Phonon dispersio dash vertical5/17/2023 While plasmonic structures have been extensively studied, the ultimate limits of phonon polariton squeezing, in particular enabling the confinement (the ratio between the excitation and polariton wavelengths) exceeding 102, is yet to be explored. Recently this has brought interest to highly confined plasmon and phonon polaritons. Improvements in device density in photonic circuits can only be achieved with interconnects exploiting highly confined states of light. The coupling strength is found to gradually increase along the superconducting dome up to the intermediate regime, suggesting bipolaronic pairing in 2D superconductivity. The unexpected short-range nature of electron-phonon (e-ph) coupling in MoS2 can be explained by its valley degeneracy that enables strong intervalley coupling mediated by acoustic phonons. Using a high-resolution band mapping of charge carriers, we found strong band renormalizations collectively identified as a hitherto unobserved spectral function of Holstein polarons. Here, we report the discovery of Holstein polarons in a surface-doped layered semiconductor, MoS2, where a puzzling 2D superconducting dome with the critical temperature of 12 K was found recently. Holstein polaron is a small composite particle of an electron carrying a cloud of self-induced lattice deformation (or phonons), which has been proposed to play a key role in high-temperature superconductivity and carrier mobility in devices. Carrier doping to 2D semiconductors can be used to modulate manybody interactions and to explore novel composite particles. Two-dimensional (2D) crystals have emerged as a class of materials with tuneable carrier density. These findings may facilitate the development of TMDC-based opto-valleytronic devices. Moreover, the developed framework was successfully applied to engineer the valley polarization of excitons in 1L-WSe2. We show that the temperature-dependent exciton valley relaxation times in 1L-WSe2 under various exciton and carrier densities can be understood using a unified framework of intervalley exciton scattering via momentum-dependent long-range electron-hole exchange interactions screened by 2D carriers that depend on the carrier density and the exciton linewidth. The exciton valley relaxation times were examined using polarization- and time-resolved photoluminescence spectroscopy at temperatures ranging from 10 to 160 K. Here, we demonstrate that Coulomb screening by 2D carriers plays a critical role in excitonic valley pseudospin relaxation processes in naturally carrier-doped WSe2 monolayers (1L-WSe2). Monolayers of transition metal dichalcogenides (TMDC) have recently emerged as excellent platforms for exploiting new physics and applications relying on electronic valley degrees of freedom in two-dimensional (2D) systems. The results will be the basis for future experimental and theoretical work regarding electron-phonon interactions, intervalley scattering, as well as phonons in related 2D materials. Supported by first-principles calculations, we determine the phonon displacement patterns, symmetry properties, and scattering intensities. Our results underline the two-dimensional nature of MoS$_2$. Here we present the phonon dispersion of bulk MoS$_2$ in the high-symmetry directions of the Brillouin zone, determined by inelastic X-ray scattering. However, experimental data of the complete phonon dispersion relations in these materials is missing. Phonons are fundamental to many of the underlying physical processes, like carrier and spin relaxation or exciton dynamics. Their remarkable physical properties make them promising for applications in optoelectronic, spintronic, and valleytronic devices. They are van-der-Waals crystals with highly anisotropic properties, which allows exfoliation of individual layers. Transition metal dichalcogenides like MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$ have attracted enormous interest during recent years.
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