Doctoral defence: Kaja Pae "Electron-phonon interactions in local degenerate electronic states in solids“

On 20 June at 14:00 Kaja Pae will defend her doctoral thesis "Electron-phonon interactions in local degenerate electronic states in solids" for obtaining the degree of Doctor of Philosophy (in Physics).

Prof. Vladimir Hizhnyakov, University of Tartu

Prof. Boris Tsukerblat, Ben-Gurion University of the Negev(Israel)


Herein lie the principal findings of the thesis concerning the theory of electronphonon interaction in molecular systems and impurity centres in crystals with degenerate electronic states.  The doctoral thesis deals with the interaction between local nuclear vibrations and the motion of electrons in solid and molecular systems, which is called the vibronic interaction. Often this interaction is small and is neglected to simplify calculations. However, there are systems where the consideration of this interaction is necessary for an adequate description of the characteristic properties of the system, and it is responsible for important quantum effects. The main focus of the doctoral thesis is to study the impurity centre of the crystal in the excited state, in which there is a vibronic interaction and which in turn interacts with the vibrations of the crystal lattice, the numerical magnitude of which is ~1023. A purely quantum mechanical theory and corresponding calculations are presented to describe the interaction between the crystal impurity centre and the crystal lattice vibrations. The interaction of degenerate electronic and vibrational states causes spontaneous symmetry breaking and mixing of electronic and vibrational states in the system. A remarkable manifestation of the vibronic interaction in systems with electronic degeneracy (or quasi-degeneracy) is the existence of conical intersections within the adiabatic potentials of these systems. Extensive literature examines the role of conical intersections in chemical transformations, as it largely determines the photochemical properties of the systems. The E ⊗ e JahnTeller system serves as an archetype of such systems with a conical intersection. As shown by Slonczewski, a distinctive quantum phenomenon emerges from a conical intersection – high-energy quantum states in the upper (conical) part of the potential with a mixed electron-vibrational (vibronic) origin. These states are called Slonczewski resonances. A clear manifestation of these states was independently discovered in 1972 by V. Loorits (University of Tartu) and M. C. M. O’Brien and S. N. Evangelou (University of Oxford) in a numerical study of the optical spectra of E ⊗ e-systems. A complementary numerical study of these resonances in optical spectra was pursued in my bachelor thesis, with particular focus on states demonstrating high pseudorotational momentum. In Publication VI was proposed an analytical theory of these resonant states, employing Airy functions with appropriately formulated boundary conditions as the basic states. The obtained analytical theory is consistent with the numerical results.