Title: Method of successive separation and summing of multiplicative diagrams of mass operator for the multi-level quasiparticle interacting with polarization phonons
Author(s):

	M.V. Tkach (Yuriy Fedkovych Chernivtsi National University, 2 Kotsyubinsky St., 58012 Chernivtsi, Ukraine) ,
	Ju.O. Seti (Yuriy Fedkovych Chernivtsi National University, 2 Kotsyubinsky St., 58012 Chernivtsi, Ukraine) ,
	O.M. Voitsekhivska (Yuriy Fedkovych Chernivtsi National University, 2 Kotsyubinsky St., 58012 Chernivtsi, Ukraine) ,
	V.V. Gutiv (Yuriy Fedkovych Chernivtsi National University, 2 Kotsyubinsky St., 58012 Chernivtsi, Ukraine)

The theory of renormalized energy spectrum of a multi-level quasiparticle interacting with polarization phonons at T = 0 K is developed within the Feynman-Pines diagram technique in a new approach. It permits a successive separation of multiplicative diagrams from non-multiplicative ones for all orders of mass operator, and their partial summing as well. The obtained mass operator is presented as a sum of branched chain fractions, which effectively take into account the multi-phonon processes. For the two-level quasiparticle it is shown that just the interlevel (non-diagonal) interaction with phonons fundamentally changes the properties of the spectrum. In the vicinity of all threshold energies, the quasi-equidistant phonon satellite bands (i.e., groups of energy levels) are formed. They correspond to the complexes of bound states of a quasi-particle with many phonons.

Key words: diagram technique, quasiparticle, mass operator, phonon, spectrum
PACS: 71.38.-k, 72.10.Di, 63.20.kk

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