Meetings with Roland Zimmermann

Haphazardly, Roland Zimmermann and I shared through most of our lives the same research interests, namely the understanding of optically excited systems of electron–hole pairs by many‐body theory. In the seventies and eighties, when nanosecond laser pulses have been used, the quasi‐equilibrium Green function theory was the appropriate theoretical tool to study the highly excited semiconductors. In my field Roland Zimmermann was the most prominent and influential theoretician from East Germany. After the fall of the Wall our interests shifted as the laser pulses got shorter to the understanding of the nonequilibrium properties as observed by time‐resolved nonlinear spectroscopy. Either non‐equilibrium Green functions or reduced density matrices had to be used. Roland Zimmermann became a very convinced convert from Green functions to density matrices. Modern crystal growth techniques allowed to study also low‐dimensional systems such as the quasi‐two dimensional quantum wells. Because interface fluctuations are decisive for many properties of these quantum confined nanostructures, Roland Zimmermann rather consequently devoted himself in the last years to the study of disorder in these systems.