Dynamical screening in monolayer transition-metal dichalcogenides and its manifestations in the exciton spectrum

Abstract

Monolayer transition-metal dichalcogenides (ML-TMDs) offer exciting opportunities to test the manifestations of many-body interactions through changes in the charge density. The two-dimensional character and reduced screening in ML-TMDs lead to the formation of neutral and charged excitons with binding energies orders of magnitude larger than those in conventional bulk semiconductors. Tuning the charge density by a gate voltage leads to profound changes in the optical spectra of excitons in ML-TMDs. On the one hand, the increased screening at large charge densities should result in a blueshift of the exciton spectral lines due to reduction in the binding energy. On the other hand, exchange and correlation effects that shrink the band-gap energy at elevated charge densities (band-gap renormalization) should result in a redshift of the exciton spectral lines. While these competing effects can be captured through various approximations that model long-wavelength charge excitations in the Bethe-Salpeter Equation, we show that a novel coupling between excitons and shortwave charge excitations is essential to resolve several experimental puzzles.Unlike ubiquitous and well-studied plasmons, driven by collective oscillations of the background charge density in the long-wavelength limit, we discuss the emergence of shortwave plasmons that originate from the short-range Coulomb interaction through which electrons transition between the K and −K valleys. The shortwave plasmons have a finite energy-gap because of the removal of spin-degeneracy in both the valence-arXiv:1801.06217v2 [cond-mat.mtrl-sci] 2. Properties of monolayer transition-metal dichalcogenides ML-TMDs are direct band-gap semiconductors with the conduction band and valence band edges at the K and K = −K points [19]. These time-reversed points define two separate low-energy pockets/valleys in the Brillouin zone. Due to the lack of space inversion symmetry and strong spin-orbit coupling arising from the d orbitals of the transition-metal atoms, the spin degeneracy in these valleys is lifted [24], as shown in Figs. 1(a,c). Whereas the spin splitting ∆ c in the conduction band is typically at most a few 10s of meV, the spin splitting in the valence band can be several 100s meV [24,54,55,56,57]. There are two important aspects to this band structure: First, timereversal symmetry results in a coupling between the spin and valley degrees of freedom, such that the spin ordering in opposite valleys is reversed as illustrated in Figs. 1(a,c). Second, the spin ordering of the conduction band in Mo-based compounds is opposite to the one in W-based compounds, as can be seen by comparing Figs. 1(a) and (c) [24,57,58]. This difference will be shown below to have profound consequences on their optical properties. Due to the large spin-splitting energy of the valence band, one can distinguish between two different series of neutral excitons in absorption experiments: One series involving the top valleys in the valence band, usually denoted as A excitons, and one involving it...

Strain-tunable orbital, spin-orbit, and optical properties of monolayer transition-metal dichalcogenides

“…(1) and fitted parameters given in Table II, we compute the excitonic spectra of the strained monolayer TMDCs for different bright excitonic states (the s-like excitons) that can be directly probed in experiments. We use the effective Bethe-Salpeter equation (BSE) [63,[90][91][92][93][94] with the electron-hole interaction mediated by the Rytova-Keldysh potential [95][96][97][98]. The screening lengths of the TMDCs are taken from the study of Berkelbach et al [98].…”

Magnetic field mixing and splitting of bright and dark excitons in monolayer MoSe 2

“…We note that in addition to the contribution from spin-orbit coupling, the value of ∆ CB is affected from band-gap renormalization in electron-doped samples due to many-body exchange interactions. 24,25 The experimental results and the analysis we have provided in this paper deal with intrinsic (un-doped) monolayers.…”

Band nesting and exciton spectrum in monolayer MoS2

“…There is currently great interest in van der Waals materials, including semiconductors, topological insulators, semimetals, superconductors and ferromagnets [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] . The weak bonding of atomic layers in bulk materials allows to peel off single layers from the bulk and reassemble them into new combinations not found in nature 3 .…”