PCSI2024 Session PCSI-MoM2: 2D Materials and Graphene I

The interactions between charges and excitons involve complex many-body interactions at high densities. The exciton-polaron model has been adopted to understand the Fermi sea screening of charged excitons in monolayer transition metal dichalcogenides (TMD). The results provide good agreement with absorption measurements, which are dominated by dilute bright exciton responses. The Fermi-polaron model treats the quasiparticle responses of a single mobile impurity in a surrounding Fermi sea. In comparison, the exciton density in monolayer TMD can be tuned by laser fluence and be comparable to or exceed the charge density, where the analogy to a single mobile impurity no longer applies. The modification to Fermi sea screening at high exciton densities, however, is still not well understood. Apart from the bright excitons previously studied in reflection contrast measurements, different spin and momentum dark exciton species have been established, which are also expected to have many-body interactions with charges. The coupling between these different species of exciton-polarons has not yet been experimentally investigated.

Strain engineering is a powerful tool that strongly influences electronic structure and exciton states in 2D transition metal dichalcogenides (2D TMDs). Among 2D TMDs, monolayer WSe₂ has gained attention as a host for quantum emitters due to its lowest lying dark exciton state that hybridizes with mid-gap defect states under tensile strain, giving rise to bright single photon emitters. The creation of a hybridized state consisting of dark excitons and mid-gap defect states results in the radiative recombination of dark excitons that is otherwise forbidden due to spin and momentum conservation. At cryogenic temperatures, the energetic alignment and coupling of the two abovementioned states results in localized defect emission which possess significant characteristics of single photon emitters. Strain-tunable devices are crucial for investigating the nature of TMD-based single photon emitters which can be beneficial for quantum information processing and secure communications.

In this study, we demonstrate strain modulation of monolayer WSe₂ suspended over a hole-patterned substrate via electrostatic deflection and characterize the resulting photoluminescence. This approach enables the creation of strain-tunable WSe₂ devices that can be operated at cryogenic temperatures, wherein strain fields are generated by applying a bias voltage to the suspended monolayer WSe₂ membrane. We observe a significant monolayer deflection of ~50 nm at 15V gate bias and a ~20 meV redshift of dark exciton peak as the applied is increased to 20V, corresponding to a 0.2% increase in tensile strain of WSe₂. Sharp localized emitters, typically associated with single photon emitters, were observed at 4K which showed less dependence on strain as the applied bias increased. Thus, we attribute these localized emitters to the presence of localized defects which are only weakly influenced by strained lattice environment. We also observe luminescence lifetimes of ~3 ns and saturation of luminescence intensity with incident power, both of which are also characteristic of single photon emitters. These realizations are critical for understanding the origin of single photon emitters based on strained monolayer WSe₂.

One-dimensional (1D) graphene superlattice (GSL) has drawn considerable research interest as it is promising for realizing the electron lensing effect [1]. Despite the intensive theoretical studies, 1D GSL has only been realized experimentally via periodic dielectric gates in previous studies [2, 3], which yields a moderate Kronig-Penney (KP) potential profile that is not viable to achieve electron supercollimation.

In this work, we demonstrate 1D GSL in the high KP potential limit exploiting nanoscale domains patterning in a ferroelectric bottom gate [4]. We work with 50 nm (001) PbZr_0.2Ti_0.8O₃ (PZT) films deposited on 10 nm La_0.67Sr_0.33MnO₃ buffered SrTiO₃ substrates. Monolayer graphene field-effect transistors with top h-BN global gates are fabricated on PZT prepatterned with periodic polarization up (P_up) and down (P_down) stripe domains, with the period L varying from 200 to 300 nm (Fig. 1a). The polarization shifts the Fermi level of graphene, leading to a KP potential V₀ of about 0.9 eV at 2 K. We fabricate 1D GSL samples in two configurations, with current along the SL vector ŝ and perpendicular to ŝ. For the former samples, additional Dirac points (DP) emerge in the sheet resistance (R_xx) vs. top-gated induced electron doping δn (Fig. 1b), from which emanates multiple Landau fan branches in the magnetic field (Fig. 1c). This feature is absent in the latter configuration (R_yy), which can be attributed to the SL modulated band crossings along ŝ. The carrier density between consecutive DP positions (Δn_DP) scales with the SL period as Δn_DP \propto L^β, with β = -1.18±0.06 (Fig. 1d), which closely resembles the inversely proportional relation predicted for the high KP potential limit. Figure 1e shows the simulated 1D GSL band structure for our ferroelectric doping scheme, with dimensionless KP potential u = V₀L/\hbarv_F = 90π, which reveals a highly flattened band that can potentially host electron lensing effect.

[1] C. H. Park, et al., Nano Letters 8, 2920 (2008)

[2] S. Dubey, et al., Nano Letters 13, 3990 (2013).

[3] Y. Li, et al., Nature Nanotechnology 16, 525 (2021).

Historically, magnetic fields have played an essential role in revealing the properties of semiconductors, and the many-body physics that can emerge when they are doped with mobile carriers. However, for atomically-thin ‘transition metal dichalcogenide’ (TMD) semiconductors such as MoS₂ and WSe₂, the relevant field scale is substantial (of order 100 tesla!) due to heavy carrier masses, huge exciton binding energies, and typically large Fermi energies.Fortunately, modern pulsed magnets can achieve this scale. This talk will discuss a few recent optical studies that probe the physics of -- and many-body correlations between -- excitons, electrons, and holes in TMD monolayers. These experiments used dual-gated TMD monolayers, assembled via van der Waals stacking directly atop single-mode optical fibers to enable polarized absorption spectroscopy at low temperatures in 60-100T fields.

*In charge-neutral monolayers, spectroscopy up to ~90T reveals the diamagnetic shifts of the neutral exciton’s 1s ground state and its excited 2s, 3s,… ns Rydberg states, revealing exciton masses, radii, binding energies, dielectric properties, and free-particle bandgaps – essential ingredients for the rational design of optoelectronic van der Waals structures. [1]

*In hole-doped monolayer WSe₂, high-field spectroscopy of both neutral and charged exciton transitions revealed the (often-hypothesized) spontaneous valley polarization of mobile holes, due to exchange interactions, occurring at ~40T. [2]

*In electron-doped WSe₂ monolayers, the ordering of the conduction bands in the K and K’ valleys allows studies of not only neutral excitons (X0) and charged excitons (X- trions) at low carrier density, but also many-body states that can emerge at higher doping. We investigate the so-called X-’ resonance that emerges at high electron density, known since 2013 but never understood. The data suggest that X-’ is, in fact, a six-particle “hexciton” state that arises when the photoexcited electron-hole pair couples simultaneously to two Fermi seas having quantum-mechanically distinguishable spin/valley quantum numbers. This state also appears in WS₂ and may appear in MoS₂, and appears in MoSe₂ at the B-exciton resonance [3,4]

[1] Nat. Comm.10, 4172 (2019); [2] PRL125, 147602 (2020);[3] Nano Letters22, 426 (2022);[4] PRL129, 076801 (2022).