Source: https://www.nature.com/articles/s41567-018-0282-x?error=cookies_not_supported&code=25af9687-744b-4798-b18a-24291eb4142f
Timestamp: 2019-04-24 22:10:26+00:00

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The optical properties of atomically thin transition metal dichalcogenide (TMDC) semiconductors are shaped by the emergence of correlated many-body complexes due to strong Coulomb interaction. Exceptional electron–hole exchange predestines TMDCs to be used to study fundamental and applied properties of Coulomb complexes such as valley depolarization of excitons and fine-structure splitting of trions. Biexcitons in these materials are less well understood and it has been established only recently that they are spectrally located between excitons and trions. Here we show that biexcitons in monolayer TMDCs exhibit a distinct and rich fine structure on the order of millielectronvolts due to electron–hole exchange. Ultrafast pump–probe experiments on monolayer WSe2 reveal decisive biexciton signatures and a fine structure in excellent agreement with a microscopic theory. We provide a pathway to understand the complex spectral structure of higher-order Coulomb complexes in TMDCs going beyond the usual classification scheme in terms of four-particle configurations.
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This work was supported by Deutsche Forschungsgemeinschaft (DFG) within CRC 1558 and RTG 2247. A.W.A. acknowledges funding from DFG via grant no. AC290-2/1. The spectroscopic experiments were jointly supported by NSF DMR1306878 (A. Singh) and the NSF MRSEC program DMR-1720595 (K.T.). X.L. gratefully acknowledges support from the Welch foundation (F-1662) and the Alexander von Humboldt Foundation, which facilitated the collaboration with TU-Berlin. A. Steinhoff and M.F. would like to acknowledge P. Gartner for fruitful discussions. We thank G. Schönhoff, M. Rösner and T. Wehling for providing material-realistic band structures and bare as well as screened Coulomb matrix elements.
A. Steinhoff and M.F. performed analytical and numerical calculations of the biexciton spectra. A. Singh, K.T. and M.K. designed and performed the experiments. K.T. exfoliated the sample. N.O., A. Singh, A.W.A. and S.H. analysed the data. N.O., A. Steinhoff, M.F. and A. Singh prepared the manuscript. U.W., F.J. and X.L. initiated and coordinated the project. All authors contributed to the discussion and the writing of the manuscript.

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