Properties of shallow-level D−-centers in polar semiconductors

The energy of the lowest singlet and triplet terms of shallow-level D− centers (two electrons bound at a singly charged Coulomb center) in semiconductors with ion bonding is theoretically analyzed. Electron-phonon interaction is described by the Fröhlich Hamiltonian. The D−-center energy is calculated by the Bu\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} $$\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\smile}$}}{l} $$ \end{document}mistrov-Pekar method of canonical transformations for an arbitrary coupling force with phonons. It was shown that, for the entire range of electron-phonon interaction parameters, the Bu\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} $$\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\smile}$}}{l} $$ \end{document}mistrov-Pekar method gives the lowest values of the ground-state energy of the D− centers and the free bipolaron in comparison with the best available numerical computations of these values which were performed using direct variational methods. The calculations showed the absence of both bound metastable triplet states, corresponding to the lowest triplet term of a D− center, and a bipolaron for the entire range of parameters of electron-phonon interaction. This is consistent with the Hill theorem concerning the absence of bound excited states of an H− ion.