title: Negative electronic compressibility and tunable spin splitting in WSe2.
abstract: Tunable bandgaps, extraordinarily large exciton-binding energies, strong light-matter coupling and a locking of the electron spin with layer and valley pseudospins have established transition-metal dichalcogenides (TMDs) as a unique class of two-dimensional (2D) semiconductors with wide-ranging practical applications. 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 effect. 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.
authors.affiliation: SUPA, School of Physics and Astronomy, University of St Andrews, St Andrews, Fife KY16 9SS, UK.|Diamond Light Source, Harwell Campus, Didcot OX11 0DE, UK.
authors.forename: J M
authors.initials: JM
authors.lastname: Riley
authors.affiliation: School of Physics, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand.|NANOTEC-SUT Center of Excellence on Advanced Functional Nanomaterials, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand.
authors.forename: W
authors.initials: W
authors.lastname: Meevasana
authors.affiliation: SUPA, School of Physics and Astronomy, University of St Andrews, St Andrews, Fife KY16 9SS, UK.
authors.forename: L
authors.initials: L
authors.lastname: Bawden
authors.affiliation: Materials and Structures Laboratory, Tokyo Institute of Technology, Kanagawa 226-8503, Japan.
authors.forename: M
authors.initials: M
authors.lastname: Asakawa
authors.affiliation: Department of Physics, University of Tokyo, Hongo, Tokyo 113-0033, Japan.|Max Planck Institute for Solid State Research, Stuttgart 70569, Germany.
authors.forename: T
authors.initials: T
authors.lastname: Takayama
authors.affiliation: School of Physics, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand.
authors.forename: T
authors.initials: T
authors.lastname: Eknapakul
authors.affiliation: Diamond Light Source, Harwell Campus, Didcot OX11 0DE, UK.
authors.forename: T K
authors.initials: TK
authors.lastname: Kim
authors.affiliation: Diamond Light Source, Harwell Campus, Didcot OX11 0DE, UK.
authors.forename: M
authors.initials: M
authors.lastname: Hoesch
authors.affiliation: Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley California 94720, USA.
authors.forename: S-K
authors.identifier: http://orcid.org/0000-0003-0711-8514
authors.initials: SK
authors.lastname: Mo
authors.affiliation: Department of Physics, University of Tokyo, Hongo, Tokyo 113-0033, Japan.|Max Planck Institute for Solid State Research, Stuttgart 70569, Germany.
authors.forename: H
authors.initials: H
authors.lastname: Takagi
authors.affiliation: Materials and Structures Laboratory, Tokyo Institute of Technology, Kanagawa 226-8503, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Sasagawa
authors.affiliation: Quantum-Phase Electronics Center and Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan.|RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan.
authors.forename: M S
authors.initials: MS
authors.lastname: Bahramy
authors.affiliation: SUPA, School of Physics and Astronomy, University of St Andrews, St Andrews, Fife KY16 9SS, UK.
authors.forename: P D C
authors.identifier: http://orcid.org/0000-0002-6523-9034
authors.initials: PD
authors.lastname: King