Emergent Trion-Phonon Coupling in Atomically-Reconstructed MoSe2_2-WSe2_2 Heterobilayers

In low-temperature resonant Raman experiments on MoSe$_2$-WSe$_2$ heterobilayers, we identify a hybrid interlayer shear mode (HSM) with an energy, close to the interlayer shear mode (SM) of the heterobilayers, but with a much broader, asymmetric lineshape. The HSM shows a pronounced resonance with the intralayer hybrid trions (HX$^-$) of the MoSe$_2$ and WSe$_2$ layers, only. No resonance with the neutral intralayer excitons is found. First-principles calculations reveal a strong coupling of Q-valley states, which are delocalized over both layers and participate in the HX$^-$, with the SM. This emerging trion-phonon coupling may be relevant for experiments on gate-controlled heterobilayers.Comment: 6 pages, 3 figure

Moiré phonons in twisted MoSe 2 –WSe 2 heterobilayers and their correlation with interlayer excitons

Abstract: We report about the investigation of twisted MoSe2 homo- and MoSe2–WSe2 heterobilayers by means of low-frequency Raman spectroscopy (LFRS) and low-temperature micro photoluminescence (µPL). In room-temperature LFRS experiments on both, twisted MoSe2 homobilayers and twisted MoSe2–WSe2 heterobilayers, we observe moiré phonons, i.e. folded acoustic phonon modes due to the moiré superlattice. In the heterobilayers, we can identify moiré phonons of both materials, MoSe2 and WSe2. While the twist angles for the homobilayers are relatively precisely known from the applied tear-and-stack preparation method, the twist angles of the heterobilayers have to be determined via second-harmonic-generation microscopy on monolayer regions of the samples, which has significant uncertainties. We show that by the moiré phonons of the heterobilayers, the relative twist angles can be determined on a local scale with much higher precision. We apply our technique for the investigation of a large area H-type (twist angle θ = 60∘ + δ) MoSe2–WSe2 heterobilayer. These investigations show that spatial regions, which can be identified to be atomically reconstructed (i.e. δ = 0∘) by the observation of an interlayer shear mode in LFRS experiments, exhibit a strong, momentum-allowed interlayer-exciton signal in low-temperature µPL. On the contrary, regions, where moiré phonons are observed, i.e. which can be identified to be rigidly twisted by a misalignment angle in the range of 5°≲|δ|≲6° , exhibit no significant interlayer-exciton signals

Large-Scale Mapping of Moiré Superlattices by Hyperspectral Raman Imaging.

Moiré superlattices can induce correlated-electronic phases in twisted van der Waals materials: strongly correlated quantum phenomena emerge, such as superconductivity and the Mott-insulating state. However, moiré superlattices produced through artificial stacking can be quite inhomogeneous, which hampers the development of a clear correlation between the moiré period and the emerging electrical and optical properties. Here, it is demonstrated in twisted-bilayer transition-metal dichalcogenides that low-frequency Raman scattering can be utilized not only to detect atomic reconstruction, but also to map out the inhomogeneity of the moiré lattice over large areas. The method is established based on the finding that both the interlayer-breathing mode and moiré phonons are highly susceptible to the moiré period and provide characteristic fingerprints. Hyperspectral Raman imaging visualizes microscopic domains of a 5° twisted-bilayer sample with an effective twist-angle resolution of about 0.1°. This ambient methodology can be conveniently implemented to characterize and preselect high-quality areas of samples for subsequent device fabrication, and for transport and optical experiments

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements