What is an exciton for laypeople



02.11.2016 10:47

On the trail of quasiparticles

University communication University communication
Swiss Federal Institute of Technology Zurich (ETH Zurich)

Electrons in solids can combine to form so-called quasiparticles, which create new phenomena. ETH physicists have now investigated previously unidentified quasiparticles in a new class of solids that consist of only one atomic layer. With their results, the researchers correct a previously prevalent misinterpretation.

In order to understand and predict weather phenomena, it makes little sense to look at the behavior of individual water droplets or air molecules. Instead, meteorologists (and laypeople too) speak of clouds, winds and precipitation - objects that result from the complex interplay of small particles. Physicists who deal with optical properties or the conductivity of solids do it in a very similar way. Here, too, the tiniest particles - electrons and atoms - are responsible for the most varied of phenomena, but an informative picture only emerges when many of them are grouped together to form “quasiparticles”.

Finding out exactly which quasiparticles are formed in a material and how they influence each other is no easy task and resembles an enormous puzzle, the pieces of which are gradually put together through lengthy research. Ataç Imamoğlu and his colleagues at the Institute for Quantum Electronics at ETH Zurich have now succeeded in finding a new piece of the puzzle in a combined experimental and theoretical study that also moves a previously incorrectly placed piece into the right place.

Excitons and polarons

In solids, for example, quasiparticles are formed when a light particle is absorbed. The kinetic energy of electrons that cavort in a solid body can only assume values ​​that are in clearly defined areas known as bands. A light particle can now transport an electron from a lower to a higher energy band, leaving a “hole” in the lower band.

The excited electron and the resulting hole attract each other through the electrostatic Coulomb force, and if this attraction is strong enough, the electron-hole pair can be viewed as a quasiparticle - an «exciton» is born. If, on the other hand, two electrons and a hole bind to each other, a trion is formed. But if there are excitons and a large number of free electrons in the material at the same time, a new quasiparticle called the Fermi polaron is needed to describe its qualitatively new - or "emergent" - properties.

Quasiparticles in semiconductors

Imamoğlu and his colleagues now wanted to study the properties of quasiparticles that occur in a certain type of semiconductor in which electrons can only move in two dimensions. To do this, they took a single layer of molybdenum diselenide, only a thousandth of a micrometer thick, which was embedded between two boron nitride disks. They added a layer of graphene to this in order to apply an electrical voltage, with the help of which the density of electrons in the material could be controlled. Finally, the arrangement was packed between two micro-mirrors, which together form an optical resonator.

With this complex experimental set-up, the Zurich physicists were able to study in detail how strongly the material absorbs light under different conditions. In doing so, they found that optical excitation forms Fermi polarons in the semiconductor structure and not, as previously assumed, excitons or trions. "The data available at the time have always been misinterpreted by research - including my own -", Imamoglu admits. "With our new experiment we have now adjusted the previously valid picture."

Team effort with visiting researcher

“The whole thing was a team effort, in which Harvard Professor Eugene Demler played a key role. He worked with us for several months as an ITS Fellow, ”says Meinrad Sidler, doctoral student in Imamoglu's group. The Institute for Theoretical Studies (ITS) at ETH has been committed to promoting interdisciplinary research at the interface between mathematics, theoretical physics and computer science since 2013. The aim is to facilitate research based primarily on pure scientific curiosity, with the aim of finding the best ideas in unexpected places.

In the study by Imamoğlu and colleagues, which has now been published in the journal Nature Physics, this principle has already borne fruit. In his research, Eugene Demler actually deals with ultra-cold atoms and examines how mixtures of bosonic and fermionic atoms behave. "Through his understanding of polarons in atomic gases and solids, Demler gave our research important and interesting impulses that we probably would not have come up with on our own," says Imamoğlu.

Light-induced superconductivity

The knowledge gained now will keep Imamoğlu and his colleagues busy for a while, because the interaction of bosonic (e.g. excitons) and fermionic particles (electrons) is the focus of a large research project for which Imamoğlu received an Advanced Grant from European Research last year Council (ERC) and that also from the National Research Center for Quantum Science and Technology (NCCR QSIT)
is promoted. A better understanding of such mixtures of quasiparticles would have important effects on basic research on the one hand, but also exciting applications on the other. For example, a central goal of the ERC project is to show how superconductivity can be controlled with the help of laser light.

Bibliography

Sidler M, Back P, Cotlet O, Srivastava A, Fink T, Kroner M, Demler E, Imamoglu A: Fermi polaron-polaritons in charge-tunable atomically thin semiconductors. Nature Physics, October 31, 2016, doi: 10.1038 / nphys3949 [http://dx.doi.org/10.1038/nphys3949]


Additional Information:

https: //www.ethz.ch/de/news-und-veranstaltungen/eth-news/news/2016/10/den-quasit ...


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