Understanding the dynamics of electrons in graphene under strong magnetic fields could lead to development of new types of broadband lasers. An international team led by Helmholtz-Zentrum Dresden-Rossendorf (HZDR) exposed graphene to a 4-Tesla magnetic field, which forced electrons in the material into certain energy states called Landau levels. These energy levels were then examined with a free-electron laser. Electron redistribution through Auger scattering has been discovered in graphene. Courtesy of Voigt/HZDR. “The laser pulse excites the electrons into a certain Landau level. A temporally delayed pulse then probes how the system evolves,” said doctoral candidate Martin Mittendorff. The researchers found the Landau level into which new electrons were pumped by the laser gradually emptied in an unexpected way, and that collisions between electrons were the cause. “Imagine a librarian sorting books on a bookshelf with three shelves,” Dr. Stephan Winnerl said by way of analogy. “She places one book at a time from the lower shelf onto the middle shelf. Her son is simultaneously ‘helping’ by taking two books from the middle shelf, placing one of them on the top shelf, the other on the bottom. The son is very eager and now the number of books on the middle shelf decreases even though this is precisely the shelf his mother wishes to fill.” Winnerl said the researchers did not expect the effect, known as Auger scattering, to be so strong or to deplete an energy level. With intense light from the HZDR’s free-electron lasers, materials can be examined on the atomic level. Courtesy of Frank Bierstedt/HZDR. While a number of effects have been discovered with graphene in magnetic fields, the dynamics of electrons in such a system have not been studied before. The researchers said the phenomenon they discovered could enable lasers that can produce light with arbitrarily adjustable wavelengths in the IR and terahertz ranges. “Such a Landau-level laser was long considered impossible, but now with graphene, this semiconductor physicists’ dream could become a reality,” Winnerl said. The work was funded by the DFG German Research Foundation. The research was published in Nature Physics (doi: 10.1038/nphys3164). For more information, visit www.hzdr.de.