др Јанез Бонча

24. јуна 2022.

У оквиру семинара Центра за изучавање комплексних система Института за физику у Београду, у понедељак, 27. јуна 2022. године у 14 часова у читаоници „др Драган Поповић“, др Јанез Бонча (Факултет за математику и физику, Универзитет у Љубљани; Институт „Јожеф Стефан“, Љубљана), одржаће предавање:

Dynamical properties  of a polaron coupled to dispersive optical phonons


In the first part we will present the  study of static and dynamic properties of an electron coupled to dispersive quantum optical phonons in the framework of the Holstein model defined on a one–dimensional lattice [1]. Calculations are performed using the Lanczos algorithm based on a highly efficient construction of the variational Hilbert space. Even small phonon dispersion has a profound effect on the low energy optical response. While the upward phonon dispersion broadens the optical spectra due to single phonon excitations, the downward dispersion has the opposite effect. With increasing dispersion a multi–phonon excitation (MPE) state becomes the lowest excited state of the system at zero momentum and determines the low–frequency response of the optical conductivity where the threshold for optical absorption moves below the single–phonon frequency. Low–energy MPEs should be
observable in systems with strong optical phonon dispersion in optical as well as angle resolved photoemission experiments.

In the second part we will discuss  Holstein polaron spectral function using the finite–temperature (T) Lanczos method [2]. With increasing T additional features in the  spectral function emerge even at temperatures below the phonon frequency. We observe a substantial spread of the spectral weight towards lower frequencies and the broadening of
the quasiparticle (QP) peak. In the weak coupling regime the QP peak merges with the continuum in the high-T limit. In the strong coupling regime the main features of the low–T spectral function remain detectable up to the highest T used in our calculations. We will also present results of the electron removal spectral function as relevant for angle resolved photo emission experiments.

If time permits,  we will also discuss some relaxation properties of the electron coupled to various bosonic excitations [3].

[1] J. Bonča, S. A. Trugman, Phys. Rev. B 103, 054304 (2021)
[2] J. Bonča, S. A. Trugman, and M. Berçiu, Phys. Rev. B 100, 094307
[3] J. Kogoj, M. Mierzejewski and J. Bonča, Phys. Rev. Lett., 117,
227002 (2016).