Max Planck Institutes and Experts

Extreme haze episodes shrouded Beijing during the winter of 2013, causing major environmental and health problems. We show that the severe winter haze was driven by stable synoptic meteorological conditions rather than by an abrupt change of emissions; the fast build-up of PM_2.5 in Beijing was mainly controlled by the atmospheric transport; and the production of secondary aerosols is enhanced during the haze periods. This enhancement cannot be explained by the weakened photochemistry suggesting a missing source of PM_2.5, which is likely the heterogeneous reaction.

Phase field models are a crucial tool in the modeling of complex phenomena. In this context, simulation can help to avoid or reduce the number of costly experiments. For this it is necessary to work with efficient algorithms. Here, we describe iterative solvers that deal with the discretized differential equation models and thus allow for an accurate solution of the problems.

The complex structures which emerge when a fluid invades a porous medium are of great relevance for many problems in the geosciences as well as in technology, engineering, and everyday life. Nevertheless, about fifty years of intense research have not been able to identify the dominant mechanisms at work. We have recently found that the solution is much simpler than anticipated. The mechanism is well hidden, but so elementary that high-school math is sufficient to come up with quantitative predictions.

Phases of matter are usually characterized by their symmetry breaking. With the Quantum Hall Effect, a completely new class of topological phases was discovered, which cannot be characterized by symmetry breaking. These phases have highly non-local excitations that could serve as ideal building blocks for a fault-tolerant quantum computer. To understand topological phases in realistic model systems, complicated quantum-many body systems have to be solved. This can be achieved by using new efficient algorithms based on insights from the field of quantum information.

In recent years, theoretical physicists discovered that the topology of a material can lead to interesting new quantum properties. This simple concept can be applied to the electronic structure of semiconducting materials in which relativistic effects are important. In 2015, several materials like NbP, NbAs, TaP, TaAs, und MoTe₂ have been suggested by theoreticians as promissing, so-called Weyl semimetals, and have been physically characterized just a bit later. The special feature is that Weyl fermions, which occur in these materials as quasiparticles, exist in two chiralities.