# Generalized method of Feynman-Pines diagram technique in the theory of energy spectrum of two-level quasiparticle renormalized due to multi-phonon processes at cryogenic temperature

## TL;DR

This paper develops a generalized Feynman-Pines diagram technique to analyze the energy spectrum of a two-level quasiparticle interacting with phonons at cryogenic temperatures, accounting for multi-phonon processes.

## Contribution

It introduces a compact branched chain fraction for the mass operator, effectively summing infinite diagrams to include multi-phonon effects in the spectrum analysis.

## Key findings

- Multi-phonon processes significantly alter the spectrum depending on energy resonance.
- Non-resonant systems show renormalized energies and phonon satellite levels.
- Resonant systems exhibit a renormalized ground state with satellite groups.

## Abstract

Theory of the spectrum of localized two-level quasi-particle renormalized due to interaction with polarization phonons at cryogenic temperature is developed using the generalized method of Feynman-Pines diagram technique. Using the procedure of partial summing of infinite ranges of the main diagrams, mass operator is obtained as a compact branched chain fraction, which effectively takes into account multi-phonon processes. It is shown that multi-phonon processes and interlevel interaction of quasiparticle and phonons cardinally change the renormalized spectrum of the system depending on the difference of energies of two states, which either resonates with phonon energy or does not. The spectrum of non-resonant system contains renormalized energies of the main states and two similar infinite series of groups of phonon satellite levels. The spectrum of a resonant system contains a renormalized ground state and infinite series of satellite groups.