Negative electronic compressibility and tunable spin splitting in WSe2

Abstract

Semiconductors are typically considered weakly interacting systems, well described by conventional band theory. The exchange and correlation energies arising from electronelectron interactions can, however, dominate the kinetic energy in the dilute doping limit.This stabilises a small regime of negative electronic compressibility (NEC), κ = weak anti-localisation 17 and a density-tuned dome of superconductivity. 18 A detailed understanding of the underlying gate-induced electronic structure evolution driving such emergent properties has, however, remained elusive.Here, we mimic the effects of field-effect doping in the TMD WSe 2 by the sub-monolayer 2 deposition of alkali metals at the vacuum-cleaved surface. Such "chemical gating" leaves the surface accessible for detailed spectroscopic measurements. From angle-resolved photoemission (ARPES), we uncover how the resulting charge accumulation drives a pronounced reconstruction of the bulk electronic structure, not only mediating the formation of a multivalley 2DEG and a giant tuneable valence band spin splitting, but also inducing a pronounced decrease of the surface chemical potential with increasing electron doping. This direct spectroscopic observation of NEC, which we find persists to remarkably high electron densities, reveals a dominant role of many-body interactions shaping the underlying electronic landscape of electrostatically-tuned TMDs.In Figure 1, we show the occupied electronic structure of bulk and chemically-gated WSe 2 as measured by ARPES. No electronic states cross the Fermi level for the pristine cleaved material ( Fig. 1(b)), consistent with its semiconducting bulk. While the uppermost valence bands near the zone centre are strongly three-dimensional, those at the zone-corner K point have negligible dispersion along k z , with electronic wavefunctions localised to single Se-W-Se monlayers (half of the unit cell). 8,19 These two-dimensional states, which form the lowest energy band extrema in monolayer TMDs, are strongly spin-polarised even in the bulk. 6,8The spin is coupled to the valley degree of freedom, alternating sign at neighbouring corners of the Brillouin zone just as for monolayer MoS 2 and WSe 2 . 5,7 For the 2H structure, spin also becomes locked to the layer pseudospin, reversing sign for neighbouring Se-W-Se layers. 6-8An energetic degeneracy of the states in neighbouring layers thus enforces the total electronic structure to be spin degenerate, as required by the structural inversion symmetry of bulk WSe 2 ( Fig. 1(a)).We show that breaking such inversion symmetry, achieved here by our surface doping approach, drives a number of striking changes of the electronic structure ( Fig. 1(c)). Deposition of minute quantities of alkali metals, electron doping the surface, causes the conduction band states to become populated at the K point (only weakly visible) and approximately mid-way along the Γ − K direction (denoted here as T ). The latter have the larger occupied bandwidth, maintaining an indirect band gap as for bulk WSe 2 . Unl...

Narrow-band anisotropic electronic structure of ReS2

“…This, however, would be expected to lead to an increased band gap via quantum confinement of the conduction-band states, and it is inconsistent with the three-dimensional dispersions of the conductionband states that we observe. The smaller band gap observed here may instead indicate an increased electronic screening due to the high near-surface electron density, creating a strong renormalization of the electronic band gap from its value in the undoped semiconductor, as has been observed in other TMDC compounds [47,48]. This possibility requires further exploration, both experimentally and theoretically, but it may point to the presence of rather strongly bound excitons even in bulk ReS 2 .…”

Renormalization of the quasiparticle band gap in doped two-dimensional materials from many-body calculations

“…Third, free carriers in low-dimensional systems form a low-energy acoustic plasmon which can dynamically couple with quasiparticles. These effects result in an enhanced many-body renormalization of quasiparticles energy, as shown from previous theoretical GW calculations in both semiconducting carbon nanotubes [21,22] and 2D transition metal dichalcogenides (TMDs) [23], and from experimental measurements [24][25][26][27]. More recently, beyond the nonlinear quasiparticle band gap renormalization of several hundred meV, the optical gap of monolayer TMDs was predicted to stay nearly constant due to a cancellation with the renormalization of exciton binding energy [28].…”

Universal Mechanism of Band-Gap Engineering in Transition-Metal Dichalcogenides

“…Finally, we note that the mechanism revealed here should be carefully taken into account in analysing negative electron compressibility in 2H-TMDs 24 and related van-der-Waals crystals.…”

“…24). This is a direct spectroscopic signature of lifting spin degeneracy by breaking interlayer symmetry in the unit bilayer of 2H-TMDs.…”