Kolloquium der Theoretischen Physik

From Institute for Theoretical Physics II / University of Erlangen-Nuremberg

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approximately every second Tuesday from 16:15 to 17:15 in Lecture Hall F

  • 24.10.2017 Markus Heyl (MPIPKS Dresden) Dynamical quantum phase transitions

The theory of phase transitions plays a central role for the understanding of equilibrium physical systems. In this talk I will introduce a dynamical analogue, termed dynamical quantum phase transitions, occuring during nonequilibium quantum real-time evolution. I will summarize the recent achieved advances including the first experimental observations in ultracold atoms as well as trapped ions. Furthermore, I will discuss to which extent important concepts of equilibrium criticality can be generalized to these dynamical transitions. In particular, I will show that it is possible to extend the concepts of scaling and universality by considering the specific example of the Ising model.

  • 21.11.2017 Karin Everschor-Sitte (Uni Mainz) Topology, Skyrmions and their dynamics - "Banana kicks in magnetism

The Nobel Prize in Physics 2016 was awarded to David J. Thouless (1/2), F. Duncan M. Haldane (1/4) and J. Michael Kosterlitz (1/4) “for theoretical discoveries of topological phase transitions and topological phases of matter”. By incorporating concepts of topology into physics the Nobel Laureates opened the path towards several current research fields including topological insulators, topological superconductors and topological magnetic textures like magnetic “skyrmions”. Skyrmions are topologically stable whirls that are realized in different areas of physics and were initially discovered by Tony Skyrme in particle physics in the 1960's. Here we will focus on skyrmions that occur in magnetic systems, which were observed experimentally for the first time in 2009. Due to their interesting physics based on their topological properties magnetic skyrmions have become a very active research area. For example, the peculiar twist of the magnetization in skyrmions leads to a very efficient coupling to electric currents making skyrmions also interesting for spintronics and allowing for “banana kicks” in analogy to the ones in soccer.

  • 28.11.2017 Stefan Wessel (RWTH Aachen) Excitations and Frustration in Coupled Spin Dimer Systems

The spin-1/2 Heisenberg model of coupled spin dimers provides a basic example to explore quantum critical behavior in quantum many-body systems. The equilibrium properties of such systems have been intensively investigated in the past, in both 2D and 3D coupled dimer arrays, as well as experimentally in several coupled spin dimer compounds. Here, we examine in detail the modifications in the excitation spectrum across quantum phase transitions between magnetically ordered phases and quantum disordered regimes in such systems. In particular, we identify the longitudinal amplitude ("Higgs") mode in the ordered phase of 2D coupled dimer systems, and examine the logarithmic scaling corrections in the excitation gap scaling in 3D coupled dimer arrays. Furthermore, we report on new results for a fully frustrated 2D coupled dimer system, wherein a discontinuous quantum phase transition is shown to extend up as a finite temperature first-order transition line that ends in a Ising critical end point in a fully SU(2)-symmetric system.

  • 12.12.2017 Fernando Barbero (Instituto de Estructura de la Materia, Madrid) The distribution of the eigenvalues of the area operator: A game of partitions?

I will discuss the distribution of the eigenvalues of the area operator in Loop Quantum Gravity. I will pay special attention to the several problems involving integer partitions that have been traditionally used to shed some light on this issue. As I will show they are only partially successful so new methods must be deveolped. During the talk I will give some background on the history of the partition problem.