In Order

Materials can assume strange states under the right conditions. For instance, conductors below a certain temperature suddenly turn into superconductors, allowing the flow of electric current without any resistance, and fluids take on properties of a state referred to by scientists as superfluidity . Its characteristics: The fluid no longer has any inner friction and flows smoothly even through the narrowest capillaries.

The physical effects behind these states fall into the realm of quantum physics: "Microscopic excitations play a decisive role in understanding the macroscopic physical properties of this form of matter, which is subject to the laws of quantum mechanics. They provide the basis for comprehending certain phenomena, such as superconductivity and superfluidity," explains Sven Höfling.

Sven Höfling is a research associate at the Department of Technical Physics of the University of Würzburg. "Working in close cooperation with their colleagues from Stanford (USA) and Tokyo (Japan), physicists of the University of Würzburg, namely Andreas Löffler, Sven Höfling and Alfred Forchel, have now succeeded in demonstrating the existence of such an elusive microscopic excitation in a two-dimensional condensate: a vortex-antivortex pair.

The experiment

The starting point for the observation was a "top quality sample without imperfections", as Höfling explains. This sample was produced in the Microstructure Laboratory of the University of Würzburg. For this purpose, Höfling and his colleagues applied two-dimensional semiconductor layers of the highest perfection to a carrier substrate by means of "vapor deposition".

In this sample, the scientists took a closer look at the so-called exciton-polaritons. What these are? "When an electron binds to an electron hole at a position where an electron usually exists, the resulting state is what physicists call an exciton," explains Höfling. If you add photons – that is to say light – a polariton can be generated from this. From the perspective of physicists, exciton-polaritons are "quasiparticles, arising from a strong coupling of cavity photons with excitons in a quantum well". These quasiparticles have a very small mass and interact with each other.

Vortices in two dimensions

Furthermore, they can also rotate – similarly to the swirling water in a draining bath tub. Vortices in two dimensions can have either of two rotational directions: clockwise or anticlockwise. Under normal circumstances, vortices and antivortices are scattered randomly (see left picture); this represents a state of great disorder. However, a highly ordered state is a necessary condition for a given substance to assume superfluid characteristics to allow the particles to flow without resistance.

So how can order be introduced into the system? "If the temperature falls below a certain critical value, these free vortices spontaneously form vortex-antivortex pairs, the total sense of rotation of which is canceled out by the opposite sense of rotation of the constituent vortices," explains Höfling (see right picture). This effect leads to a stabilization of the order in the system. To put it in simple terms: On the whole, there is order in the system despite the existence of some localized small-scale disorders.

The importance of the discovery

"Basic physical research": This is how Sven Höfling describes the research result, which has been reported in the scientific journal Nature Physics. The experiment establishes the importance of topological order, sheds light on certain phenomena in nature and it is of great significance for future studies of quantum phase transitions. At the moment, the research cannot be put into direct practical use, but the existence of a vortex-antivortex pair is an important piece of evidence for confirming the theory of the superfluid phase in two-dimensional systems. While the observed single pair in the experiment was imprinted on the condensate by means of a laser, the latest findings even point to the spontaneous formation of multiple vortex-antivortex pairs and the associated superfluid phase in the condensate.

"Single vortex–antivortex pair in an exciton-polariton condensate", Georgios Roumpos, Michael D. Fraser, Andreas Löffler, Sven Höfling, Alfred Forchel und Yoshihisa Yamamoto, Nature Physics, doi:10.1038/NPHYS1841

Contact person
Sven Höfling, T +49 (0)931 31-83613, sven.hoefling@physik.uni-wuerzburg.de