Physicists from Switzerland and Germany have unveiled fingerprints of the long-sought particle known as the Mahan exciton in the room temperature optical response of the popular methylammonium lead halide perovskites.

The optical properties of semiconductors are governed by the so-called "excitons," which are bound pairs of negative electrons and positive holes. Excitons are important because they transport energy (with no net charge) across materials and thus they play a crucial role in a number of optoelectronic devices. The ability to control the excitonic properties of semiconductors (by tuning parameters such as temperature, pressure, , electric and magnetic fields) is key to broadening the range and diversity of applications. In particular, when the of charge carriers (electrons and holes) increases, excitons tend to melt and a semiconductor eventually turns into a metal at the so-called Mott density.

However, back in 1967, Gerald Mahan predicted that a different type of exciton can still persist above the Mott density. Despite years of research, this so-called Mahan has not been observed, let alone under the normal operating conditions of devices.

This has now just been achieved by the group of Majed Chergui at EPFL, in collaboration with Alexander Steinhoff (University of Bremen), Ana Akrap (University of Fribourg), and the group of László Forró (EPFL). Publishing in Nature Communications, the teams uncovered signatures of Mahan excitons in the very popular lead-bromide organic-inorganic . The researchers mapped how the material's optical properties modify at increasing densities of charge carriers with a temporal resolution of tens of femtoseconds (one femtosecond is one millionth of a billionth of a second). Mahan excitons emerged in the optical properties with the distinctive features predicted by theory.

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