Giant Exciton Mott Density in Anatase TiO2

Physical Review Letters 125, 116403 (2020)

Giant Exciton Mott Density in Anatase TiO2

Edoardo Baldini, Tania Palmieri, Adriel Dominguez, Angel Rubio,, Majed Chergui

Elucidating the carrier density at which strongly bound excitons dissociate into a plasma of uncorrelated electron-hole pairs is a central topic in the many-body physics of semiconductors. However, there is a lack of information on the high-density response of excitons absorbing in the near-to-mid ultraviolet, due to the absence of suitable experimental probes in this elusive spectral range. Here, we present a unique combination of many-body perturbation theory and state-of-the-art ultrafast broadband ultraviolet spectroscopy to unveil the interplay between the ultraviolet-absorbing two-dimensional excitons of anatase TiO2 and a sea of electron-hole pairs. We discover that the critical density for the exciton Mott transition in this material is the highest ever reported in semiconductors. These results deepen our knowledge of the exciton Mott transition and pave the route toward the investigation of the exciton phase diagram in a variety of wide-gap insulators.

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We thank Alexander Steinhoff and David Snoke for insighful discussions and comments, and Simon Moser and Marco Grioni for providing the sample used for this study. We acknowledge support by the Swiss National Science Foundation via the NCCR:MUST, Contract No. 154056 (PNR 70, “Energy turnaround”) and the R’EQUIP Contract No. 206021-157773. We acknowledge financial support from the European Research Council (Grant No. ERC-2015-AdG-694097), the Deutsche Forschungsgemeinschaft (DFG) through RTG2247, and the Cluster of Excellence Advanced Imaging of Matter (AIM) Grant No. EXC 336 2056-390715994. The Flatiron Institute is a division of the Simons Foundation. Support by the Max Planck Institute—New York City Center for Non-Equilibrium Quantum Phenomena is acknowledged

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