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discovered 02_2012

discovered 02.12 FOCUS WWW.Hzdr.DE What has Stephan Winnerl newly discovered about the strange life of electrons in graphene? Working with colleagues from the Helmholtz-Zentrum Dresden-Rossendorf and with scientists from Technische Universität Berlin, the High Field Magnet Lab in Grenoble, France, and the Georgia Institute of Technology, USA, he determined the lifetimes of electrons in graphene within low energy ranges. This scenario had never been researched before. For their experiments, the scientists shone infrared light from the free-electron laser at the HZDR onto their graphene samples. In the relatively long-wave range, they adjusted the laser energy very precisely to the energy bands in the graphene, very close to the Dirac point, i.e. the point where the apexes of the two inverted “ice cream cones” meet. They discovered that the relationship between the energy of the photons and the vibrations of the atomic lattice significantly influences the lifetime of the electrons: If the energy of the photons is higher than the energy of the lattice vibrations, then the electrons change their energy state faster and live less long. Conversely, the electrons remain longer at a given energy level if the laser energy is lower than that of the lattice vibrations. Model calculations at TU Berlin confirm the experimental data from Dresden and, accordingly, the international research team has contributed towards a better understanding of the electronic and optical properties of graphene. Yet there are still many open questions about the wonder material graphene to be researched, and the HZDR scientists are excited to be conducting more experiments together with the theoreticians from TU Berlin and scientists from the High Field Magnet Lab in Grenoble to observe the unique behavior of electrons in graphene under the influence of magnetic fields. Initial experiments there have revealed that it takes only a relatively low magnetic field to fundamentally modify the band structure of graphene. This modification prevents the electrons from moving freely and instead forces them into an orbit lying within the plane. In this state, they very much resemble their “comrades” held captive in quantum dots. Confined inside pyramids Unlike the especially fast and freely mobile electrons that graphene possesses in its normal state, electrons inside quantum dots live a very constrained life. In current experiments, the Dresden researchers took a closer look at such dots in indium arsenide and indium gallium arsenide by firing infrared light from HZDR’s free-electron laser at these tiny, pyramid-like dots. Each nano-pyramid contains only two or three electrons. These electrons are essentially imprisoned, in that their freedom of movement is considerably restricted. One could say that the electrons “feel” the confining walls of the pyramid. Translated into physics, that means the energy of the electrons no longer extends over the large areas of the bands in proportion to their kinetic energy. Instead, they reside at highly specific energy levels inside the pyramids, where the FREE-ELECTRON LASER: Inside the undulator (the structure in front), electrons from the ELBE accelerator are forced onto a wiggling motion – forcing them to emit light. Image credit: Sven Claus