Mahan excitons in room-temperature methylammonium lead bromide perovskites

Abstract

In a seminal paper, Mahan predicted that excitonic bound states can still exist in a semiconductor at electron-hole densities above the insulator-to-metal Mott transition. However, no clear evidence for this exotic quasiparticle, dubbed Mahan exciton, exists to date at room temperature. In this work, we combine ultrafast broadband optical spectroscopy and advanced many-body calculations to reveal that organic-inorganic lead-bromide perovskites host Mahan excitons at room temperature. Persistence of the Wannier exciton peak and the enhancement of the above-bandgap absorption are observed at all achievable photoexcitation densities, well above the Mott density. This is supported by the solution of the semiconductor Bloch equations, which confirms that no sharp transition between the insulating and conductive phase occurs. Our results demonstrate the robustness of the bound states in a regime where exciton dissociation is otherwise expected, and offer promising perspectives in fundamental physics and in room-temperature applications involving high densities of charge carriers.

Giant Exciton Mott Density in Anatase TiO2

“…We believe that the photoexcited carrier density in our experiment lies below the actual value of n M . Persistence of excitonic correlations above n M have been recently demonstrated in several semiconductors [7,8,10] and the emergence of Mahan excitons has been invoked [10]. However, in such a scenario, the Wannier exciton feature would be accompanied by the enhancement of the continuum absorption, which is instead absent in our data.…”

“…More refined theoretical analysis [4,6] and extensive experimental work [7][8][9][10][11][12][13] have instead revealed a rich phase diagram of exotic states persisting above n M . Notable examples include robust excitonic and biexcitonic correlations [7][8][9], emergent Mahan excitons [10], anomalous metallic states [11], e-h droplets [12,13], and possible Bose condensates of photoexcited e-h Cooper pairs [4]. Therefore, the identification of n M in semiconductors acquires a crucial importance for discovering hitherto unobserved phenonena and clarifying how excitons react to the large carrier densities present in many optoelectronic devices.…”

“…We obtain n ∼ 5 × 10 19 cm −3 , i.e. below the theoretically predicted n M but sufficiently high compared to the density at which excitons dissociate in most solids [10,[62][63][64][65]. Subsequently, we monitor the relative changes in the material reflectivity (ΔR=R) over a broad spectral range covering the bound exciton feature [grey shaded area in Fig.…”

Exciton and Carrier Dynamics in Two-Dimensional Perovskites

“…29 Recently, it was found that excitons are surprisingly persistent in bulk CH3NH3PbBr3 perovskite above the Mott density, which is attributed to Mahan excitons. 30 It is therefore unclear whether an electron-hole plasma is stabilized at densities in the range of 10 18 -10 19 cm -3 for 2D perovskites. This motivates an analysis of the THz absorption spectra at different time delays to discern the evolution of free carriers into excitons.…”

“…If we were in a regime where the Mott transition was starting to play a role, then Eb should decrease with fluence as the screening becomes stronger. 30 This would translate into a blueshift of the emission peak since the excitons are less stabilized. However, we do not observe any blueshift of the peak with fluence.…”