Fast two-dimensional long-wavelength plasmons in atomically thin Mo S 2

Résumé fourni par la source

Plasmons are bosonic wavelike charge oscillations in solids and the plasmon group velocity ( v g ), the first derivative of momentum ( q )-dependent dispersions, depicts the many-body electron motions of critical electronic importance. The profound characteristics of v g in either three-dimensional (3D) or two-dimensional (2D) electron systems are nonetheless short of quantitative scrutiny. Using q -resolved electron energy loss spectroscopy, we tackle the q -dependent v g of π and π + σ plasmons in 3D-bulk and 2D-monolayer (ML) Mo S 2 semiconductors, with the 2D Mo S 2 being the model transition-metal dichalcogenide (TMD) for emergent atomically thin electronics. v g along with the underlining electronic parameters of the charge density, effective mass, and screening length in 2D for the plasmon dispersions is quantitatively elaborated. 2D v g is further compared to those evaluated from metallic ML-TMD Ta S 2 and graphene, and the scaling over the single-particle Fermi velocity ( v F ) unveils a v g / v F function generally obeyed by the semiconducting and metallic TMD and graphene of distinctly different massive and relativistic characters, respectively. Beyond the pertinent critical q (on the order of 10 − 2 Å − 1 ) where one observes v g = v F , electric-field lines connecting the oscillating 2D charges would turn from vacuum into the atomically thin matters, concomitant with the increasing plasmon damping and reducing v g ( v g < v F ). Long-wavelength 2D plasmons below the critical q characterize the field lines largely in vacuum and are persistent and fast ( v g > v F ), with high-speed application merits. Conversely, 3D plasmons in bulk Mo S 2 never exceed v F .

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