Paarerzeugung in semiklassischer und quantenfeldtheoretischer Beschreibung

Die vorliegende Arbeit beschäftigt sich mit der Paarerzeugung von Elektron-Positron-Paaren in starken elektromagnetischen Feldern. Es werden zwei Regime der Paarerzeugung untersucht. Das erste Regime ist das sogenannte Tunnelregime der Paarerzeugung. Für dieses wird das vorhandene Tunnelbild zur Paarerzeugung um die Effekte eines Magnetfeldes und eines zusätzlichen, störungstheoretisch behandelten Photons erweitert. Beide Effekte werden in der semiklassischen Näherung behandelt. Für das zweite Regime der Paarerzeugung, welches untersucht wird, existiert bisher noch kein einfacher Zugang. Deshalb werden unter Verwendung des in/out-Formalismus für externe Felder numerische Berechnungen durchgeführt. Die numerischen Simulationen zeigen nicht-triviale Effekte, die in diesem Regime auftreten können. Speziell wird auf die Abhängigkeit der erzeugten Paare von der Polarisation des elektromagnetischen Feldes sowie auf die Bildung von Mehrpaarzuständen eingegangen

Domain-wall melting in ultracold boson systems with holes and spin-flip defects

Quantum magnetism is a fundamental phenomenon of nature. As of late, it has garnered a lot of interest because experiments with ultracold atomic gases in optical lattices could be used as a simulator for phenomena of magnetic systems. A paradigmatic example is the time evolution of a domain-wall state of a spin-1/2 Heisenberg chain, the so-called domain-wall melting. The model can be implemented by having two species of bosonic atoms with unity filling and strong on-site repulsion U in an optical lattice. In this paper, we study the domain-wall melting in such a setup on the basis of the time-dependent density matrix renormalization group (tDMRG). We are particularly interested in the effects of defects that originate from an imperfect preparation of the initial state. Typical defects are holes (empty sites) and flipped spins. We show that the dominating effects of holes on observables like the spatially resolved magnetization can be taken account of by a linear combination of spatially shifted observables from the clean case. For sufficiently large U, further effects due to holes become negligible. In contrast, the effects of spin flips are more severe as their dynamics occur on the same time scale as that of the domain-wall melting itself. It is hence advisable to avoid preparation schemes that are based on spin-flips.Comment: 15 pages, 12 figures. Supplemental Material: 2 animations (avi) comparing the domain-wall melting with and without defects, corresponding to figures 3, 4 and the discussion in section V.B; minor improvements; published versio

Relativistic tunneling picture of electron-positron pair creation

The common tunneling picture of electron-positron pair creation in a strong electric field is generalized to pair creation in combined crossed electric and magnetic fields. This enhanced picture, being symmetric for electrons and positrons, is formulated in a gauge-invariant and Lorentz-invariant manner for quasistatic fields. It may be used to infer qualitative features of the pair creation process. In particular, it allows for an intuitive interpretation of how the presence of a magnetic field modifies and, in particular cases, even enhances pair creation. The creation of electrons and positrons from the vacuum may be assisted by an energetic photon, which can also be incorporated into this picture of pair creation

Solitary excitations in one-dimensional spin chains

International audienc