the accuracy of the IPA approximation, i.e., the role of vertex corrections [2,3].

Acknowledgments
This research was supported by the Science Fund of the Republic of Serbia, Grant No. 5468, Polaron Mobility in Model Systems and Real Materials – PolMoReMa.

References
[1] P. Mitric, V. Jankovic, N. Vukmirovic, and D. Tanaskovic, Phys. Rev. B 107, 125165 (2023)
[2] P. Mitric, V. Jankovic, N. Vukmirovic, and D. Tanaskovic, Phys. Rev. B 113, 155119 (2026)
[3] V. Jankovic, P. Mitric, D. Tanaskovic, and N. Vukmirovic, Phys. Rev. B 109, 214312 (2024)