Negative electronic compressibility and tunable spin splitting in WSe2

• Published in: Nature nanotechnology Vol. 10; no. 12; pp. 1043 - 1047
• Main Authors: Riley, J. M., Meevasana, W., Bawden, L., Asakawa, M., Takayama, T., Eknapakul, T., Kim, T. K., Hoesch, M., Mo, S.-K., Takagi, H., Sasagawa, T., Bahramy, M. S., King, P. D. C.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01-12-2015; Nature Publishing Group
• Subjects: 639/301/119/1000/1018; 639/766/119/544; 639/766/119/995; 639/925/357/1018; Chemical potential; CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; letter; MATERIALS SCIENCE; Nanotechnology; Nanotechnology and Microengineering
• ISSN: 1748-3387; 1748-3395
• DOI: 10.1038/nnano.2015.217

Angle-resolved photoemission measurements of electron-doped layers of tungsten diselenide reveal signatures of negative electronic compressibility that survive to much higher carrier densities than in conventional 2D electron gases.
Tunable bandgaps
1
, extraordinarily large exciton-binding energies
2
,
3
, strong light–matter coupling
4
and a locking of the electron spin with layer and valley pseudospins
5
,
6
,
7
,
8
have established transition-metal dichalcogenides (TMDs) as a unique class of two-dimensional (2D) semiconductors with wide-ranging practical applications
9
,
10
. Using angle-resolved photoemission (ARPES), we show here that doping electrons at the surface of the prototypical strong spin–orbit TMD WSe
2
, 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 effect
11
,
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.