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Researchers at the Ruhr University Bochum and the University of Duisburg-Essen have gained new knowledge about quantum dots. These are small islands in semiconductors in which individual electrons can be locked. They could one day form the information units in quantum computers. By finely adjusting the semiconductor properties, the physicists created special, long-lived energy states inside the quantum dots, which are normally too unstable to be measured. The results are published in "Physical Review B" on January 15, 2018. The magazine recognized the contribution as a highlight article.

A research team led by Dr. Arne Ludwig from the Bochum Chair for Applied Solid State Physics with Prof. Dr. Björn Sothmann from the University of Duisburg-Essen as part of the Materials Chain Research Association of the University Alliance Ruhr.

Electrons occupy certain energy levels

Quantum dots are around ten nanometers in size. Due to their small size, the movement of the electrons inside is severely restricted. As a result, the energy in a quantum dot cannot have continuous values; instead, the electrons only occupy certain energy levels - similar to atoms in which the electrons occupy certain shells.

Usually a quantum dot is in thermodynamic equilibrium; that is, the electrons contained occupy the available energy states one after the other from lowest to highest. These energy levels can be measured with the help of so-called capacitance-voltage spectroscopy. States in non-equilibrium - in which the electrons do not obey the normal rules for the occupation of the energy levels - could not yet be detected with this method. This is exactly what the team from Duisburg-Essen and Bochum succeeded in doing.

"Non-equilibrium processes are very useful for tailoring materials for certain technical applications," explains Prof. Dr. Andreas Wieck, co-author and head of the Chair for Applied Solid State Physics. “But they can also be fatal. Therefore, knowledge of the generation and control of these processes is important. "

Pairs of electrons and electron holes are created

The key was to keep the unstable non-equilibrium states alive long enough to be able to measure them. For their experiments, the researchers produced quantum dots in a semiconductor with finely adjusted material properties. In the quantum dots they created pairs of electrons and holes, the latter being created by the targeted removal of individual electrons and which in the solid state can be viewed as antiparticles of the electrons. If the electron and the hole meet, they cancel each other out. The energy released is emitted in the form of a photon, i.e. a single light particle.

This is how you can imagine quantum dots in a solid.
© Sascha Valentin

The researchers used the opposite phenomenon to produce the electron-hole pairs: They irradiated the quantum dot with light, which created electrons and holes inside.

Measured non-equilibrium states and described in the model

The physicists had tailored the material structure of the quantum dots and their surroundings so that they could now use the electron holes to specifically extinguish individual electrons. This left electrons in a non-equilibrium state, i.e. electrons that occupy atypical energy levels. The team measured these states with capacitance-voltage spectroscopy. In addition to the experiments, the scientists created a model that theoretically describes the measurements of the non-equilibrium states.

“It was obvious to interpret the experimental results as non-equilibrium phenomena,” says Bochum's first author Sascha Valentin. "Developing a physical model, however, meant bigger pull-ups that could only be done with the help of our theoretical colleagues."

"Surprisingly, we also found out that an established model that describes the dynamics of the measurement of equilibrium processes needs to be corrected," adds Arne Ludwig.

The profile focus Materials Chain of the University Alliance Ruhr combines the know-how of more than 200 research groups in the Ruhr area; including internationally renowned experts and highly qualified young scientists. The Ruhr University Bochum, the Technical University Dortmund, the University of Duisburg-Essen and their partners offer a variety of research facilities - from characterization in atomic resolution to component production - that cover all areas of the materials chain.

The work was financially supported by the NRW Ministry for Innovation, Science and Research as part of the NRW returnee program, the Federal Ministry for Education and Research as part of the "Quantum communication program" (16KIS0109), the German Research Foundation as part of the special research area TRR 160, the Franco-German University (CDFA-05-06) and the Research School of the Ruhr University Bochum.

Sascha Valentin, Jonathan Schwinger, Pia Eickelmann, Patrick Labud, Andreas Wieck, Björn Sothmann, Arne Ludwig: Illumination-induced nonequilibrium charge states in self-assembled quantum dots, in: Physical Review B, 2018, DOI: 10.1103 / PhysRevB.97.045416

Dr. Arne Ludwig
Chair of Solid State Physics
Faculty of Physics and Astronomy
Ruhr-University Bochum
Tel .: 0234 32 25864
Email: [email protected]

Prof. Dr. Bjorn Sothmann
Theoretical physics
university Duisburg-Essen
Tel .: 0203 379 1578
Email: [email protected]

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