Holstein polaron in a valley-degenerate two-dimensional semiconductor


Two-dimensional (2D) crystals have emerged as a class of materials with tunable carrier density1. Carrier doping to 2D semiconductors can be used to modulate many-body interactions2 and to explore novel composite particles. The Holstein polaron is a small composite particle of an electron that carries a cloud of self-induced lattice deformation (or phonons)3,4,5, which has been proposed to play a key role in high-temperature superconductivity6 and carrier mobility in devices7. Here we report the discovery of Holstein polarons in a surface-doped layered semiconductor, MoS2, in which a puzzling 2D superconducting dome with the critical temperature of 12 K was found recently8,9,10,11. Using a high-resolution band mapping of charge carriers, we found strong band renormalizations collectively identified as a hitherto unobserved spectral function of Holstein polarons12,13,14,15,16,17,18. The short-range nature of electron–phonon (e–ph) coupling in MoS2 can be explained by its valley degeneracy, which enables strong intervalley coupling mediated by acoustic phonons. The coupling strength is found to increase gradually along the superconducting dome up to the intermediate regime, which suggests a bipolaronic pairing in the 2D superconductivity.

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Fig. 1: Holstein polaron of MoS2.
Fig. 2: Spectral function of Holstein polarons.
Fig. 3: Doping dependence of polarons.
Fig. 4: Strength of e–ph coupling and superconductivity.


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This work was supported by the National Research Foundation of Korea (grants no. 2017R1A5A1014862 and no. 2017R1A2B3011368), Future-leading Research Initiative 2017-22-0059 of Yonsei University, and the POSCO Science Fellowship of POSCO TJ Park Foundation. This work was carried out with the support of the Diamond Light Source (beamline I05). The work at the Advanced Light Source was supported by the US Department of Energy, Office of Sciences under contract no. DE-AC02-05CH11231. M.K. acknowledges the Samsung Scholarship from Samsung Foundation of Culture. We thank A. Bostwick, C. Jozwiak and E. Rotenberg for help in the ARPES experiments, and R. Comin for helpful discussions.

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M.K. conducted experiments and analysed data with help from W.J.S., Y.S., S.H.R., T.K.K. and M.H. S.W.J. performed spectral-function simulations. K.S.K. directed the project. M.K. and K.S.K wrote the manuscript with contributions from all the other co-authors.

Correspondence to Keun Su Kim.

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Kang, M., Jung, S.W., Shin, W.J. et al. Holstein polaron in a valley-degenerate two-dimensional semiconductor. Nature Mater 17, 676–680 (2018). https://doi.org/10.1038/s41563-018-0092-7

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