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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.
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Location & approach
The campus of TU Dortmund University is located close to interstate junction Dortmund West, where the Sauerlandlinie A 45 (Frankfurt-Dortmund) crosses the Ruhrschnellweg B 1 / A 40. The best interstate exit to take from A 45 is “Dortmund-Eichlinghofen” (closer to South Campus), and from B 1 / A 40 “Dortmund-Dorstfeld” (closer to North Campus). Signs for the university are located at both exits. Also, there is a new exit before you pass over the B 1-bridge leading into Dortmund.
To get from North Campus to South Campus by car, there is the connection via Vogelpothsweg/Baroper Straße. We recommend you leave your car on one of the parking lots at North Campus and use the H-Bahn (suspended monorail system), which conveniently connects the two campuses.
TU Dortmund University has its own train station (“Dortmund Universität”). From there, suburban trains (S-Bahn) leave for Dortmund main station (“Dortmund Hauptbahnhof”) and Düsseldorf main station via the “Düsseldorf Airport Train Station” (take S-Bahn number 1, which leaves every 15 or 30 minutes). The university is easily reached from Bochum, Essen, Mülheim an der Ruhr and Duisburg.
You can also take the bus or subway train from Dortmund city to the university: From Dortmund main station, you can take any train bound for the Station “Stadtgarten”, usually lines U41, U45, U 47 and U49. At “Stadtgarten” you switch trains and get on line U42 towards “Hombruch”. Look out for the Station “An der Palmweide”. From the bus stop just across the road, busses bound for TU Dortmund University leave every ten minutes (445, 447 and 462). Another option is to take the subway routes U41, U45, U47 and U49 from Dortmund main station to the stop “Dortmund Kampstraße”. From there, take U43 or U44 to the stop “Dortmund Wittener Straße”. Switch to bus line 447 and get off at “Dortmund Universität S”.
The AirportExpress is a fast and convenient means of transport from Dortmund Airport (DTM) to Dortmund Central Station, taking you there in little more than 20 minutes. From Dortmund Central Station, you can continue to the university campus by interurban railway (S-Bahn). A larger range of international flight connections is offered at Düsseldorf Airport (DUS), which is about 60 kilometres away and can be directly reached by S-Bahn from the university station.
The H-Bahn is one of the hallmarks of TU Dortmund University. There are two stations on North Campus. One (“Dortmund Universität S”) is directly located at the suburban train stop, which connects the university directly with the city of Dortmund and the rest of the Ruhr Area. Also from this station, there are connections to the “Technologiepark” and (via South Campus) Eichlinghofen. The other station is located at the dining hall at North Campus and offers a direct connection to South Campus every five minutes.
The facilities of TU Dortmund University are spread over two campuses, the larger Campus North and the smaller Campus South. Additionally, some areas of the university are located in the adjacent “Technologiepark”.