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TU Research Team Finds Exotic Interactions in Semiconductors

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Technical devices and green laser light in a laboratory. © 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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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.

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