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1. Gate-tunable exciton-polaron Rydberg series with strong roton effect

   Liu, Erfu; van Baren, Jeremiah; Lu, Zhengguang; Taniguchi, Takashi; Watanabe, Kenji; Smirnov, Dmitry; Chang, Yia-Chung; Lui, Chun Hung (June 2020, ArXivorg)

   null (Ed.)

   The electronic exciton polaron is a hypothetical many-body quasiparticle formed by an exciton dressed with a polarized electron-hole cloud in the Fermi sea (FS). It is predicted to display rich many-body physics and unusual roton-like dispersion. Exciton polarons were recently evoked to explain the excitonic spectra of doped monolayer transition metal dichalcogenides (TMDs), but these studies are limited to the ground state. Excited-state exciton polarons can exhibit richer many-body physics due to their larger spatial extent, but detection is challenging due to their inherently weak signals. Here we observe gate-tunable exciton polarons for the 1s - 3s excitonic Rydberg series in ultraclean monolayer MoSe2 devices by optical spectroscopy. When the FS expands, we observe increasingly severe suppression and steep energy shift from low to high Rydberg states. Their gate-dependent energy shifts go beyond the trion description but match our exciton-polaron theory. Notably, the exciton-polaron absorption and emission bands are separated with an energy gap, which increases from ground to excited state. Such peculiar characteristics are attributed to the roton-like exciton-polaron dispersion, where energy minima occur at finite momenta. The roton effect increases from ground to excited state. Such exciton-polaron Rydberg series with progressively significant many-body and roton effect shall provide a new platform to explore complex many-body phenomena. 

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2. Interplay of valley polarized dark trion and dark exciton-polaron in monolayer WSe2

   https://doi.org/10.1038/s41467-023-41475-4  

   Cong, Xin; Mohammadi, Parisa Ali; Zheng, Mingyang; Watanabe, Kenji; Taniguchi, Takashi; Rhodes, Daniel; Zhang, Xiao-Xiao (September 2023, Nature Communications)

   Abstract 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. The results provide good agreement with absorption measurements, which are dominated by dilute bright exciton responses. Here we investigate the Fermi sea dressing of spin-forbidden dark excitons in monolayer WSe₂. With a Zeeman field, the valley-polarized dark excitons show distinct p-doping dependence in photoluminescence when the carriers reach a critical density. This density can be interpreted as the onset of strongly modified Fermi sea interactions and shifts with increasing exciton density. Through valley-selective excitation and dynamics measurements, we also infer an intervalley coupling between the dark trions and exciton-polarons mediated by the many-body interactions. Our results reveal the evolution of Fermi sea screening with increasing exciton density and the impacts of polaron-polaron interactions, which lay the foundation for understanding electronic correlations and many-body interactions in 2D systems. 

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3. Enhanced nonlinear interaction of polaritons via excitonic Rydberg states in monolayer WSe2

   https://doi.org/10.1038/s41467-021-22537-x  

   Gu, Jie; Walther, Valentin; Waldecker, Lutz; Rhodes, Daniel; Raja, Archana; Hone, James C.; Heinz, Tony F.; Kéna-Cohen, Stéphane; Pohl, Thomas; Menon, Vinod M. (December 2021, Nature Communications)

   Abstract Strong optical nonlinearities play a central role in realizing quantum photonic technologies. Exciton-polaritons, which result from the hybridization of material excitations and cavity photons, are an attractive candidate to realize such nonlinearities. While the interaction between ground state excitons generates a notable optical nonlinearity, the strength of such interactions is generally not sufficient to reach the regime of quantum nonlinear optics. Excited states, however, feature enhanced interactions and therefore hold promise for accessing the quantum domain of single-photon nonlinearities. Here we demonstrate the formation of exciton-polaritons using excited excitonic states in monolayer tungsten diselenide (WSe 2 ) embedded in a microcavity. The realized excited-state polaritons exhibit an enhanced nonlinear response ∼ $${g}_{{pol}-{pol}}^{2s} \sim 46.4\pm 13.9\,\mu {eV}\mu {m}^{2}$$ g p o l − p o l 2 s ~ 46.4 ± 13.9 μ e V μ m 2 which is ∼4.6 times that for the ground-state exciton. The demonstration of enhanced nonlinear response from excited exciton-polaritons presents the potential of generating strong exciton-polariton interactions, a necessary building block for solid-state quantum photonic technologies. 

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4. Thermodynamic behavior of correlated electron-hole fluids in van der Waals heterostructures

   https://doi.org/10.1038/s41467-023-43799-7  

   Qi, Ruishi; Joe, Andrew Y.; Zhang, Zuocheng; Zeng, Yongxin; Zheng, Tiancheng; Feng, Qixin; Xie, Jingxu; Regan, Emma; Lu, Zheyu; Taniguchi, Takashi; et al (December 2023, Nature Communications)

   Abstract Coupled two-dimensional electron-hole bilayers provide a unique platform to study strongly correlated Bose-Fermi mixtures in condensed matter. Electrons and holes in spatially separated layers can bind to form interlayer excitons, composite Bosons expected to support high-temperature exciton condensates. The interlayer excitons can also interact strongly with excess charge carriers when electron and hole densities are unequal. Here, we use optical spectroscopy to quantitatively probe the local thermodynamic properties of strongly correlated electron-hole fluids in MoSe₂/hBN/WSe₂heterostructures. We observe a discontinuity in the electron and hole chemical potentials at matched electron and hole densities, a definitive signature of an excitonic insulator ground state. The excitonic insulator is stable up to a Mott density of ~0.8 × 10¹²cm⁻²and has a thermal ionization temperature of ~70 K. The density dependence of the electron, hole, and exciton chemical potentials reveals strong correlation effects across the phase diagram. Compared with a non-interacting uniform charge distribution, the correlation effects lead to significant attractive exciton-exciton and exciton-charge interactions in the electron-hole fluid. Our work highlights the unique quantum behavior that can emerge in strongly correlated electron-hole systems. 

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5. Disorder of excitons and trions in monolayer MoSe ₂

   https://doi.org/10.1063/5.0108001  

   Wang, Jue; Manolatou, Christina; Bai, Yusong; Hone, James; Rana, Farhan; Zhu, X. -Y. (December 2022, The Journal of Chemical Physics)

   The optical spectra of transition metal dichalcogenide monolayers are dominated by excitons and trions. Here, we establish the dependence of these optical transitions on the disorder from hyperspectral imaging of h-BN encapsulated monolayer MoSe₂. While both exciton and trion energies vary spatially, these two quantities are almost perfectly correlated, with spatial variation in the trion binding energy of only ∼0.18 meV. In contrast, variation in the energy splitting between the two lowest energy exciton states is one order of magnitude larger at ∼1.7 meV. Statistical analysis and theoretical modeling reveal that disorder results from dielectric and bandgap fluctuations, not electrostatic fluctuations. Our results shed light on disorder in high quality TMDC monolayers, its impact on optical transitions, and the many-body nature of excitons and trions. 

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