Abstract
The noncentrosymmetric nature of single-layer (SL) transition-metal dichalcogenides (TMD) manifests itself in the finite piezoelectricity and valley-Zeeman coupling. We microscopically model nonlinear exciton transport in nanobubbles of SL TMDs. Thanks to the giant piezoelectric effect, we obtain an enormous internal electric field, ${E}_{\mathrm{piezo}}\ensuremath{\sim}{10}^{7}\phantom{\rule{0.16em}{0ex}}\mathrm{V}/\mathrm{m}$, resulting in a built-in dipole moment of excitons. We demonstrate that the piezo-induced dipole-dipole interaction provides a novel channel for the nonlinear exciton transport distinct from the conventional isotropic funneling of excitons and leads to the formation of a hexagon-shaped exciton droplet on the top of circularly symmetric nanobubbles. Moreover, we found that the hexagonal distribution of exciton density is preserved even for strongly elliptic nanobubbles. The effect is tunable via the bubble-size dependence of the piezoelectric field ${E}_{\mathrm{piezo}}\ensuremath{\sim}{h}_{\mathrm{max}}^{2}/{R}^{3}$ with ${h}_{\mathrm{max}}$ and $R$ being the bubble height and radius, respectively.
Highlights
The single-layer (SL) of transition-metal dichalcogenides (TMDs) represents a flatland for probing exciton-related phenomena [1] owing to the visible frequency range direct band gap
The dipole moment can be induced in TMD excitons via an external electric field [21,22,23] and in spatially indirect excitons in bilayers [24,25]
We discuss a novel mechanism for inducing an exciton dipole moment in noncentrosymmetric SL-TMD due to strain
Summary
The single-layer (SL) of transition-metal dichalcogenides (TMDs) represents a flatland for probing exciton-related phenomena [1] owing to the visible frequency range direct band gap. The dipole moment can be induced in TMD excitons via an external electric field [21,22,23] and in spatially indirect excitons in bilayers [24,25]. Two-dimensional (2D) materials are very flexible to out-ofplane deformation and strong to in-plane stretch [26,27] This unique property leads to nanobubble formation in graphene and TMDs with a wide radius range 10–500 nm. The piezoelectric constant conveys topological information about the valley-Chern number [39,42] Another consequence of broken inversion symmetry in SL-TMDs is the valley-. A giant piezoelectric effect yields in an enormous internal electric field of the order of ∼107V/m, depicted, which induces an exciton dipole moment d(r) = αE Epiezo(r), where αE stands for the exciton polarizability. Owing to the exciton nonlinearity, we predict a long-standing spatially threefold symmetric exciton density which can be directly accessed via photoluminescence measurements
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