Fig. 1: Characterizing polaron states and their interactions in monolayer WS2.
a The band ordering in monolayer WS2 gives rise to four distinct attractive polaron states, which we label by the associated singlet and triplet configurations. An exciton consisting of an electron (solid circle) and hole (empty circle) can be dressed by a Fermi sea in the lower conduction band (depicted as flattened ovals). The red (blue) coloring indicates spin-up (-down) electrons. b Schematic depiction of attractive polarons and the interactions observed. In (i), two excitons (X) are dressed by the same Fermi sea of \(\uparrow\) electrons (solid red circles), which are displaced from their equilibrium positions (empty red circles). Between the excitons, an \(\uparrow\) electron is interacting with the left exciton, making it unavailable to interact with the exciton on the right (empty white dashed circle). This phase-space filling effect leads to an effective repulsive interaction between the two polarons dressed by the same Fermi sea. By contrast, in (ii) the polarons involve distinct Fermi seas, and thus there are no phase-space filling effects and the polarons do not interact via the medium. c The pulse ordering in a 1Q rephasing MDCS experiment, and time periods between them. The fast phase oscillations during time periods t1 and t3 arise from coherent superpositions between states separated by optical photon energies, while phase oscillations during t2 arise from coherences involving states that are much closer in energy. d Co-linearly polarized 1Q rephasing 2D spectrum at t2 = 0 is obtained by Fourier transforming the full complex signal measured as a function of t1 and ω3, and correlates the absorption energy (ℏω1) with the nonlinear signal emission energy (ℏω3). Color scaling represents the normalized absolute signal amplitude (Norm. Amp.). With co-linear polarization, all resonant states are excited and all permitted interactions are present. Several peaks can be identified and attributed to the attractive polaron (AP), repulsive polaron (RP), biexciton (B), bipolaron (C), and interactions between AP and RP (AP ~ RP and RP ~ AP). The inset shows a slice perpendicular to the diagonal (indicated by dashed line), clearly showing the splitting of the attractive polaron peaks.
