Intrinsic dipole moment is an important characteristic of Janus single-layer MoSSe. Tuning the dipole moment would broaden the potential applications of Janus MoSSe in the field of piezoelectricity and molecular sensing. In this study, the dipole moments of Janus single-layer MoSSe with 3d transition-metal (TM) adatoms (Sc–Ni) are explored by using first-principles calculations. Our results demonstrate that the dipole moments of Janus MoSSe change with TM atom adsorption. For the adsorption of TM atoms on the Se surface, the dipole moments are enhanced when compared to the case of pristine MoSSe, regardless of at the M adsorption site or at the H adsorption site. However, in the case of S surface adsorption, the dipole moments are weakened or even reversed for some TM atoms. Among all the 3d TMs considered, the effect of Sc atom adsorption is the largest, while it is the smallest for Ni atom adsorption. By means of a simplified model, the total dipole moments can be regarded as the superposition of the dipole moments from the Janus MoSSe and the ionic TM atoms. Strengthening and weakening of the dipole moments depend on the direction of dipole moments from the ionic TM atoms. Thus, we could utilize TM atom adsorption to tune the dipole moments with both magnitude and direction.

：Inter-twisted bilayers of two-dimensional (2D) materials can host low-energy flat bands, which offer opportunity to investigate many intriguing physics associated with strong electron correlations. In the existing systems, ultra-flat bands only emerge at very small twist angles less than a few degrees, which poses challenge for experimental study and practical applications. Here, we propose a new design principle to achieve low-energy ultra-flat bands with increased twist angles. The key condition is to have a 2D semiconducting material with large energy difference of band edges controlled by stacking. We show that the interlayer interaction leads to defect-like states under twisting, which forms a flat band in the semiconducting band gap with dispersion strongly suppressed by the large energy barriers in the moiré superlattice even for large twist angles. We explicitly demonstrate our idea in bilayer α-In2Se3 and bilayer InSe. For bilayer α-In2Se3, we show that a twist angle ~13.2˚ is sufficient to achieve the band flatness comparable to that of twist bilayer graphene at the magic angle ~1.1˚. In addition, the appearance of ultra-flat bands here is not sensitive to the twist angle as in bilayer graphene, and it can be further controlled by external gate fields. Our finding provides a new route to achieve ultra-flat bands other than reducing the twist angles and paves the way towards engineering such flat bands in a large family of 2D materials.

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In this work we study the stochastic resonance (SR) effect in a driven dissipative Jaynes-Cummings model. The SR effect is systematically studied in the semiclassical and full quantum frameworks and in both cases we find that SRs simultaneously occur for the optical and atomic freedoms. In particular, at zero temperature quantum SR can be induced merely by vacuum fluctuations. The qualitative features of semiclassical SR and quantum SR are similar, but the parameter region of quantum SR are shifted from the semiclassical region due to the widely-used factorization in obtaining semiclassical equations of motion. Our results provide a theoretical basis for experimentally observing and studying the SR phenomenon of the Jaynes-Cummings model in the quantum regime.

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