Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Hot on the heels of quasiparticles

02.11.2016

If one tries to understand weather phenomena, it's not much use looking at the behaviour of single water droplets or air molecules. Instead, meteorologists (and also laymen) speak of clouds, winds and precipitation - objects that result from the complex interplay between small particles. Physicists dealing with the optical properties or the conductivity of solids use much the same approach. Again, tiny particles - electrons and atoms - are responsible for a multitude of phenomena, but an illuminating picture only emerges when many of them are grouped into "quasiparticles".

However, finding out precisely what quasiparticles arise inside a material and how they influence one another is not a simple task, but more akin to a large puzzle whose pieces fit together, little by little, through arduous research. In a combination of experimental and theoretical studies, Ataç Imamoglu and his collaborators at the Institute for Quantum Electronics at the ETH in Zurich have now succeeded in finding a new piece of the puzzle, which also helps to put a previously misplaced piece in its correct position.


A polaron (orange) is formed amidst the electrons (violet) inside a solid.

Credit: ETH Zurich / Meinrad Sidler

Excitons and polarons

In solids quasiparticles can be created, for instance, when a photon is absorbed. The motional energy of electrons teeming about in a solid can only take values within well-defined ranges known as bands. A photon can promote an electron from a low-lying to a high-lying energy band, thus leaving behind a "hole" in the lower band.

The excited electron and the resulting hole attract each other through the electrostatic Coulomb force, and if that attraction is strong enough, the electron-hole pair can be viewed as a quasiparticle - an "exciton" is born. Two electrons and a hole can bind together to form a trion. When excitons and a large number of free electrons are simultaneously present however, the description of the qualitatively new - or "emergent" - properties of the material requires the introduction of new type of quasiparticles called Fermi polarons.

Quasiparticles in a semiconductor

Imamoglu and his colleagues wanted to find out the nature of quasiparticles that appear in a certain type of semiconductors in which electrons can only move in two dimensions. To do so, they took a single layer of molybdenum diselenide that is thousand times thinner than a micrometer and sandwiched it between two disks of boron nitride. They then added a layer of graphene in order to apply an electric voltage with which the density of electrons in the material could be controlled. Finally, everything was placed between two mirrors that formed an optical cavity.

With this complex experimental setup the physicists in Zurich could now study in detail how strongly the material absorbs light under different conditions. They found that when the semiconductor structure is optically excited, Fermi-polarons are formed - and not, as previously thought, excitons or trions. "So far, researchers - myself included - have misinterpreted the data available at the time in that respect", admits Imamoglu. "With our new experiments we are now able to rectify that picture."

Team effort with a guest scientist

"This was a team effort with essential contributions by Harvard professor Eugene Demler, who collaborated with us over several months when he was an ITS fellow", says Meinrad Sidler who is a doctoral student in Imamoglus group. Since 2013 the Institute for Theoretical Studies (ITS) of the ETH has endeavoured to foster interdisciplinary research at the intersection between mathematics, theoretical physics and computer science. In particular, it wants to facilitate curiosity-driven research with the aim of finding the best ideas in unexpected places.

The study by Imamoglu and his colleagues, now published in "Nature Physics", is a good example for how this principle can be successful. In his own research, Eugene Demler deals with ultracold atoms, studying how mixtures of bosonic and fermionic atoms behave. "His insight into polarons in atomic gases and solids have given our research important and interesting impulses, which we may not have come up with on our own", says Imamoglu.

Light induced superconductivity

The insights they have gathered will most likely keep Imamoglu and his collaborators busy for some time to come, as the interactions between bosonic (such as excitons) and fermionic (electrons) particles are the topic of a large research project for which Imamoglu won an Advanced Grant of the European Research Council (ERC) last year, and is also supported by the National Centre of Competence in Research Quantum Science and Technology (NCCR QSIT). A better understanding of such mixtures would have important implications for basic research, but exciting applications also beckon. For instance, a key goal of the ERC project is the demonstration of control of superconductivity using lasers.

###

Reference

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, 31 October 2016, doi: 10.1038/nphys3949 [http://dx.doi.org/10.1038/nphys3949]

Media Contact

Atac Imamoglu
imamoglu@phys.ethz.ch
41-446-334-570

 @ETH_en

http://www.ethz.ch/index_EN 

Atac Imamoglu | EurekAlert!

Further reports about: ERC ETH ETH Zurich Nature Physics polarons semiconductor superconductivity

More articles from Physics and Astronomy:

nachricht What happens when we heat the atomic lattice of a magnet all of a sudden?
18.07.2018 | Forschungsverbund Berlin

nachricht Subaru Telescope helps pinpoint origin of ultra-high energy neutrino
16.07.2018 | National Institutes of Natural Sciences

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: First evidence on the source of extragalactic particles

For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.

To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...

Im Focus: Magnetic vortices: Two independent magnetic skyrmion phases discovered in a single material

For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.

Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...

Im Focus: Breaking the bond: To take part or not?

Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.

A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...

Im Focus: New 2D Spectroscopy Methods

Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.

"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....

Im Focus: Chemical reactions in the light of ultrashort X-ray pulses from free-electron lasers

Ultra-short, high-intensity X-ray flashes open the door to the foundations of chemical reactions. Free-electron lasers generate these kinds of pulses, but there is a catch: the pulses vary in duration and energy. An international research team has now presented a solution: Using a ring of 16 detectors and a circularly polarized laser beam, they can determine both factors with attosecond accuracy.

Free-electron lasers (FELs) generate extremely short and intense X-ray flashes. Researchers can use these flashes to resolve structures with diameters on the...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

Leading experts in Diabetes, Metabolism and Biomedical Engineering discuss Precision Medicine

13.07.2018 | Event News

Conference on Laser Polishing – LaP: Fine Tuning for Surfaces

12.07.2018 | Event News

11th European Wood-based Panel Symposium 2018: Meeting point for the wood-based materials industry

03.07.2018 | Event News

 
Latest News

Global study of world's beaches shows threat to protected areas

19.07.2018 | Earth Sciences

New creepy, crawly search and rescue robot developed at Ben-Gurion U

19.07.2018 | Power and Electrical Engineering

Metal too 'gummy' to cut? Draw on it with a Sharpie or glue stick, science says

19.07.2018 | Materials Sciences

VideoLinks
Science & Research
Overview of more VideoLinks >>>