Quasi-steady electron-excitonic complexes coupling in a two-dimensional semiconductor

Abstract

Excitons and their complexes govern optical-related behaviors in semiconductors. Here, using angle-resolved photoemission spectroscopy (ARPES), we have elucidated the light-matter interaction mediated by quasi-steady excitonic complexes within a monolayer of the prototypical two-dimensional (2D) semiconductor WSe2. Under continuous incident light, we have observed the generation of quasi-steady excitons and their complexes, encompassing ground and excited state excitons, trions, as well as their intricate interplay. We further show spectral evidence of electronic excitation states within the background of quasi-steady excitonic complexes, characterized by valence band (VB) effective mass renormalization, the enhanced spin-orbit coupling (SOC), the formation of an excitonic gap near the Fermi level (EF ) of the conduction band (CB), and intervalley excitonic band folding. Our findings not only unveil a quasi-steady excitonic complex background for the creation of diverse electronic excitations in 2D semiconductors but also offer new insights into the role of excitons in the charge density wave (CDW) formation mechanism and facilitate the advancement of correlated electronic state engineering based on the coupling between electrons and excitonic complexes in a quasi-equilibrium state.

Figures (4)

• Figure 1: (a-b) Photo-generated holes in ARPES measurements on pristine WSe$_2$ ML in real and momentum space, respectively. (c) Excitons are formed by photo-generated holes and doped electrons. (d) The orange lines represent the ARPES signatures of bright and dark excitons, respectively. (e) Trions are formed by photo-generated holes and electron-hole excitations near $E_F$. (f) The SVB$_1$ represents the ARPES signature of trions, and the VB edge undergoes renormalization. (g) Excited 2s-states of exciton emerges, accompanied by exciton-trion interactions. (h) The red lines and SVB$_2$ represent the ARPES signatures of exciton 2s-states and exciton-trion interactions, respectively. Folded SVBs emerge at the Q-valley.
• Figure 2: (a) Doping-dependent ARPES intensity plots along the red line direction, with the bare CB appended. The CB, 1s-state and 2s-state of dark excitons are denoted as $\alpha$, $\alpha_1$ and $\alpha_2$, respectively. (b) Doping-dependent EDCs at the Q-point and fitting analysis. (c) Doping-dependent symmetrized EDCs at $k_F$ (blue dashed lines in (a)). The energy gap $\Delta$ is marked by the arrow. (d) ARPES intensity plot for $n = 9.6 \times 10^{13} \, \text{cm}^{-2}$ along the $\Gamma$-K direction, with MDC$\#1$ of the 1s-states of excitons and fitting analysis. (e) Plot similar to (d) but for $n = 1.4 \times 10^{14} \, \text{cm}^{-2}$, with MDC$\#2$ of the 2s-states of excitons. (f) Symmetrized constant energy maps. (g) Doping-dependent gap value $\Delta$ and the photoemission intensity of excitons, respectively.
• Figure 3: (a) Doping-dependent VBs around the K-valley along the $\Gamma$-K direction, with the bare VBs appended. The VBs $\beta$, $\gamma$, and SVBs $\beta_1$, $\beta_2$ are marked by arrows, respectively. (b) Corresponding second-derivatives of (a). (c) Doping-dependent EDCs at the K point, with the energy positions of VBs and SVBs marked, respectively. (d) Enlarged view of the box in (c). The energy interval $\Delta E$ between SVBs $\beta_1$ and $\beta_2$ is marked by the arrow. (e) $E_{b,\mathrm{T}}$ and 2$\Delta$ (extracted from Fig. 2(g)), are plotted as a function of the electron carrier density, respectively. All ARPES plots are background-subtracted for clarity, with raw data available in SM SI Sec. S10.
• Figure 4: (a) ARPES intensity plot along the $\Gamma-K$ direction for pristine sample, with the bare band $\beta$ appended. (b) Plot similar to (a), but for $n = 1.4 \times 10^{14} \, \text{cm}^{-2}$, with background subtracted (for details, see SM SI Sec. S11). (c) MDCs at the energies of SVBs $\beta_1$ and $\beta_2$, as indicated by the horizontal lines in (b). EDCs at the Q-valley and K-valley, respectively. Dashed lines mark the peak positions. (d) Illustration of intervalley electron-hole coupling with total momentum transfer $\vec{\mathrm{QK}}$, mediated by dressed photo-generated holes at the K-valley VB $\beta$, inducing the folding of SVBs $\beta_1$ and $\beta_2$ from the K- to the Q-valley.