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Sunset at the North Campus © Roland Baege​/​TU Dortmund
PUBLICATION IN NATURE COM­MU­NI­CA­TIONS

TU Research Team Finds Exotic Interactions in Semiconductors

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There are several devices in a laboratory and green laser beams are shining. © Felix Schmale​/​TU Dortmund
The team led by Dr. Marc Aßmann tailored two laser beams to precisely examine the interactions of Rydberg excitons.
A research team led by Dr. Marc Aßmann in TU Dortmund University’s Department of Physics, in cooperation with university partners at Rostock, Aarhus, and Harvard, has investigated extraordinarily strong interactions among Rydberg excitons in cuprous oxide. In the process the group discovered a blockade effect between excitons, which, with a size of several micrometers, seem like giants in the quantum mechanical system. The ability to control such effects is highly relevant for optical circuits and quantum information processing. The results have been published in the renowned journal Nature Communications.

Excitons are hydrogen-like bound states made up of negatively charged electrons and so-called “holes” – positively charged electron voids – in a semiconductor. They play a role in such diverse areas as organic solar cells, photosynthesis, and semiconductor lasers. Excitons are analogous to hydrogen in that they too possess excited states. Excitons in highly excited states, the Rydberg excitons, exhibit astonishing properties that are stronger the higher the quantum number of the excited state is: Thus the volume of an exciton in the twentieth excited state is already 64 million times as large as in its ground state, while the polarizability – that is, the sensitivity to external electric fields – is 1.2 billion times as great. These properties make Rydberg excitons very interesting for precision sensor technology.

Investigations with custom-tailored laser beams

Dr. Julian Heckötter investigated the interactions between several such Rydberg excitons in different excited states as part of his doctoral research, for which he was awarded the Else Heraeus Dissertation Prize of Dortmund’s Department of Physics. To accomplish this, he tailored two laser beams in such a way that each beam generates one strictly defined Rydberg exciton state, enabling him to make precise measurements of the interactions between the two states. Here he was able to demonstrate a complex blockade effect. “We determined that, around every exciton, a sphere forms within which no additional excitons can be generated,” says Dr. Marc Aßmann. “The excitons must keep a certain minimum distance between them, which can become as large as several micrometers.”

A systematic asymmetry also appeared, which depends on whether the effects are examined on a larger or a smaller exciton. Theoretical physicists Dr. Valentin Walther from Harvard, Prof. Thomas Pohl from Aarhus, and Prof. Stefan Scheel from Rostock were able to elucidate this phenomenon. Detailed computer simulations showed that its cause lies in Van der Waals interactions. These are the same forces credited with giving geckos the ability to walk along walls and ceilings.

The findings of the interdisciplinary research team were recently published in the renowned scientific journal Nature Communications. The project was funded in part within the framework of the joint German-Russian Collaborative Research Center TRR 160, in which research institutions in Dortmund and St. Petersburg are participating.

Original publication

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