Benchmarking Nonequilibrium Green's Functions against Configuration Interaction for time-dependent Auger decay processes

We have recently proposed a Nonequilibrium Green's Function (NEGF) approach to include Auger decay processes in the ultrafast charge dynamics of photoionized molecules. Within the so called Generalized Kadanoff-Baym Ansatz the fundamental unknowns of the NEGF equations are the reduced one-particle density matrix of bound electrons and the occupations of the continuum states. Both unknowns are one-time functions like the density in Time-Dependent Functional Theory (TDDFT). In this work we assess the accuracy of the approach against Configuration Interaction (CI) calculations in one-dimensional model systems. Our results show that NEGF correctly captures qualitative and quantitative features of the relaxation dynamics provided that the energy of the Auger electron is much larger than the Coulomb repulsion between two holes in the valence shells. For the accuracy of the results dynamical electron-electron correlations or, equivalently, memory effects play a pivotal role. The combination of our NEGF approach with the Sham-Schl\"uter equation may provide useful insights for the development of TDDFT exchange-correlation potentials with a history dependence.Comment: 7 pages, 3 figure

Nonadiabatic Electron Dynamics in Tunneling Junctions: Lattice Exchange-Correlation Potential

The search for exchange-correlation functionals going beyond the adiabatic approximation has always been a challenging task for time-dependent density-functional theory. Starting from known results and using symmetry properties, we put forward a nonadiabatic exchange-correlation functional for lattice models describing a generic transport setup. We show that this functional reduces to known results for a single quantum dot connected to one or two reservoirs and furthermore yields the adiabatic local-density approximation in the static limit. Finally, we analyze the features of the exchange-correlation potential and the physics it describes in a linear chain connected to two reservoirs where the transport is induced by a bias voltage applied to the reservoirs. We find that the Coulomb blockade is correctly described for a half-filled chain, while additional effects arise as the doping of the chain changes

Transient charge and energy flow in the wide-band limit

The wide-band limit is a commonly used approximation to analyze transport through nanoscale devices. In this work we investigate its applicability to the study of charge and heat transport through molecular break junctions exposed to voltage biases and temperature gradients. We find that while this approximation faithfully describes the long-time charge and heat transport, it fails to characterize the short-time behavior of the junction. In particular, we find that the charge current flowing through the device shows a discontinuity when a temperature gradient is applied, while the energy flow is discontinuous when a voltage bias is switched on and even diverges when the junction is exposed to both a temperature gradient and a voltage bias. We provide an explanation for this pathological behavior and propose two possible solutions to this problem.Comment: 11 pages, 9 figure

Real-time dynamics of Auger wavepackets and decays in ultrafast charge migration processes

The Auger decay is a relevant recombination channel during the first few femtoseconds of molecular targets impinged by attosecond XUV or soft X-ray pulses. Including this mechanism in time--dependent simulations of charge--migration processes is a difficult task, and Auger scatterings are often ignored altogether. In this work we present an advance of the current state-of-the-art by putting forward a real--time approach based on nonequilibrium Green's functions suitable for first-principles calculations of molecules with tens of active electrons. To demonstrate the accuracy of the method we report comparisons against accurate grid simulations of one-dimensional systems. We also predict a highly asymmetric profile of the Auger wavepacket, with a long tail exhibiting ripples temporally spaced by the inverse of the Auger energy.Comment: 11 pages, 7 figure

Efficient computation of the second-Born self-energy using tensor-contraction operations

In the nonequilibrium Green’s function approach, the approximation of the correlation self-energy at the second-Born level is of particular interest, since it allows for a maximal speed-up in computational scaling when used together with the generalized Kadanoff-Baym ansatz for the Green’s function. The present day numerical time-propagation algorithms for the Green’s function are able to tackle first principles simulations of atoms and molecules, but they are limited to relatively small systems due to unfavorable scaling of self-energy diagrams with respect to the basis size. We propose an efficient computation of the self-energy diagrams by using tensor-contraction operations to transform the internal summations into functions of external low-level linear algebra libraries. We discuss the achieved computational speed-up in transient electron dynamics in selected molecular systems.ACKNO</h4

Effiziente ab-initio Beschreibung von Ladungsträgerdynamik in wechselwirkenden Vielteilchensystemen basierend auf der Nicht-Gleichgewichts Greenschen Funktion

The study of ultrafast electron dynamics has drawn much attention in the past few years with the advent of advanced experimental techniques to access physical observables at the femtoseconds timescale. Many applications in physics, chemistry and biology are based on electron dynamics and understanding and controlling the flow of electrons would not only answer fundamental questions about physical processes, but further technological advances. While, in principle, solving the nonrelativistic Schrödinger equation would completely determine any physical phenomenon, an arbitrarily accurate resolution is essentially impossible for any case other than Hydrogen-like systems. For this reason, great effort has been put in the resolution of the many-electron problem, but many questions still go unanswered due to the highly complex nature of electronic correlations and the lack of computationally efficient methods. In this thesis, a study of the role of electron correlations is carried out, with focus on the description of ultrafast electron dynamics in finite systems. To this extent, a first-principle non-equilibrium Green’s function (NEGF) approach based on the Generalized Kadanoff-Baym Ansatz (GKBA) for the study of ultrafast electron dynamics is put forward. The method is built upon approximations aimed at increasing the efficiency of the otherwise computationally cumbersome NEGF equations, while correctly describing physical phenomena. By trimming down on the complexity of the calculation of the correlation self-energy one can analyze the ultrafast dynamics of systems with up to tens of electrons, otherwise inaccessible with standard methods, which lack the required efficiency or accuracy. To complement this study, an analysis of correlation has been carried out in the context of Time-Dependent Density-Functional Theory. A construction for the exchange correlation potential, encoding electronic correlations and memory effects, is developed, to study transport in nanoscale devices. In conclusion, this thesis aims to deepen our understanding of nonequilibrium electron dynamics by providing the methodology to study never-before simulated phenomena and gain insight into the role of correlations in electron-electron interaction.Die Untersuchung ultraschneller Elektronendynamik hat in den letzten Jahren viel Aufmerksamkeit erregt, insbesondere durch die Entwicklung fortschrittlicher experimenteller Techniken, welche physikalische Beobachtungen im Zeitraum von Femtosekunden ermöglichen. Viele Anwendungen in Physik, Chemie und Biologie basieren auf der Dynamik der Elektronen, und das Verständnis und die Kontrolle des Elektronenflusses würde nicht nur grundlegende Fragen zu physikalischen Prozessen beantworten, sondern auch technologischen Fortschritt ermöglichen. Während die Lösung der nicht-relativistischen Schrödingergleichung prinzipiell jedes physikalische Phänomen vollständig bestimmt, ist eine beliebig genaue Lösung in der Praxis, für alles außer wasserstoffähnliche Systeme, unmöglich. Deshalb sind große Anstrengungen unternommen worden, um das Vielteilchenproblem wechselwirkender Elektronen zu lösen, aber viele Fragen bleiben aufgrund der hohen Komplexität elektronischer Korrelationen und des Fehlens effizienter numerischer Methoden unbeantwortet. Im Rahmen dieser Arbeit wird eine Studie über die Rolle der Korrelationen von Elektronen durchgeführt, mit Schwerpunkt auf der Beschreibung ultraschneller Elektronendynamik in endlichen Systemen. Im Speziellen wird eine first-principle, non-equilibrium Green’s Function (NEGF) Methode, basierend auf der Grundlage des Generalized Kadanoff-Baym Ansatzes (GKBA), zur Untersuchung ultraschneller Elektronendynamik vorgeschlagen. Das Verfahren basiert auf Näherungen, welche darauf abzielen die numerische Effizienz der Lösung der rechenintensiven NEGF-Gleichungen zu erhöhen, und gleichzeitig physikalische Phänomene korrekt zu beschreiben. Durch Reduzierung der Komplexität der Berechnung der KorrelationsSelbstenergie kann ultraschnelle Dynamik von Systemen mit zehn oder mehr Elektronen analysiert werden, was mit Standardmethoden, welche nicht über die erforderliche Effizienz oder Genauigkeit verfügen, bisher nicht möglich ist. Zur Ergänzung dieser Studie wird eine Analyse der Beschreibung von Korrelation im Rahmen der zeitabhängigen Dichtefunktionaltheorie durchgeführt. Dazu wird eine Näherungen für das sogenanntes Austauschkorrelationspotenzial konstruiert, welche elektronische Korrelationen und Retardationseffekte beschreibt, um Elektronentransport durch Schaltkreisen im Nanobereich zu untersuchen. Das Hauptziel dieser Arbeit ist es unser Verständnis von Nichtgleichgewichtsprozessen und Dynamik von Elektronen zu vertiefen, indem Methoden entwickelt und zur Verfügung gestellt werden, um nie zuvor simulierte Phänomene zu untersuchen und Einblicke in die Rolle von Korrelationen in der Elektronenwechselwirkung zu gewinnen

Real-time observation of a correlation-driven sub 3 fs charge migration in ionised adenine

Sudden ionisation of a relatively large molecule can initiate a correlation-driven process dubbed charge migration, where the electron density distribution is expected to rapidly move along the molecular backbone. Capturing this few-femtosecond or attosecond charge redistribution would represent the real-time observation of electron correlation in a molecule with the enticing prospect of following the energy flow from a single excited electron to the other coupled electrons in the system. Here, we report a time-resolved study of the correlation-driven charge migration process occurring in the nucleic-acid base adenine after ionisation with a 15–35 eV attosecond pulse. We find that the production of intact doubly charged adenine – via a shortly-delayed laser-induced second ionisation event – represents the signature of a charge inflation mechanism resulting from many-body excitation. This conclusion is supported by first-principles time-dependent simulations. These findings may contribute to the control of molecular reactivity at the electronic, few-femtosecond time scale

Ultrafast dynamics of adenine following XUV ionization

The dynamics of biologically relevant molecules exposed to ionizing radiation contains many facets and spans several orders of magnitude in time and energy. In the extreme ultraviolet (XUV) spectral range, multi-electronic phenomena and bands of correlated states with inner-valence holes must be accounted for in addition to a plethora of vibrational modes and available dissociation channels. The ability to track changes in charge density and bond length during ultrafast reactions is an important endeavor toward more general abilities to simulate and control photochemical processes, possibly inspired by those that have evolved biologically. By using attosecond XUV pulses extending up to 35 eV and few-femtosecond near-infrared pulses, we have previously time-resolved correlated electronic dynamics and charge migration occurring in the biologically relevant molecule adenine after XUV-induced sudden ionization. Here, using additional experimental data, we comprehensively report on both electronic and vibrational dynamics of this nucleobase in an energy range little explored to date with high temporal resolution. The time-dependent yields of parent and fragment ions in the mass spectra are analyzed to extract exponential time constants and oscillation periods. Together with time-dependent density functional theory and ab-initio Green's function methods, we identify different vibrational and electronic processes. Beyond providing further insights into the XUV-induced dynamics of an important nucleobase, our work demonstrates that yields of specific dissociation outcomes can be influenced by sufficiently well-timed ultrashort pulses, therefore providing a new route for the control of the multi-electronic and dissociative dynamics of a DNA building block