J. G. McHugh | Searcharxiv

arXiv2023

Moiré Superstructures in Marginally-Twisted NbSe$_2$ Bilayers

The creation of moiré superlattices in twisted bilayers of two-dimensional crystals has been utilised to engineer quantum material properties in graphene and transition metal dichalcogenide (TMD) semiconductors. Here, we examine the structural relaxation and electronic properties in small-angle twisted bilayers of metallic NbSe$_2$. Reconstruction appears to be particularly strong for misalignment angles $θ_P$ < 2.9$^o$ and $θ_{AP}$ < 1.2$^o$ for parallel (P) and antiparallel (AP) orientation of monolayers' unit cells, respectively. Multiscale modelling reveals the formation of domains and domain walls with distinct stacking, for which density functional theory (DFT) calculations are used to map the shape of the bilayer Fermi surface and the relative phase of the CDW order in adjacent layers. We find a significant modulation of interlayer coupling across the moiré superstructure and the existence of preferred interlayer orientations of the CDW phase, necessitating the nucleation of CDW discommensurations at superlattice domain walls.

cond-mat.mes-hall↗

arXiv2022

Self-organised quantum dots in marginally twisted MoSe$_2$/WSe$_2$ and MoS$_2$/WS$_2$ bilayers

Moiré superlattices in twistronic heterostructures are a powerful tool for materials engineering. In marginally twisted (small misalignment angle, $θ$) bilayers of nearly lattice-matched two-dimensional (2D) crystals moiré patterns take the form of domains of commensurate stacking, separated by a network of domain walls (NoDW) with strain hot spots at the NoDW nodes. Here, we show that, for type-II transition metal dichalcogenide bilayers MoX$_2$/WX$_2$ (X=S, Se), the hydrostatic strain component in these hot spots creates quantum dots for electrons and holes. We investigate the electron/hole states bound by such objects, discussing their manifestations via the intralayer intraband infrared transitions. The electron/hole confinement, which is strongest for $θ<0.5^{\circ}$, leads to a red-shift of their recombination line producing single photon emitters (SPE) broadly tuneable around 1\,eV by misalignment angle. These self-organised dots can form in bilayers with both aligned and inverted MoX$_2$ and WX$_2$ unit cells, emitting photons with different polarizations. We also find that the hot spots of strain reduce the intralayer MoX$_2$ A-exciton energy, enabling selective population of the quantum dot states.

cond-mat.mes-hall↗