Tunable bandgaps1, extraordinarily large exciton-binding energies2,3, strong light–matter coupling4 and a locking of the electron spin with layer and valley pseudospins5,6,7,8 have established transition-metal dichalcogenides (TMDs) as a unique class of two-dimensional (2D) semiconductors with wide-ranging practical applications9,10. Using angle-resolved photoemission (ARPES), we show here that doping electrons at the surface of the prototypical strong spin–orbit TMD WSe2, akin to applying a gate voltage in a transistor-type device, induces a counterintuitive lowering of the surface chemical potential concomitant with the formation of a multivalley 2D electron gas (2DEG). These measurements provide a direct spectroscopic signature of negative electronic compressibility (NEC), a result of electron–electron interactions, which we find persists to carrier densities approximately three orders of magnitude higher than in typical semiconductor 2DEGs that exhibit this effect11,12. An accompanying tunable spin splitting of the valence bands further reveals a complex interplay between single-particle band-structure evolution and many-body interactions in electrostatically doped TMDs. Understanding and exploiting this will open up new opportunities for advanced electronic and quantum-logic devices.
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This work was supported by the Engineering and Physical Sciences Research Council, UK (Grant Nos EP/I031014/1, EP/M023427/1, EP/L505079/1 and EP/G03673X/1), TRF-SUT Grant RSA5680052 and NANOTEC, Thailand, through the Centres of Excellence Network. P.D.C.K. acknowledges support from the Royal Society through a University Research Fellowship. M.S.B. was supported by a Grant-in-Aid for Scientific Research (S) (No. 24224009) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, US Department of Energy, under Contract No. DE-AC02-05CH11231. We thank the Diamond Light Source for access to beamline I05 (proposal numbers SI9500 and SI11383) that contributed to the results presented here.
The authors declare no competing financial interests.
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Riley, J., Meevasana, W., Bawden, L. et al. Negative electronic compressibility and tunable spin splitting in WSe2. Nature Nanotech 10, 1043–1047 (2015). https://doi.org/10.1038/nnano.2015.217
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