Many-body Green's function theory for electron-phonon interactions: the Kadanoff-Baym approach to spectral properties of the Holstein dimer

We present a Kadanoff-Baym formalism to study time-dependent phenomena for systems of interacting electrons and phonons in the framework of many-body perturbation theory. The formalism takes correctly into account effects of the initial preparation of an equilibrium state, and allows for an explicit time-dependence of both the electronic and phononic degrees of freedom. The method is applied to investigate the charge neutral and non-neutral excitation spectra of a homogeneous, two-site, two-electron Holstein model. This is an extension of a previous study of the ground state properties in the Hartree (H), partially self-consistent Born (Gd) and fully self-consistent Born (GD) approximations published in Ref. [arXiv:1403.2968]. We show that choosing a homogeneous ground state solution leads to unstable dynamics for a sufficiently strong interaction, and that allowing a symmetry-broken state prevents this. The instability is caused by the bifurcation of the ground state and understood physically to be connected with the bipolaronic crossover of the exact system. This mean-field instability persists in the partially self-consistent Born approximation but is not found for the fully self-consistent Born approximation. By understanding the stability properties, we are able to study the linear response regime by calculating the density-density response function by time-propagation. This functions amounts to a solution of the Bethe-Salpeter equation with a sophisticated kernel. The results indicate that none of the approximations is able to describe the response function during or beyond the bipolaronic crossover for the parameters investigated. Overall, we provide an extensive discussion on when the approximations are valid, and how they fail to describe the studied exact properties of the chosen model system.Comment: 12 figure

Kadanoff-Baym approach to double-excitations in finite systems

We benchmark many-body perturbation theory by studying neutral, as well as non-neutral, excitations of finite lattice systems. The neutral excitation spectra are obtained by time-propagating the Kadanoff-Baym equations in the Hartree-Fock and second Born approximations. Our method is equivalent to solving the Bethe-Salpeter equation with a high-level kernel while respecting self-consistently, which guarantees the fulfillment of a frequency sum rule. As a result, we find that a time-local method, such as Hartree-Fock, can give incomplete spectra, while already the second Born, which is the simplest time-nonlocal approximation, reproduces well most of the additional excitations, which are characterized as double-excitations.Comment: 20 pages, 10 figure

Master School-artikkeleita opinnäytetöistä 2023

Vuoden 2023 Kajaanin Ammattikorkeakoulun Master Schoolin opinnäytetöistä kirjoitetut artik-kelit kiinnittyvät neljään teemaan, jotka ovat asiakkuuksien -, palvelujen -, henkilöstön osaamisen - ja koulutusorganisaatioiden työelämäyhteyksien kehittäminen. Opinnäytetöitä valmistuu lähes sata ja tähän julkaisuun kootut artikkelit ovat katsaus niistä. Asiakaskeskeisen organisaation kehittäminen ja hoitotyön näkökulmat, kuten eksistentiaalisen kärsimyksen tukeminen palliatiivisessa hoidossa, ovat hyvin tärkeitä. Samoin palvelujen kehittä-minen, kuten digipalveluiden käyttöönotto ja kustannustietoinen lähestymistapa ravitsemispal-veluihin, ovat merkittäviä aiheita. Palvelujen jatkuva parantaminen ja innovointi ovat avainasemassa nykypäivän markkinoilla. Tämä teema keskittyy uusien palveluiden luomiseen, nykyisten palveluiden tehostamiseen, tek-nologian hyödyntämiseen, jotka edesauttavat asiakastyytyväisyyttä