The absorption of light by semiconductors is characterised by the so-called excitons - bound electron-hole pairs - which play a crucial role in several applications by transporting electrical charge in a neutral way. Semiconductors are basically insulating but can conduct electricity in a non-negligible way, when the material is doped by atomic impurities that give off their electrons. Alternatively, one can irradiate the material with light, a process called photodoping. When the concentration of injected electrons passes a certain threshold, known as the Mott density, the semiconductor can turn metallic, i.e. it becomes a conductor. Usually in this regime, excitons vanish, yielding a wider spectral region over which the material is transparent. The value of the Mott density varies greatly among semiconductors and it is also temperature-dependent.

The lowest energy exciton of the cheap and abundant titanium dioxide is already known for its robustness against temperature, defects and impurities. In a new study published in Physical Review Letters, a team of scientists led by Majed Chergui at EPFL found that this exciton is in addition exceptionally stable against high electron concentrations at room temperature. This observation was possible using a unique instrument generating ultrashort deep-ultraviolet pulses., allowing to combine intense photodoping of the material with monitoring the response of excitons. The team found that titanium dioxide can withstand electron concentrations over an order of magnitude larger than the highest Mott densities ever reported (in the case of another metal oxide, ZnO). This remarkable behaviour and the coexistence of a bound exciton and metallicity open exciting perspectives for applications and fundamental science.

On the practical side, the robustness of the excitons with respect to high electron densities is crucial applications such as Transparent Conductive Oxides (TCOs), systems that are used in displays and lighting devices. Although transparency and electrical conductivity are incompatible, Titanium dioxide exhibits both simultaneously, offering a wide window of transparency. On the fundamental side, these results enrich our understanding of many-body correlations in a class of materials that was previously inaccessible.

“Exciton effects in metal oxides have remained elusive for a long time due to the lack of suitable tools covering the deep-ultraviolet range” says Edoardo Baldini, the leading author of the research, now Postdoctoral Fellow at the Massachusetts Institute of Technology. “Now we have a tool that can explore the behaviour of ultraviolet-absorbing excitons in a wide class of exciting materials, such as antiferromagnetic or ferroelectric insulators.” The experimental results are fully backed by advanced calculations performed by Adriel Dominguez (Computational Science and Applied Research Institute, Shenzhen, China) and Angel Rubio (Max Planck Institute for the Structure and Dynamics of Matter, Hamburg, Germany).

Reference: Baldini, E., Palmieri, T., Dominguez, A., Rubio, A., and Chergui, M. (2020). Giant Exciton Mott Density in Anatase TiO2. PhysRev Lett 125, 116403 (10.1103/PhysRevLett.125.116403)

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